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So I have this problem (which I translated to English as accurately as could):
When studying a compact molecular cloud with radiotelescope, at a wavelength of 18 cm, hydroxyl (OH) maser(ic) radiation was registered. Radiation intensity changes at about 5 min cycle. Evaluate the maximum diameter of the cloud.
Actually we learned very little about molecular clouds at high school and I have no idea how to solve this problem.
If the intensity change cycle is wave period, I can calculate wave velocity, which would be very low (as well as the frequency). I'm not asking for the answer, some hints would be enough. Thanks.
The principle that you apply is that the properties of a body cannot change faster than it the time it takes for light to cross the body. So pulsars can be very small, which we know from the high frequency of their variation.
A body which varies with a period of 5 minutes can't be larger than about 5 light-minutes across.
Abstract: Formation and evolution of galaxies are mainly driven by mergers and interactions. The signatures of these interactions are expected to be imprinted on their structure and stellar populations of these galaxies and hence, are excellent probes to understand galaxy evolution. In the low redshift, minor mergers dominate. Magellanic System is one of the nearest examples of a minor merger event. In this talk, I will present the 3D structure and kinematics of the stellar populations of the Small Magellanic Cloud obtained using the data from the VISTA Survey of the Magellanic Clouds and Gaia and the implications of the results on our understanding of the evolution of the Magellanic System.
Dr Hannah Worters
Abstract: The SAAO hosts 24 optical/infrared telescopes in the semi-arid desert region of the Karoo. I will give an informal overview of what we have (including the 10-m SALT telescope), what we're working on (a bunch of new instruments, remote observing and AI), and what you can apply to use (without any membership requirements or fees).
Dr Vanessa Moss
Title: Survey of Weak Intensity Southern HI: Uncovering the hidden iceberg structure of the Galactic halo.
Abstract: How the Milky Way gets its gas and keeps its measured star formation rate going are both long-standing mysteries in Galactic studies, with important implications for the relationship of galaxies to their circumgalactic media throughout the Universe. I will present our discovery of two populations of neutral hydrogen (HI) in the halo of the Milky Way: 1) a narrow line-width dense population typical of the majority of bright high velocity cloud (HVC) components, and 2) a fainter, broad line-width diffuse population that aligns well with the population found in very sensitive pointings such as in Lockman et al. (2002). This discovery of diffuse HI, which appears to be prevalent throughout the halo, takes us closer to solving the Galactic mystery of accretion and reveals a gaseous neutral halo hidden from the view of most large-scale surveys. We have carried out deep Parkes observations as part of the Survey of Weak Intensity of Southern HI (SWISH) to investigate these results further, in order to truly uncover the nature of the diffuse HI and determine whether the 3:1 diffuse/dense ratio (based on the limited existing data) is consistent with what is seen when Parkes and the 140 ft Green Bank telescope are employed at comparable sensitivity. With these data, through a combination of both known and new sightline measurements, we aim to reveal the structure of the Galactic halo in more detail than ever before and connect our results to the recent studies of ionised UV absorption towards HVCs.
Ms Amy Whitney
Title: Unbiased size evolution in the GOODS fields.
Abstract: We present a new analysis of a sample of galaxies from the GOODS North and South fields of CANDELS using distance independent relative surface brightness metrics to determine an unbiased measure of the size evolution over the redshift range 1<z<7. We introduce a new method of removing background objects from images of galaxies, used in order to reduce the effect extraneous objects have on measuring the size of a galaxy. Using the Petrosian radius, we are able to determine whether the size of a galaxy increases most in the inner or outer regions. We find a slower evolution of the inner regions of galaxies compared to the outer regions which imply mass is added to the outer edges of a galaxy as it evolves and therefore suggests inside-out formation. Our results place new limits on the history of galaxy structural evolution through the past 12 Gyr of cosmic evolution.
Prof. Andy Bunker
"Exploring the high redshift Universe with JWST&rdquo
Prof. Shu-ichiro Inutsuka
Title: Filament Paradigm and Star Formation in the Galactic Disk.
Ms Stefania Barsanti
Title: SAMI: bulge and disk stellar populations in cluster galaxies.
Dr Lorenzo Spina
Title: The Milky Way disk that we have never observed.
Prof. Toshifumi Futamase
Title: Subaru Weak lensing Survey of Dark Matter Subhalos in Coma and nearby clusters of galaxy.
Abstract: The hierarchical structure formation scenario based on the cold dark matter (CDM) paradigm has been very successful in explaining various cosmological observations such as the large scale distribution of galaxies, and thus became the standard scenario for structure formation. However there is almost no observational evidence to test it on the Mpc scale where mass assembly history becomes important. We have performed weak lensing observation of COMA cluster at z = 0.027. and other nearby
Dylan Michelson Paré
Title: A VLA Polarimetric Study of the Galactic Center Radio Arc: Characterizing Polarization, Rotation Measure, and Magnetic Field Properties".
Abstract: "The Galactic Center (GC) is a unique observational target containing molecular cloud densities, magnetic field strengths, and gas temperatures more extreme than found elsewhere in the Galaxy. One way to study the properties of the strong magnetic field in this region is to analyze the unique non-thermal filaments (NTFs) -- extended, synchrotron-emitting structures. The most prominent set of NTFs is known as the Radio Arc. In this talk I present the results of my VLA study of the Radio Arc and compare my findings with those obtained for other NTFs in the GC. I detail the complex region the Arc NTFs are embedded within and also detail the polarized intensity structure of the Arc NTFs. We analyze the distribution of RM values for the source and determine the rotation is likely caused by external media along the line of sight. After correcting for Faraday rotation, the intrinsic magnetic field orientation is found to generally trace the extent of the NTFs. However, the intrinsic magnetic field in several regions of the Radio Arc shows an ordered pattern that is rotated by about 60 degrees to the extent of the NTFs. This changing pattern may be caused by an additional magnetized structure local to the Radio Arc, so that we observe two field systems superposed in our observations."
Dr Alessandro Maini
Title: Radio-Quiet AGN: What brews into the cauldron?
Abstract: The radio-quiet/radio-loud dichotomy of Active Galactic Nuclei is an outstanding and long-lasting open question in Astrophysics research, dating back to the '60s of the past century. Only some 30 years ago the Astrophysics community started to shed some light on the topic, but the more we dug into the dichotomy the more the questions that were raised. Today, we can say that it is quite clear what radio-loud AGN are, but the same answer for radio-quiet objects eludes us. In this talk, I will briefly recall the historical evolution of the dichotomy topic, to show how it started and where we are now. I will also show my personal contribution to the research on the topic, focusing on findings about the nature of some radio-quiet AGN I worked on during and after my PhD work.
Dr Meridith Joyce
Title: Better Stellar Modeling: Numerical Tools and Techniques for the Modern Observational Landscape.
Abstract: These data will help us calibrate stellar models!" is an oft-quoted refrain in justifications for scientific projects, yet we hear less frequently and in less detail about the ways large surveys of stars or precision measurements of individual systems actually do drive improvement in our stellar models. In this talk, I will highlight a few recent cases in which the contemporaneous expansion of our modeling capabilities and the increased availability of observational constraints have aligned to provide us with exciting new insights on stars and stellar physics.
I will discuss the seismic and evolutionary modeling of T Ursae Minoris using the Modules for Experiments in Stellar Astrophysics (MESA) code and advertise the new 1D-MESA2HYDRO-3D software package just released this month. The latter is a Python tool for projecting 1D MESA profile models into 3D particle distributions that can be used as initial conditions for hydrodynamical modeling with Phantom smoothed-particle hydrodynamics program, designed with the help of collaborators here at Macquarie University and at Monash University.
Mr Chikaedu Ogbodo
Title: A-Masing Star is born!: When magnetic fields don't go Gaga!
Abstract: Observed at the Galactic scale, magnetism plays an essential role at the onset of star formation, going from weak diffuse interstellar fields to amplified compressed fields. Weak, large-scale Galactic magnetic fields in diffuse gas have been extensively studied. In contrast, high density regions of the Galactic spiral arms which host high-mass star forming (HMSF) regions have typical magnetic field strengths order(s) of magnitude stronger. I study the correlation between orientations observed in the diffuse large-scale magnetic field and the Galactic-scale magnetic field directions traced in high-density regions, using observations of four ground-state OH masers towards 554 sites of 6.7-GHz methanol masers (exclusive tracers of HMSF). I use the Zeeman effect to measure the in-situ magnetic field direction and strength in these high-density regions. An expected outcome is to determine if the information about field orientation is retained after contraction from weak large-scale magnetic fields to the high densities found in high-mass star forming regions. I'll present the complete polarimetric and detection results for the rarer 1720-MHz OH maser transition, and the Galactic magnetic field distribution revealed by the in-situ magnetic field orientation of the 1720-MHz maser transition.
Ms Rachel Rayner
Title: Science Communication
Abstract: The field of science communication is growing exponentially but what does it look like and how can it help obtain research goals? By exploring methods of science communication used on land, air and sea, we can extract tools for the communication of astronomy to larger and broader audiences.
Prof Isabel Perez Martin
Title: Are galactic bars efficient at stirring and mixing their discs?
Abstract: Bars are believed to be major drivers of secular evolution and radial mixing in spiral galaxies through its dynamical interaction with the disc and the halo. Bars are present in around 70% of dic galaxies and they are thought to be key to the rearrangement of material and angular momentum as well as in the building up of the central bulge. I will review our current understanding of how bars affect their hosting galaxies throughout time, placing in this context the Milky Way bar. I will present recent observational results based on integral field spectroscopy of 2D and radial distribution of the stellar and the ionised gas properties of a large sample of galaxies. I will also show some recent observational results on the issues of bar formation and evolution to conclude on the importance of bars in the galaxy makeover.
Dr Robert Harris
Title: Astrophotonics at the Landessternwarte, Heidelberg.
Abstract: As astrophotonic instruments increase in maturity they are moving from small prototypes to capable scientific instruments. This development is leading to further challenges of long term stability and increased performance. In this talk, I will describe the on-going work at the Landessternwarte, Heidelberg, to model and develop some of these devices and instruments. I will detail our astrophotonic group&rsquos work on modelling past experiments, the development of a 3D printed microlens ring for use as a tip-tilt sensor for the iLocater on the Large Binocular Telescope and the development of the MCIFU, a multi-core fibre fed integral field spectrograph. I will also discuss our future plans for the instruments.
Ms Anita Petzler
Title: Getting excited about excitation: Making sense of a scary molecule
Dr Daniel Cotton
Title: A New Age of Stellar Polarimetry.
Abstract: We have entered a new age of stellar polarimetry, and are poised to answer some of the outstanding questions in stellar evolution. These questions include the interior structure of stars, their rotation and the complications of binarity, which are currently the biggest sources of uncertainty in stellar evolution models (e.g. Heger 2000, Georgy et al. 2017). In this talk, I will give an overview of the technique of polarimetry, the progress we have made on these questions and the future direction of this research.
Since the discovery of interstellar polarisation in the 1940s, the applications of optical stellar linear polarimetry have been largely limited to examining dust and gas in interstellar and circumstellar processes. Yet, in his seminal paper, that spurred the development of the first modern instruments, Chandrasekhar (1946) envisioned studying the atmospheres of eclipsing binaries. Since then Harrington & Collins (1968) have shown polarimetry can be used to measure the true rotational speeds of stars, and Odell (1979) that polarimetry has the power to determine the modes of oscillation in heavy stars &ndash revealing the structure of their interiors. For 70 years instrumental imprecision has prevented the pursuit of these endeavours.
Over the last 5 years, my colleagues and I have been at the forefront of the development and application of a new generation of high-precision stellar polarimeters. With these instruments, we have pushed the limits of their conventional uses, contributed to the study of exoplanet atmospheres with them, and with the aid of radiative transfer models, pioneered their application to stellar atmospheres. We have made the first measurement of intrinsic polarisation resulting from rapid rotation (Cotton et al. 2017), and most recently observed polarimetric variation due to non-radial pulsation in beta Cephei stars. We have also made the first determination of true reflection in binary star systems (Bailey et al. 2019) &ndash an effect that was never predicted, and yet is readily observable with our instrument using even an amateur-sized telescope!
References: | Heger et al. (2000) ApJ 528, 1. | Georgy et al. (2017) ABSC Conf., 37. |Chandrasekhar (1946) ApJ 103, 351. | Harrington & Collins (1968) ApJ 151, 1051. | Odell (1979) PASPC 91, 326. | Cotton et al. (2017) Nature Ast. 1, 690. | Bailey et al. (2019) Nature Ast. in press.
Dr Joanne Drazkowska
Title: How do planets form?
Abstract: Recent years brought a paradigm shift for how planets form. It has become clear that planet formation is a rule rather than an exception. Nevertheless, this process remains poorly understood. I will present the most recent discoveries and concepts emerging in the planet formation community. I will particularly focus on the earliest stages of planet formation when small dust grains present in disks surrounding young stars grow towards the building blocks of planets, called planetesimals.
Dr Andy Sheinis
Title: Update on the Maunakea Spectroscopic Explorer.
Abstract: Australia is a partner in the Maunakea Spectroscopic Explorer (MSE), a massively multiplexed pectroscopic survey facility that will be built in the coming decade on the Canada-France-Hawaii telescope site on Maunakea, Hawaii. MSE will be a dedicated, 11.25 m, wide-field telescope that will observe more than 4000 targets distributed over 1.5 square degrees in every pointing. MSE will use 8 fibre-fed spectrographs to capture
3000 low resolution spectra (R
1000 high-resolution spectra (R
40,000) covering the full 1.5 sq. degree field contiguously with each resolution. MSE will have a survey speed that is
20X faster than MOONS based on perture size x field of view x multiplexing x observing time. Furthermore, it will produce the same number of spectra as the full SDSS Legacy Survey every 7 weeks. Some of the initial science goals will be to identify the astrophysical location and details of stellar nucleosynthesis unveil the composition and dynamics of the faint universe through chemical abundance studies of stars in the outer Galaxy measure the masses of thousands of black holes at the cores of galaxies weigh neutrinos and test exotic models of cosmology where dark energy properties vary at high redshift. Australian scientists make up approximately 10 % of the MSE Science Team that now involves close to 400 astronomers from 30 countries. Furthermore, Australia has been picked to develop the fibre positioner system based on the systems AAO produced for Subaru and 4Most. Here, I will review the technical aspects of the facility and discuss scientific potential of the only dedicated 10-meter class spectroscopic facility planned for the coming decade.
Mr Abdelbassit Senhadji
Title: A new channel for the formation and evolution of subdwarf stars.
Abstract: We present a detailed analysis of the conditions required to form subdwarf stars in wide binaries via stable Roche Lobe overflow. Starting with an evolutionary grid of almost 4000 primordial binaries with component masses between approximately 1 to 8 Msun and initial orbital periods of
1 to 200 days, many of these binaries experience an Algol-like phase of evolution and a substantial fraction of those produce binaries containing subdwarfs with orbital periods of between
20 to 500 days. We conclude that: (1) the final period is largely dependent on the assumed physics of non-conservative mass transfer (2) the donor star (subdwarf progenitor) typically has a mass of between about 3 and 6 solar masses (3) there is a very natural evolution of subdwarfs from a long-lived sdB stage (
30 Myr) and, (4) the range of masses and effective temperatures of sdB and sdO stars can overlap substantially depending on the properties of the primordial binary. Specifically, we find that the masses can range from
0.4 to 0.8 Msun, and that sdBs have temperatures in the range of 15000 < Teff (K) < 45000 while the range for sdOs is about 25000 < Teff (K) < 100000. One example of a post-Algol binary that is evolving towards the subdwarf stage is MWC882 (Zhou et al. 2018). The observational implications of this channel are also discussed.
Mr James Tocknell
Title: Winds from Protoplanetary Discs
Abstract: Magnetically-driven disc winds have significant effects on the evolution of protoplanetary discs, via the removal of angular momentum and mass from the disc. However, existing models typically ignore non-ideal magnetohydrodynamic effects, such as Hall drift, but these are known to operate inside these discs, and affect their structure and evolution, for example suppressing magnetically-driven turbulence and magneto-rotational instability. In my talk, I will present preliminary results of self-similar disc wind models which include non-ideal magnetohydrodynamic effects within the disc.
Mr Greg Goldstein
Title: The Distribution of Star Formation Across Galaxi Disks
Abstract: The distribution of the star formation across galaxy disks using integral field spectroscopy is studied to determine if the distribution varies according to the location of a galaxy on, above or below the main sequence of star formation. Radial profiles of the star formation rate surface density demonstrate that central suppression of star formation may occur in galaxies in all locations. It is considered there is an evolutionary sequence moving from MS, to quenching-in-progress, to quenched allowing tests of proposed mechanisms for quenching. The findings favor quenching models such as the compaction model, and bar-driven quenching that involve a sequence of processes including: disk instabilities that trigger gas inflows, central starbursts, stellar feedback and gas outflows, and central quenching associated with gas depletion.
Mr Hiep Nguyen
Title: &ldquoExploring the properties of warm and cold atomic hydrogen in the Taurus and Gemini regions".
Abstract: We report Arecibo 21 cm absorption-emission observations to characterise the physical properties of neutral hydrogen (HI) in the proximity of five giant molecular clouds (GMCs): Taurus, California, Rosette, Mon OB1, and NGC 2264. Strong HI absorption was detected toward all 79 background continuum sources in the
60x20 square degree region. Gaussian decompositions were performed to estimate temperatures, optical depths and column densities of the cold and warm neutral medium (CNM, WNM). The properties of individual CNM components are similar to those previously observed along random Galactic sightlines and in the vicinity of GMCs, suggesting a universality of cold HI properties. The CNM spin temperature (Ts) histogram peaks at
50K. The turbulent Mach numbers of CNM vary widely, with a typical value of
4, indicating that their motions are supersonic. About 60% of the total HI gas is WNM, and nearly 40% of the WNM lies in thermally unstable regime 500-5000K. The observed CNM fraction is higher around GMCs than in diffuse regions, and increases with increasing column density (NHI) to a maximum of
75%. On average, the optically thin approximation (N*(HI)) underestimates the total N(HI) by
21%, but we find large regional differences in the relationship between N(HI) and the required correction factor, f=N(HI)/N*(HI). We examine two different methods (linear fit of f vs log10(N*(HI)) and uniform Ts) to correct for opacity effects using emission data from the GALFA-HI survey. We prefer the uniform Ts method, since the linear relationship does not produce convincing fits for all subregions.
Prof Rupert Croft
Title: Relativistic probes of galaxies and the large-scale structure of the Universe
Abstract: In the next 5 years, the number of galaxies with measured redshifts will increase into the tens of millions. This will enable us to map the large-scale structure of the Universe with great precision. New tests of dark energy, dark matter and the nature of gravity will become possible. One avenue is to search for evidence of relativistic effects, which alter the relationship between the intrinsic and observed properties of galaxies. These include the gravitational redshift (first seen in Earth-bound laboratories in 1960), that depends on the depth of galaxy potential wells. Another is relativistic beaming, which is sensitive to the interplay between galaxy peculiar velocities and their spectra. The first large surveys of galaxies have begun to make measurements of these effects possible, and they offer us a new way look at the relationships between galaxies and their surrounding dark matter. I will describe how to measure these effects, including results from hydrodynamic simulations as well as the first measurements from both large scale structure survey data and individual galaxies.
Prof Tiziana Di Matteo
Title: A Universe of Black Holes
Abstract: Massive black holes are fundamental constituents of our cosmos. Understanding their formation at cosmic dawn, their growth, and the emergence of the first, rare quasars in the early Universe remains one of our greatest theoretical and observational challenges. Hydrodynamic cosmological simulations self-consistently combine the processes of structure formation at cosmological scales with the physics of smaller, galaxy scales. They capture our most realistic understanding of massive black holes and their connection to galaxy formation. I will focus on the predictions for the first quasars and their host galaxies in the BlueTides simulation. Next generation facilities and the advent of multi-messenger astrophysics brings new exciting prospects for tracing the origin, growth and merger history of massive black holes across cosmic ages.
Mr Abner Zapata
Title: &ldquoFIDEOS spectrograph: radial velocity stability results at the ESO1m&rdquo
Abstract: The Fiber Dual Echelle Optical Spectrograph (FIDEOS) is a high-resolution spectrograph developed by the Center of Astro-engineering UC, Chile, and installed in the ESO1m telescope at La Silla Observatory. We present the results of the commissioning and early science. The radial velocity (RV) precision and stability were improved during the early operation reaching values as good as 5 m/s in a single night and long term stability better than 10 m/s. Also, we present a brief overview of our ongoing and future projects.
Dr. Esha Kundu
Title: SNe IIb in Radio
Abstract: Massive stars that loss most of their hydrogen envelope explode as Type IIb supernovae (SNe IIb). The progenitors of these SNe may be single massive stars that undergo huge mass loss due to strong winds. Alternatively, the mass stripping can happen due to an interaction with a companion star in a binary system. A useful way to investigate the pre-SN systems is to look for radio emission from the interaction of SN ejecta with the circumstellar medium (CSM). The flux of radio emission is roughly proportional to the density of the particle in the CSM, which, in general, is shaped by the mass ejection from the pre-SN star. Therefore, by studying this radiation one can map the mass-loss history of the progenitor star. In this talk, I will mainly focus on radio emission from two very well observed SNe IIb, SN 1993J and SN 2011dh, and discuss how the detailed modelling of their radio emission has enabled us to gain vital information about the evolution of their progenitors before explosion.
Dr Simon Ellis
Title: Making the near-infrared night sky dark.
Abstract: The near-infrared night sky is extremely bright due to emission from atmospheric OH molecules. The high surface brightness makes sky-subtraction intrinsically noisy. Furthermore, the OH emission is highly variable, spatially and temporally, leading to large systematic errors in sky-subtraction. This long-standing problem has severely hindered near-infrared astronomy for decades, but we are now close to a ground-based solution for the first time. PRAXIS is a unique near-infrared spectrograph currently being commissioned on the AAT, which selectively filters the atmospheric OH lines from the incoming light using fibre Bragg gratings, rendering the sky dark. I will describe the principles of OH suppression with fibre Bragg gratings, and the development of this novel technology up to the present day, culminating in PRAXIS. I will present preliminary results from the first two commissioning runs, and describe possible future developments.
Dr Elaina Hyde
Title: Science vs. DataScience Astrophysics Face-Off: Who Will Win?
Abstract: This talk will cover my journey through Astrophysics and Data Science with some tips and tricks that I picked up along the way as well as some of the processes that I use in my work. I will compare Data Science and Science methodologies and discuss what it means for Astrophysics. I will additionally cover some of the tools that have helped me build my career in STEM so far. If we have time, we will even play a Machine Learning game.
Mr Georges Georgevits
Title: Searching for Kuiper Belt objects by stellar occultation.
Abstract: The Kuiper Belt is the region of the solar system extending from the orbit of Neptune to
50AU. A large number of objects, known as Kuiper Belt Objects (KBOs), are thought to reside there. Only the largest of these can be seen by direct observation, since they are so distant and faint. Small KBOs, too faint to be seen by direct observation, can be detected by stellar occultation when they pass in front of a suitable background star.
Our work reports the results obtained from a ground-based stellar occultation survey using the 1.2m UK Schmidt telescope fitted with 100 optical fibres feeding a CCD camera with continuous readout and 10msec time resolution. We accumulated 6,500 star-hours of data on the ecliptic, including Neptune&rsquos L4 Lagrangian region. Our setup is capable of detecting KBOs with radii greater than about
0.25km. This is two orders of magnitude smaller than that achievable by direct observation.
We report one detection event---a KBO of radius
0.43km at a distance of 46AU. This is the best resolved KBO occultation event reported to date, and arguably the first credible ground based detection. From our survey results we constrain the implied cumulative population density for KBOs with radii greater than 0.25km within ±2 degrees of the ecliptic to
10^7 objects per square degree of sky.
Dr Lucyna Kedziora-Chudczer
Title: Light scattered from the atmosphere of a planet and its surface is polarised.
Abstract: In contrast, light from solar-type stars is largely unpolarised. Therefore polarimetry can be used for the detection and characterisation of extrasolar planets around such stars. The degree of polarisation due to reflected starlight depends strongly on the composition and physical properties of planetary atmospheres. Ultimately observations of polarisation could provide the clues about the water droplets in the planetary atmosphere and possible detection of liquid water on the planetary surface via glint reflection.
I will discuss processes that lead to polarisation of light. Next I will talk about techniques of polarisation measurements and describe polarimetric observations of hot Jupiters with the high precision polarimeter, HIPPI built for the AAT at the UNSW. I will also describe capabilities of the newly developed polarimeter, HIPPI-2, to be used on the 8-metre Gemini telescope and will briefly mention other projects carried out with the HIPPI polarimeter.
Professor Di Li
Title: "How to Catch the Cold Gas"
Abstract: Star formation is the key process in producing 'luminous' matter in galaxies. To allow gravity to drive matter toward nuclear fusion, interstellar medium (ISM) needs to evolve from atomic to molecular forms. H2 formation, mainly happening in cold gas, is hard to observe and thus not well constrained nor understood, as demonstrated by the current controversy regarding the so-called 'dark gas' (Grenier et al. 2005). I report here our measurements of cold gas and H2 formation, mostly through absorption techniques. Based on the HI Narrow Self-Absorption (HINSA: Li et al. 2003) method, we published the first clear detection of the birth of a molecular dark cloud (Zuo et al. 2018). The formation time scale, and thus the lower limit of star formation time scale, is longer than 6 million years. In the region where a stable abundance of CO is not established, we found that OH is a good tracer of the total H2 content (Xu et al. 2016). We measure the OH excitation temperature through quasar absorption to be close to that of the Galactic background (Li et al. 2018), explaining why OH, the first radio-discovered and abundant molecule, remains elusive. Upcoming sensitive radio facilities, namely ASKAP, FAST, MeerKAT, SKA1, etc., will provide a complete inventory of Galactic cold gas through absorption (McClure-Griffith et al. 2015).
Ms Rebecca Davies
Title: Resolved Properties and Demographics of Ionized Gas Outflows at z
Abstract: Outflows are ubiquitous at the peak epoch of star formation (z
1-3), and are likely to play an important role in shaping the growth and evolution of galaxies. Near-IR integral field spectroscopy is a powerful tool to investigate the physical properties of galactic winds at this epoch because it enables us to kinematically disentangle them from gravitational motions, and to map the launch sites, extent, and geometry of outflows. In this talk I will present the most recent results from our integral field studies of outflows at z
1-3. I will summarise the properties and scaling relations of outflows resolved on 1-2 kpc scales using SINFONI with adaptive optics, and place these results in the context of our study of global outflow properties and demographics from the KMOS^3D survey.
Dr Samyaday Choudhury
Title: Study of sparse star clusters and metallicity maps of the Magellanic Clouds.
Abstract: I will present our studies directed towards two frotiners: (1) understanding sparse star clusters and their importance in our neighbouring galaxies, the Magellanic Clouds (MCs, the LMC & SMC), and (2) understanding the metallicity variation in these two galaxies. The MCs apart from rich populous clusters also host poor/sparse star clusters. Our work on sparse star clusters in the LMC, is aimed to increase our understanding of such objects, using deep Washington photometric data of 45 star clusters. A systematic study was performed to estimate their parameters, and they were grouped the into two categories based on their genuineness. The sizes and masses of these inconspicuous clusters emphasizes that the LMC has a significant population of clusters, which are similar to the open clusters in our Galaxy. Motivated by the above finding, a larger team is looking into the low mass open cluster like systems (identification, cataloging, estimating properties) in the LMC & SMC using existing large area surveys (e.g. OGLE III). To understand the metallicity variation within the LMC and the SMC, we created first of its kind high-spatial resolution metallcity map with field Red Giant Branch (RGB) stars as the tool, using two large scale photometric surveys: the MCPS and OGLE III data. These maps reveal the metallicity trend across the inner
5 deg and 2.5 deg field region of the LMC and SMC respectively. We have used the maps to estimate the mean metallicity within different regions of the MCs, their metallcity gradient, as well as identified outliers, which are important in the context of understanding the chemical evolution of these galaxies. The metallicity gradient of the LMC is found to be almost constant within the bar region and falls off beyond that, indicating that the bar might have been active in the past. Whereas, a shallow but gradual metallicity gradient of the SMC presents a different story of evolution compared to the LMC.
Dr Andrew Cameron
Title: "The latest results from the HTRU-S Low Latitude Pulsar Survey: a zoo of new and exciting pulsars"
Abstract: "Pulsars, rapidly-rotating and highly-magnetised neutron stars, can be utilised as tools in the study of many aspects of fundamental physical, most notably in the application of binary pulsars to the study of gravitational theories such as General Relativity. The discovery of ever-more relativistic binary systems than those presently known will allow for such tests to probe even deeper into the nature of gravity. Here, I will present results from the processing of 44% of the the HTRU-South Low Latitude pulsar survey (HTRU-S LowLat), the most sensitive blind survey of the southern Galactic plane taken to date. This includes the discovery and long-term timing of 40 new radio pulsars identified through the continued application of a novel &ldquopartially-coherent segmented acceleration search&rdquo technique, which was specifically designed to discover highly-relativistic binary systems. These pulsars display a range of scientifically-interesting behaviours including glitching, pulse-nulling and binary motion, and appear to comprise a population of generally older, lower-luminosity pulsars as compared to the previously-known population. In addition, I will also present an in-depth report on PSR J1757-1854, the only relativistic binary pulsar to have been discovered in HTRU-S LowLat to date. This extreme binary system (which remains the most accelerated pulsar binary ever discovered) promises to provide new insights into gravitational theories within the coming years."
Oxygen is the third most abundant element in the universe, after hydrogen and helium, so a basic knowledge of oxygen chemistry in molecular clouds is essential in order to understand the chemical structure, thermal balance, and diagnostic line emission from star-forming molecular gas in galaxies. Early gas-phase chemical models (e.g., Langer & Graedel 1989 Millar 1990 Bergin et al. 1998) predicted large abundances of H2O (
10 −5 –10 −4 ) relative to hydrogen nuclei in molecular gas well shielded from far-ultraviolet (FUV, 6 eV <hν < 13.6 eV) photons. If gas-phase H2O and O2 were that abundant, they would be important coolants for dense gas (Goldsmith & Langer 1978 Hollenbach 1988 Neufeld et al. 1995). However, the Submillimeter Wave Astronomy Satellite (SWAS) made pointed observations in low-energy transitions of ortho-H2O (the 110–101 transition at 557 GHz) and O2 (the 33–12 transition at 487 GHz) toward numerous dense (but unshocked) molecular cores and determined that the line-of-sight-averaged and beam-averaged (SWAS beam
4') abundance of H2O is of order 3 × 10 −8 (Snell et al. 2000) and that of O2 is 10 −6 (Goldsmith et al. 2000). More recent observations by the Odin mission set more stringent upper limits on O2, 10 −7 (Pagani et al. 2003), with a reported detection at the
2.5 × 10 −8 level in ρ Oph (Larsson et al. 2007). Although the water abundance derived from the observed water emission depends inversely on the gas density, and therefore is somewhat uncertain, understanding the two order of magnitude discrepancy between the gas-phase chemical models and the observations is essential to astrochemistry and to the basic understanding of the physics of molecular clouds.
Previous attempts to explain the low abundance of H2O and O2 observed by SWAS showed that time-dependent gas-phase chemistry by itself would not be sufficient (Bergin et al. 1998, 2000). Starting from atomic gas, a dense (n(H2)
10 5 cm −3 ) cloud only took 10 4 years to reach H2O abundances
10 −6 , close to the final steady-state values and much greater than observed.
The best previous explanation involved time-dependent chemistry linked with the freeze-out of oxygen species on grain surfaces and the formation of substantial water ice mantles on grains (Bergin et al. 2000). In these models, all of the oxygen in the molecular gas which is not tied up in CO adsorbs quickly (in
10 5 years for a gas density of
10 4 cm −3 ) to grain surfaces, forms water ice, and remains stuck on the grain as an ice mantle. As a result, the gas-phase H2O abundance drops from
10 −8 . The grains are assumed to be too warm for CO to freeze as a CO ice mantle, so that for a period from about 10 5 years to about 3 × 10 6 years, the gas-phase H2O abundance remains at the
10 −8 level, fed by the slow dissociation of CO into O and the subsequent reaction of some of this O with H + 3, which then ultimately forms gas-phase H2O. The slow dissociation of CO is driven by cosmic-ray ionization of He the resultant He + reacts with CO to form O and C + . After
3 × 10 6 years, even the gas-phase CO abundance drops significantly as the dissociation process bleeds away the oxygen that ends up as water ice on grain surfaces. Therefore, after about 3 × 10 6 years, the gas-phase H2O also drops from
10 −8 to <10 −9 . This previous scenario explained the observed H2O emission as arising from the central, opaque regions of the cloud, where the abundance has dropped to the observed values but has not had time to reach the extremely low steady-state values. The model relied, then, on a "tuning" of molecular cloud timescales so that they are long enough for the freeze-out of existing gas-phase oxygen that is not in CO onto grains, but not so long that the CO is broken down and the resultant O converted to water ice, which would cause the gas-phase H2O abundances to drop below the observed values. 5 The model also relied on CO not freezing out on grains in the opaque cloud centers.
To add to the problems of modeling the SWAS data, the SWAS team observed a number of strong submillimeter continuum sources, such as SgrB, W49, and W51, and found the 557 GHz line of H2O in absorption, as the continuum passed through translucent clouds (AV
1–5) along the line of sight (Neufeld et al. 2002, 2003 Plume et al. 2004). The absorption measurement provided an even better estimate of the H2O column, N(H2O) through these clouds because the absorption line strengths are only proportional to N(H2O), whereas the emission line strengths are proportional to n(H2)N(H2O)e −27K/T since the line is subthermal, effectively optically thin, and lies ΔE/k = 27 K above the ground state. Therefore, to obtain columns from emission line observations requires the separate knowledge of both the gas density and the gas temperature. The absorption measurements showed column-averaged abundances of H2O of
10 −7 –10 −6 with respect to H2 using observed CO columns and multiplying by 10 4 to obtain H2 columns. In the context of the time-dependent models with freeze-out, the difference between the abundances measured in absorption compared with those measured in emission was attributed to the presumed lower gas densities in the absorbing clouds compared to the emitting clouds. Because the freeze-out time depends on n −1 , it was assumed that the lower density clouds had not had time to freeze as much oxygen from the gas phase, so that gas-phase H2O abundances were higher.
A variation of this model is that of Spaans & van Dishoeck (2001) and Bergin et al. (2003), where it was noticed that the water emission seemed to trace the photodissociation region (PDR see Hollenbach & Tielens 1999), which lies on the surface (AV 5) of the molecular cloud. A two-component model was invoked in which the water froze out as ice in the dense clumps, but remained relatively undepleted in the low-density interclump gas because of the longer timescales for freeze-out. Thus, the average gas-phase water abundance was derived from a mix of heavily depleted and undepleted gas.
We propose in this paper a new model of the H2O and O2 chemistry in a cloud. In this model, we assume that the molecular cloud lifetime is sufficiently long to allow freeze-out 6 and reduce the gas-phase H2O and O2 abundances to very low values in the central, opaque regions of the cloud. The key to understanding the H2O emission and the O2 upper limits observed by SWAS and Odin is to model the spatially dependent H2O and O2 abundances through each cloud. At the surface of the cloud, the gas-phase H2O and O2 abundances are very low because of the photodissociation by the ISRF or by the FUV flux from nearby OB stars. Near the cloud surface, the dust grains have little water ice because of photodesorption by the FUV field. Deeper into the cloud, the attenuation of the FUV field leads to a rapid rise of the gas-phase H2O and O2 abundances, which peak at a hydrogen nucleus column Nf
10 21.5 –10 22 cm −2 (or depths AVf
several into the cloud) and plateau at this value for ΔAV
several beyond AVf, insensitive to the gas density n and the incident FUV flux G0 (scaling factor in multiples of the avearage local interstellar radiation field). At these intermediate depths, photodesorption of some of the water ice by FUV photons keeps the gas-phase water abundance high (the "f" in Nf and AVf signifies the onset of water ice freeze-out, as will be discussed in Sections 2.5 and 3.3). The FUV is strong enough to keep some of the ice off the grains, but, due to efficient water ice formation followed by photodesorption, it is not strong enough to dissociate all gas-phase water. At greater depths, gas-phase reactions other than photodissociation begin to dominate gas-phase H2O destruction, and the steady-state gas-phase H2O and O2 abundances plummet as all the gas-phase elemental O is converted to water ice. Thus, the overall structure of a molecular cloud has three subregions: at the surface is a highly "photodissociated region," at intermediate depths is the "photodesorbed region," and deep in the cloud is the "freeze-out region."
In this new model, the H2O and O2 emission mainly arises from the layer of high-abundance gas in the plateau that starts at Nf and extends somewhat beyond, so that the emission is a (deep) "surface" process rather than a "volume" process. For clouds with columns N > Nf, the H2O and O2 emissions become independent of cloud column, assuming the incident FUV field and gas density are fixed. The model gives column-averaged abundances through the dense cores of 10 −8 for H2O and O2 for low values of G0 < 500, but the local abundances peak at values that are at least an order of magnitude larger than these values. For cloud columns N > Nf, the average H2O abundance scales as N −1 .
The implications of this scenario for molecular clouds may be much broader than this particular model of H2O and O2 chemistry in clouds. Other molecules, such as CO, CS, CN, and HCN, require a spatial model of their distribution in a cloud, with photodissociation and photodesorption as the dominant processes near the cloud surface, and the freezing out of the molecules the dominant process deeper into the cloud. In addition, the adsorption process and creation of abundant ice mantles changes the relative gas-phase abundances of the elements. In the case considered in this paper, the C/O ratio in the gas may span from 0.5 at the cloud surface to unity or greater in the H2O freeze-out region. Such changes in the gas-phase C/O ratio have major implications on all gas-phase chemistry (e.g., Langer et al. 1984 Bergin et al. 1997).
The interesting and important caveat to this relatively simple steady-state model is the effect of time dependence in raising abundances of, for example, gas-phase H2O, O2, and CO above steady-state values in the opaque (AV > 5–10) interiors of clouds. One such effect is that at low densities, n 10 3 cm −3 , species do not have time to freeze out within cloud lifetimes, and therefore have much higher gas-phase abundances. We have also uncovered a new time-dependent process that may elevate the H2O and O2 abundances for times t 10 7 years in the freeze-out region at very high AV even at high cloud densities n
10 5 cm −3 . If the grains are sufficiently cold to freeze out CO (Tgr 20 K or G0 500) at high AV, a CO/H2O ice mix rapidly (t 10 5–6 years) forms. The steady-state solution has very little CO ice with most of the O in H2O ice. However, the time to convert CO ice to H2O ice is very long, and the bottleneck is the cosmic-ray desorption of CO from the CO/H2O ice mixture. This desorption provides gas-phase CO that then acts as a reservoir to produce gas-phase H2O and O2, until all the oxygen eventually freezes out as water ice (i.e., the same mechanism as described in Bergin et al. 2000 except that the gas-phase CO in this case comes from the CO ice). Depending on the assumptions regarding the CO desorption process, this cosmic-ray CO desorption timescale may range from 10 5 to 10 7 years. If the timescale is roughly 0.1–1 times the cloud age, a maximum gas-phase H2O and O2 abundance is produced at that time. Although not as large as the peak abundances produced at AVf, these abundances can be significant and can contribute to the total column of these species if the cloud has a high total column (but only if the CO desorption time is 0.1–1 times the cloud age).
This paper is organized as follows. In Section 2, we describe the new chemical/thermal model of an opaque molecular cloud illuminated by FUV radiation. The major changes implemented in our older PDR models (Kaufman et al. 1999) include the adsorption of gas species onto grain surfaces, chemical reactions on grain surfaces, and the desorption of molecules and atoms from grain surfaces. In Section 3, we show the results of the numerical code as functions of the cloud gas density n, the incident FUV flux G0, and the grain properties. We present a simple analytical model that explains the numerical results and how the abundances scale with depth and with the other model parameters. We discuss time-dependent models for the opaque cloud center. In Section 4, we apply the numerical model to both diffuse clouds and dense clouds. Finally, we summarize our results and conclusions in Section 5. For the convenience of the reader, Table 3 in the Appendix lists the symbols used in the paper.
III. TECHNICAL DEVELOPMENT
A. Central Development Laboratory
The Central Development Laboratory (CDL) in Charlottesville provides support for the Observatory in the areas of feeds, cooled front-end devices of both HFET and SIS-mixer types, and digital back-ends. Support is also provided in other areas as needs arise, for example, with some signal processing functions of the OVLBI earth station at Green Bank. During the past two years increasing attention has also been given to future requirements for the MMA, including both front-end and system design. In addition to equipment for NRAO facilities, the laboratory has provided HFET amplifiers, and in a few cases SIS mixers, to other observatories on a cost reimbursable basis when equivalent items were not available commercially.
Updating of test equipment and similar facilities is necessarily limited by available funds, but some important new items are as follows. A new cryogenic test station for HFET amplifiers, including computer control and monitoring, has been assembled and programmed. Construction of a similar facility for SIS mixers is also in progress. Sonnet em software for analyzing planar electromagnetic circuitry has been acquired and provided an important increase in accuracy over other programs available. A turbo-pump vacuum station and equipment to provide millimeter-wavelength spectrum analysis are also being obtained.
HFET Amplifier Development
Development and production of cryogenically cooled HFET amplifiers has continued with units covering almost the entire frequency range from 300 MHz to 90 GHz. At the low frequency end, a balanced design is being developed to obtain good input match without the need for a circulator. Amplifiers of this type will be used on the GBT and will cover the range 290-1230 MHz in five bands. Of these, the design for 680-920 MHz is complete and amplifiers have been produced. At the highest frequencies an amplifier for the range 60-90 GHz is under development. A prototype which performed well up to 75 GHz was tested in 1994, and modification to extend the range to 90 GHz is under study. A front end covering the 70-90 GHz band, which will use the new amplifier, is under development and will be used for testing of the high frequency performance of VLBA antennas. There is also an ongoing program to evaluate noise characteristics of HFETs from leading manufacturers as part of a continuous effort to improve the performance of existing amplifier designs.
An important action has been the purchase from Hughes of a full wafer of InP HFETs, testing of which began in early 1995. Gate widths cover a range of 30 to 400 m, and are appropriate for the full frequency coverage of NRAO amplifier designs. The number of transistors on the wafer should be sufficient to cover all amplifier construction for several years. The noise performance is very good, for example, over the range 21-26 GHz a mean noise temperature of 12 K is achieved, which is 30 percent better than obtained with previously available production HFETs. The excellent high frequency performance requires that the circuit impedances be well controlled up to 100 GHz or more to prevent oscillations at such frequencies. As a result some difficulty has been experienced when the new HFETs were retrofitted into existing amplifiers for frequencies below about 18 GHz, and investigation of necessary modifications is in progress.
SIS Mixer Development
The purpose of the SIS mixer work is to provide front ends for the 12 Meter Telescope and to develop designs that can be used for the MMA. Tunerless mixers (i.e., those that do not require mechanical adjustment of a circuit element as the local oscillator (LO) is tuned) are preferred for simplicity, especially for the MMA. A study performed in 1993 led to the conclusion that about 36 percent bandwidth, comparable to that of rectangular waveguide, is feasible in tunerless designs. New tunerless designs for 200-300 GHz have been developed during the past year.
To increase the instantaneous bandwidth of SIS mixer receivers, it has been decided to change the IF amplifier response used from 1.2-1.8 GHz to 4-6 GHz. Better noise performance of recent HFET amplifiers allows the higher intermediate frequency to be used without serious loss in sensitivity.
A project to develop a fully integrated image separation mixer on a single quartz chip was started during the past year and the design phase is now well advanced.
In 1993 a problem arose in the device fabrication system at UVa, which has been the main source of supply of SIS mixers used at NRAO. A loss in performance of devices produced was traced to an unwanted edge deposit of Nb on circuit elements. This has now been eliminated and satisfactory fabrication was resumed in late 1994.
Electromagnetic Analysis and Testing
Work in this area mainly concerns design and measurement of feeds and related equipment, and analysis of antenna performance. During the past two years much of this work has been related to the GBT project, including feeds for all bands included in the initial outfitting. For example, a feed design for 3.95-5.85 GHz provides a calculated aperture efficiency of 71 percent, and an orthomode transition to separate polarizations for this frequency band has a return loss of better than 18 dB. A tertiary reflector system for beam switching and pointing adjustment has been designed for frequencies above 22 GHz. Calculations of the effects of gravitational distortion of the main reflector surface as a function of elevation show that aperture efficiency can be maintained within one percent by lateral adjustment of the subreflector position. Noise shields have been designed for the telescope that will intercept radiation from the feed that would otherwise be directed towards the ground and reflect it into the main reflector so that it terminates on the cold sky.
Some contributions to other telescopes are as follows. A preliminary design for wideband feeds covering 1.2-40 GHz has been developed for the VLA, to use with upgraded front ends. A feed for 40-52 GHz has also been designed and will be used with a two-feed receiver for testing VLA antennas in this band. The receiver will also be used as a prototype for a multi-feed system for the GBT and will provide an opportunity for refinement of the feed cluster design to find the best compromise between close spacing of beams and loss of aperture efficiency. A special feed for the 140 Foot Telescope has been designed for Zeeman effect observations. This covers the OH line band (1600-1730 MHz). The responses in the principal planes are equal within 0.4 dB, and the measured cross polarization level is -33 dB.
2. Digital Systems
In 1993 work was just starting on a digital spectrometer for the GBT. As described in the last annual report, the total spectrometer bandwidth is 6.4 GHz and the number of correlation lag channels is 262,144. The instrument will incorporate 256 custom application specific integrated circuits (ASIC) designed by J. Canaris of the University of New Mexico for the Arecibo Telescope and other users as well as the NRAO. A feature added more recently is a pulsar observing mode with 1024 integrating time slots within the pulsar period. The first run of ASIC chips that were produced operated up to clock speeds of only 72 Mb/s, compared with the required 100 Mb/s. Detailed testing revealed that the low clock speed resulted from a small error in the chip circuitry. A run yielding satisfactory chips was obtained in the fourth quarter of 1994. In the meantime, work in Charlottesville had produced designs for the circuit cards and the sampler, and test fixtures had been completed. The sampler was tested at 2 Gb/s, and eight such units, running at 1.6 Gb/s, will be used. As a result, by mid-1995 the design of the spectrometer was complete and construction of the full system had been started. Completion of the project is expected before mid-1996.
B. Computing and Software Development
There are three main elements to the NRAO's software strategy for supporting scientific data processing and analysis. First, AIPS supports the reduction and analysis of (primarily) radio interferometric data. AIPS has extremely broad and flexible capabilities and is used in many other areas of image processing and analysis outside of ordinary interferometric radio astronomy. Second, UniPOPS supports the reduction and analysis of single dish data, with a strong emphasis on NRAO's two current single dish instruments, the 140 Foot Telescope in Green Bank and the 12 Meter Telescope on Kitt Peak. Finally, the AIPS++ project is a development effort begun in 1992 aimed at producing an analysis package which will eventually replace both the AIPS and UniPOPS packages.
1. Astronomical Image Processing System (AIPS)
There have been two releases of Classic AIPS approximately every six months during the past two years. Typically 100 copies of each release are shipped, about half electronically. The total number of institutions actively using AIPS is over 250. AIPS is now available as binary executables for selected computer architectures (Sun, IBM, Linux, DEC, HP, and SGI). Currently, AIPS is shipped only to licensed sites. Beginning with the next release in July 1995, AIPS will be distributed as copyrighted code using the GNU general public license to allow wider distribution of AIPS. A simple site registration mechanism is being implemented to help track sites using AIPS or receiving support.
During the past two years, AIPS was ported to a number of new operating systems. These are (1) Solaris, Sun's new System V based OS (2) Digital Unix (was OSF/1), used on DEC's Alpha series computers (3) HP-UX, Hewlett-Packard's UNIX (4) Linux, a public-domain UNIX OS for Intel x86 architecture personal computers and (5) IRIX, Silicon Graphics' version of UNIX. The AIPS verification and performance package DDT was modernized and used to test these and numerous other computers.
A large amount of new or improved software was added to AIPS during the two years. Major areas of concern were (1) reading VLBA correlator data, correcting for correlator artifacts (2) VLBI data processing, including fringe fitting, spectral-line polarization calibration, amplitude and bandpass calibration and fringe-rate mapping (3) single-dish data processing, especially imaging from 12 Meter OTF observations (4) automatic source finding and fitting (5) wide-field imaging (for VLA surveys) (6) general imaging problems, including significantly enhanced interferometric imaging algorithms and (7) improved access to information about the large volume of tasks available to the user. The major job of rewriting the AIPS Cookbook was begun, with introductory, calibration, display, spectral-line, VLBI, and site-specific chapters completed. Anyone who is interested in the details may consult the AIPSLetter which is issued with each release. Extensive documentation on AIPS is available on the WWW at URL http://www.cv.nrao.edu/aips/.
AIPS quarterly and annual reports, the AIPS memo series, AIPSLetters, Cookbook chapters, FAQs, software patches for recent releases, and even all current help files are available from this URL.
UniPOPS development has slowed at the NRAO, with the last major update in mid-1994 to version 3.3, followed by release 3.4 in May 1995. Version 3.4 is the default analysis system at the 140 Foot and 12 Meter Telescopes. In addition to installations at each NRAO site, UniPOPS has been distributed to over 24 sites. The latest versions have been installed at over 10 sites.
The most significant change in versions 3.3 and 3.4 from previous versions of UniPOPS is the new disk format of the basic data files. The 16-bit integer indexing was replaced with 32-bit integer indexing. This new disk format is also now the on-line disk format at the 12 Meter (replacing the VAX pdfl format). A single data file can now be made large enough to handle most observing programs. Versions 3.3 and 3.4 include the ability to access Green Bank spectral processor data as well as individual records from longer integrations. The SD-FITS writer now writes continuum as well as spectral line data. The most significant change for version 3.4 is the ability to run under the Solaris operating system. Version 3.4 is likely to be the last major release of UniPOPS future single dish software development will move to the AIPS++ project. Future releases will be limited primarily to bug fixes, minor new verbs, and updates to the procedure library.
The NRAO's top priority for UniPOPS continues to be responding to user problems (either while observing or at their home installation).
3. World Wide Web support at NRAO
NRAO is now providing on-line documentation for Internet users using the WWW. NRAO is working towards making most kinds of routine documentation available to users on-line, including facilities descriptions, proposal procedures, descriptions of major initiatives, and various NRAO memo series. Recent significant accomplishments are also documented. For example, users can access the first scientific results from the VLBA correlator, including copies of high resolution images. Plans are underway to provide a variety of documentation for the GBT, the MMA, the AIPS++ software project, and other initiatives. NRAO abstracts and preprints are also available through the Web. In late 1995 the Proceedings from IAU Symposium #170 will also be available electronically. The URL for the NRAO's master Home Page on the WWW is http://www.nrao.edu.
There were 211 workstations at NRAO at the start of 1995: 14 larger workstations for shared use on large problems, and 197 user workstations for moderate size problems. Most of the larger workstations (IBM RS/6000 560's and 580's, and Sun Sparcstation 20's) are reserved for users and visitors with large data reduction problems many Sun IPX class machines are also reserved for visitor use. Two of the larger workstations (including one being delivered in mid 1995) are dedicated servers for software development. The larger workstations are typically equipped with 128-256 MBytes of physical memory and up to 8-10 GBytes of disk space, while the smaller user workstations typically have 1-2 GBytes of disk space and 40-64 MBytes of memory. There were 109 tape drives at the NRAO at the start of 1995, mostly DAT drives and Exabyte drives (including some high-density Exabyte systems), as well as a few remaining 9-track tape drives. A specialized system with a film recorder for producing high quality hard copy has also been installed at the NRAO, with procedures implemented to allow outside users access.
A computing hardware plan is under development to address the needs of the Observatory over the next ten years in computing hardware.
C. Astronomical Information Processing System
The AIPS++ Project is being conducted by an international consortium of radio observatories with the goal of producing a modern data analysis software system suitable for data from both interferometric arrays and single dishes. The consortium members are: Australia Telescope National Facility (R.Ekers), Berkeley-Illinois-Maryland Association (R. Crutcher), Herzberg Institute for Astronomy (D. Morton), National Center for Radio Physics (V. Kapahi), National Radio Astronomy Observatory (P. Vanden Bout), Netherlands Foundation for Research in Astronomy (H. Butcher), and Nuffield Radio Astronomy Laboratories (R. Davies).
The AIPS++ Project was reviewed by an external review panel in December 1994. Following the main recommendations of the review panel, a number of changes were made in early 1995. A full-time project manager, T. Cornwell, was appointed. The AIPS++ Consortium now oversees the work of the Project via an executive committee composed of directors from the principal partner observatories. Inside NRAO, AIPS++ is now treated as a construction project with dedicated staff and budget.
The immediate goals of the Project have been defined to be consolidation and testing of the AIPS++ library and development of a few key applications chosen to provide unique astronomical capabilities. The long-term goal of the Project has been defined to be the achievement of functional equivalence to AIPS by 2000. At that point, AIPS is expected to be a small subset of AIPS++, and most applications areas will look quite different from the corresponding areas in AIPS.
A development plan for the next 12-18 months has been instituted to provide a coherent overall picture of the direction of the Project in the intermediate term. Tracking of progress in AIPS++ is now performed using a Target Dates mechanism.
The NRAO AIPS++ group split into two principal groups: one in Charlottesville concerned mainly with support of single dish processing and another in Socorro concerned with project management and synthesis telescope support. In addition, NRAO plans to locate dedicated AIPS++ programmers at both the Green Bank and Tucson sites.
The following applications are now present and are being developed further: a tool for OTF mapping using the 12 Meter Telescope in rapid scanning mode, a self-calibration/deconvolution tool used principally on the Australia Telescope Compact Array data, and a tool for plotting and manipulating data from the GBT systems integration tests on the 140 Foot Telescope.
The following infrastructure library changes have occurred: a system for class documentation is now in place, numerous improvements have been made to the Glish system used for task control and command line interface, a very capable tool for visualization (AIPSView) has been developed by the Berkeley-Illinois-Maryland Association (BIMA) group at the National Center for Supercomputing Applications.
Intellectual developments are also crucial for the long-term success of AIPS++. A partial design for UV plane calibration and imaging was completed by a team drawn from the Australia Telscope National Facility (ATNF), the Netherlands Foundation for Research in Astronomy (NFRA), and the NRAO, and a collaboration of NFRA and ATNF personnel has developed a very general formalism for the calibration of synthesis polarimetric observations.
Documentation for both users and programmers is now available via the WWW at URL http://www.nrao.edu/aips++/docs/aips++.html. General project information, such as development plans and target dates, is also available via this URL.
Connecting the ISM to TeV PWNe and PWN candidates
We investigate the interstellar medium towards seven TeV gamma-ray sources thought to be pulsar wind nebulae using Mopra molecular line observations at 7 mm [CS(1–0), SiO(1–0, v = 0)], Nanten CO(1–0) data and the Southern Galactic Plane Survey/GASS H i survey. We have discovered several dense molecular clouds co-located to these TeV gamma-ray sources, which allows us to search for cosmic rays coming from progenitor SNRs or, potentially, from pulsar wind nebulae. We notably found SiO(1–0, v = 0) emission towards HESS J1809–193, highlighting possible interaction between the adjacent supernova remnant SNR G011.0–0.0 and the molecular cloud at d ∼ 3.7 kpc. Using morphological features, and comparative studies of our column densities with those obtained from X-ray measurements, we claim a distance d ∼ 8.6 − 9.7kpc for SNR G292.2–00.5, d ∼ 3.5 − 5.6 kpc for PSR J1418–6058 and d ∼ 1.5 kpc for the new SNR candidate found towards HESS J1303–631. From our mass and density estimates of selected molecular clouds, we discuss signatures of hadronic/leptonic components from pulsar wind nebulae and their progenitor SNRs. Interestingly, the molecular gas, which overlaps HESS J1026–582 at d ∼ 5 kpc, may support a hadronic origin. We find however that this scenario requires an undetected cosmic-ray accelerator to be located at d < 10 pc from the molecular cloud. For HESS J1809–193, the cosmic rays which have escaped SNR G011.0–0.0 could contribute to the TeV gamma-ray emission. Finally, from the hypothesis that at most 20% the pulsar spin down power could be converted into CRs, we find that among the studied pulsar wind nebulae, only those from PSR J1809–1917 could potentially contribute to the TeV emission.
About this memoir
This memoir was originally published in Historical Records of Australian Science, vol.17, no.2, 2006. It was written by:
- J. B. Whiteoak, CSIRO Australia Telescope National Facility, Sydney (corresponding author)
- H. L. Sim, CSIRO Australia Telescope National Facility, Sydney
Numbers in brackets refer to the bibliography.
The authors are grateful to Tony Robinson for an email copy of the text of his eulogy, and to several people for contributing information by email and/or telephone: Jill Robinson (including comments by her children), Mal Sinclair, Dick Manchester, Dick McGee and Lyn Newton.
4. Zeeman Observational Results𠅎xtended Gas
4.1. H I, OH, and CN Zeeman Observations
The Zeeman effect in the ISM was first detectedter multiple attempts—in absorption lines of H I toward the Cassiopeia A supernovae remnant (Verschuur, 1968). Over the next 5 years only three more detections were made, toward Orion A, M 17, and Taurus A. Three of these were in the H I associated with molecular clouds, while the Taurus A line is not.
Troland and Heiles (1982a,b) and Heiles and Troland (1982) achieved the first H I Zeeman detections beyond Verschuur's original four souces, 14 years after that first detection. Further emission-line H I Zeeman observations and maps were toward the dark cloud filament L204 (Heiles, 1988), H I filaments associated with supernova or super-bubble shells (Heiles, 1989), the Ophiuchus dark cloud (Goodman and Heiles, 1994), and four dense H I clouds (Myers et al., 1995). Heiles (1997) mapped H I Zeeman toward 217 positions in the Orion-Eridanus region and carried out an extensive analysis. Finally, Heiles and Troland (2004) carried out a large H I Zeeman survey in absorption lines toward continuum sources.
The first detection was of OH absorption toward the NGC 2024 molecular cloud (Crutcher and Kazès, 1983). The OH Zeeman effect was later mapped with the VLA (e.g., Figure 3) toward several molecular clouds.
Figure 3. Left: OH Zeeman Stokes I and V profiles toward NGC2024 at the peak BLOS position from VLA mapping (Crutcher et al., 1999a). Right: Map of BLOS (color) from OH Zeeman. Contours are C 18 O intensities and yellow line segments are dust polarization directions (Hildebrand et al., 1995). The magnetic field in the plane of the sky is perpendicular to the dust polarization, hence roughly along the minor axis of the molecular cloud defined by C 18 O (horizontal in the figure).
Crutcher et al. (1993) carried out a survey of OH Zeeman toward dark clouds, achieving mostly upper limits. Bourke et al. (2001) extended attempts to detect the OH Zeeman effect, and obtained one definite and one probable new detection out of the 23 molecular clouds observed. Then, Troland and Crutcher (2008) carried out a major survey toward dark clouds, with 9 detections out of 34 positions.
Crutcher et al. (1996, 1999b) detected the Zeeman effect in a second molecular species, CN. Finally, Falgarone et al. (2008) extended the earlier work on CN Zeeman with a survey of dense molecular cores. The combined total was 14 positions observed and eight detections.
Figures 4, 5 show results for the Zeeman observations of H I, OH, and CN in extended gas.
Figure 4. H I, OH, and CN Zeeman measurements of BLOS vs. NH = NHI + 2NH2. The straight line is for a critical M / Φ = 3 . 8 × 1 0 - 21 N H / B . Measurements above this line are subcritical, those below are supercritical.
Figure 5. The set of diffuse cloud and molecular cloud Zeeman measurements of the magnitude of the line-of-sight component BLOS of the magnetic vector B and their 1σ uncertainties, plotted against nH = n(HI) or 2n(H2) for H I and molecular clouds, respectively. Different symbols denote the nature of the cloud and source of the measurement: H I diffuse clouds, filled circles (Heiles and Troland, 2004) dark clouds, open circles (Troland and Crutcher, 2008) dark clouds, open squares (Crutcher, 1999), molecular clouds, filled squares (Crutcher, 1999) and molecular clouds, stars (Falgarone et al., 2008). Although Zeeman measurements give the direction of the line-of-sight component as well as the magnitude, only the magnitudes are plotted. The dotted line shows the most probable maximum values for BTOT(nH) determined from the plotted values of BLOS by the Bayesian analysis of Crutcher et al. (2010b).
4.2. Interpretation of Zeeman Observations
The three species (H I, OH, and CN) with Zeeman detections in extended gas have resulted in measurements of BLOS that cover a large range of densities. H I emission samples the cold neutral atomic medium over densities between 1 and 100 cm 𢄣 . H I in absorption toward molecular clouds can sample densities 縐 2 -10 4 cm 𢄣 the ground-state 18 cm lines of OH sample roughly the same density range. Finally, the 3 mm emission lines of CN, which have a critical density 縐 5 cm 𢄣 , sample densities 縐 5 6 cm 𢄣 .
The astrophysical significance of Zeeman results requires determination of NH and/or nH in the regions where magnetic field strengths have been measured. For H I in absorption NH may be determined by also observing the line in emission off the continuum source so that the spin temperature and optical depth can be inferred, e.g., Heiles and Troland (2003). The associated nH may then be estimated from the mean interstellar pressure in the cold neutral diffuse medium and the spin temperature, e.g., Crutcher et al. (2010b). Since the OH line optical depths are generally small, NOH can be estimated from the observed line strengths, e.g., Crutcher (1979). To obtain NH one then uses the [OH/H] ratio determined by Crutcher (1979). To obtain nH for the regions in which OH is found one divides NH by the mean diameter of the OH region. For CN (Falgarone et al., 2008), the methods are similar to those for OH. The CN hyperfine-line ratios imply that the lines are optically thin, so N(CN) may be calculated from observed line strengths. NH then comes from [CN/H] based on studies by Turner and Gammon (1975) and Johnstone et al. (2003). The nH in the CN emitting regions must be fairly close to the critical density of the transition, since the lines are observed to be much weaker than kinetic temperatures and optically thin (no line photon trapping). Unfortunately few excitation analyses of CN excitation have been carried out, but since CN and CS have similar critical densities and map similarly, nH in the CN regions can be assumed to be about the same as obtained from CS excitation analyses. Finally, a second, independent method for determining nH comes by dividing NH by the estimated thickness of clouds from the mean extent of the CN distribution on the sky. There are certainly significant uncertainties in the estimates of nH especially, particularly as applied to individual clouds, where estimates may be off by an order of magnitude. However, in statistical studies such as those described in this paper, more important is the ensemble uncertainty. Crutcher et al. (2010b) found a statistical uncertainty of about a factor of two in nH.
Two important quantities than can be inferred from the Zeeman data are the mass to magnetic flux ratio M/Φ (∝NH/B) and κ (in the B ∝ n H κ relation (see Crutcher, 2012 for a detailed discussion). M/Φ is proportional to the ratio of gravity to magnetic pressure and informs whether magnetic fields are sufficiently strong to support clouds against gravitational contraction. A simple way to derive the expression for the critical M/㩪 at which magnetic and gravitational energies are in equilibrium is to equate the virial terms: 3GM 2 /5R = B 2 R 3 /3. Since magnetic flux Φ = πR 2 B, the critical M/Φ is:
The precise numerical value differs slightly for detailed models depending on geometry and density structure. A supercritical ratio means that magnetic pressure alone is insufficient to prevent gravitational collapse, while a subcritical ratio means collapse is prevented by magnetic pressure. The scaling of magnetic field strength with density is a prediction of many theoretical studies of the evolution of the interstellar medium and star formation. Simple examples include (1) mass accumulation along field lines without change in magnetic field strength, for which κ = 0 compression of mass perpendicular to the field with flux freezing, for which κ = 1 and spherical collapse with flux freezing and weak field strength, for which κ = 2/3 (Mestel, 1966).
4.2.1. B vs. N
First, we discuss field strength vs. column density. Bourke et al. (2001) plotted this for their OH observations and discussed the implication. Figure 4 shows BLOS vs. NH with data from the compilation by Crutcher (1999) and four later major Zeeman surveys of H I, OH, and CN (Bourke et al., 2001 Heiles and Troland, 2004 Falgarone et al., 2008 Troland and Crutcher, 2008). The data are clearly separated into three ranges in NH, corresponding to the tracers H I, OH, and CN. The straight line is the critical M/Φ line.
An essential point in interpreting Figure 4 is that only one component of the total magnetic vector B is measured. Hence, all points are lower limits on what the total magnetic field strength would be. However, for N H ≲ 1 0 21 cm 𢄢 , most of the points are above the critical line, showing that at low column densities the diffuse H I and lower column density molecular gas is subcritical. In contrast, for N H ≳ 1 0 22 cm 𢄢 , all but one of the points are below the critical line. It is possible that some of these clouds are subcritical with the magnetic field close to the plane of the sky. However, that fact that all of the points are below the critical line suggests strongly that a transition occurs at N H ~ 1 0 22 cm 𢄢 from subcritical to supercritical M/Φ. Clouds with N H ≳ 1 0 22 cm 𢄢 have a mean M/Φ that is supercritical by a factor of 2𠄳. The data strongly suggest that subcritical self-gravitating clouds are the exception and in fact none may exist. These self-gravitating clouds are the ones in the ambipolar diffusion model that should be subcritical at early stages of gravitational contraction.
Figure 4 might appear to support the ambipolar diffusion model of cloud evolution, in which initially subcritical clouds become supercritical by gravitational contraction of neutral matter through magnetic fields. However, the points with N H ≲ 1 0 21 cm 𢄢 , are lower density H I clouds. These cold H I clouds are confined by pressure from the surrounding warm ISM and are not self-gravitating, so they could not gravitationally collapse as envisioned by the ambipolar diffusion model. Heiles and Troland (2005) found that the mean BTOT is approximately the same in the cold H I medium and the warm neutral medium. Hence, the magnetic field strength does not systematically change during transitions of gas between the lower density warm and the higher density cold neutral medium. Possible explanations for this are that diffuse clouds form by flows along magnetic flux tubes or that they form preferentially from regions of lower magnetic field strength. Another process that could be important in keeping field strengths fairly constant is turbulent magnetic reconnection (Vishniac and Lazarian, 1999).
NH in the range 10 21 cm 𢄢 marks a clear transition between magnetic field strengths being statistically independent of NH and an increase in strength with column density. A similar transition is seen in Figure 5 (discussed below) at nH ≈ 300 cm 𢄣 . Assuming that these NH and nH correspond to the same clouds, the typical diameters of these clouds is 0.1𠄱 pc. These are roughly the parameters for an interstellar cloud to become self-gravitating. Gravitational contraction with flux freezing would then cause the magnetic field strength to increase with increasing NH and nH. We also note that N H ≈ 1 0 22 cm 𢄢 is also roughly the column density where the orientation of magnetic fields in the plane of the sky as mapped with polarized dust emission changes (statistically) from parallel to perpendicular with respect to the elongated mass structures on the plane of the sky (Ade et al., 2016).
Probably the main uncertainty in Figure 4 comes from the column densities. For H I the NH are very well determined, since both the line optical depths and spin temperatures are directly measured. However, for OH and CN the NH come from determinations of NOH and NCN and studies of OH/H and CN/H, which introduce possible errors. A major issue is exactly what NH the OH and CN Zeeman results sample. On the basis of ambipolar diffusion models with time-dependent astrochemistry, Tassis et al. (2012) argue that OH and CN are heavily depleted at higher densities due to chemistry and hence tend to sample the lower density outer layers of clouds rather than the cores, and that therefore the Zeeman results underestimate the magnetic field strengths in cores. If the true field strengths are higher at each NH than those plotted in Figure 4, many of the points with N H > 1 0 22 cm 𢄢 should be plotted at stronger field strengths. Such points would then lie above the critical M/Φ line, and would represent subcritical self-gravitating clouds. One issue with this conclusion is that ambipolar diffusion driven evolution is significantly slower than those for which the magnetic flux problem has been resolved by other physics such as turbulent reconnection (Vishniac and Lazarian, 1999 Lazarian et al., 2012) the chemical depletion at high densities may not have had sufficient time to be as significant as Tassis et al. (2012) find. A more direct problem with their argument is that the interpretation of Figure 4 does not depend on OH and CN sampling the highest densities of molecular cores. The Zeeman effect estimates the magnetic field strength in the regions sampled by the Zeeman tracer (OH or CN), and the relevant NH and nH for estimating M/Φ are those sampled by the Zeeman species. There is no claim that either species samples the highest densities of cores. Ideally one might use a variety of Zeeman species that sample a range of densities in order to measure the change in M/Φ from envelope to core in clouds. The fact that all Zeeman species do not trace the field in the cores, while true, does not invalidate our interpretation of Figure 4.
4.2.2. B vs. n
The above discussion was limited by the fact that only the line-of-sight component of the vector B is measured with the Zeeman effect. However, with a large number of Zeeman measurements, it is possible to infer statistical information about the total field strength. One can assume a PDF of the total field strength, P(BTOT), and compute P(BLOS), the PDF of the observable line-of-sight field strengths, assuming a random distribution of the θ. Comparison between the two lets one infer the most probable (of those assumed) P(BTOT). Heiles and Crutcher (2005) attempted this for H I Zeeman data with a frequentist approach, but found that the observations did not allow a strong discrimination among possible PDFs for the total field strength.
Crutcher et al. (2010b) used a Bayesian approach, and expanded the Zeeman data set to include H I, OH, and CN surveys (Crutcher, 1999 Heiles and Troland, 2004 Falgarone et al., 2008 Troland and Crutcher, 2008). Their model for BTOT vs. nH had BTOT, max = B0 at lower densities, based on the most probable result from Heiles and Crutcher (2005). For higher densities the maximum BTOT had a power-law dependance, B T O T , m a x = B 0 ( n / n 0 ) κ . The PDF of BTOT at each density was assumed to be flat, with the BTOT equally distributed between the BTOT, max at that nH and a lower limit BTOT = f × B0, with 0 ≤ f ≤ 1. A delta function PDF (all BTOT at each nH being the same) would have f = 1, while f = 0 would be the flat PDF between BTOT, max and 0. The results for the four free parameters in the Bayesian model (Figure 5) were B0 ≈ 10 μG, n0 ≈ 300 cm 𢄣 , κ ≈ 0.65, and f ≈ 0.
For nH > no interstellar magnetic field strengths increase with density. Possible explanations are that diffuse clouds form by accumulation of matter along magnetic field lines, which would increase the density but not the field strength, or that there is a physical process such as turbulent magnetic reconnection that acts to keep fields from increasing with density (Vishniac and Lazarian, 1999 Lazarian et al., 2012). Once densities become large enough for clouds to be self-gravitating, gravitational contraction with flux freezing may lead to the increase in field strength with increasing density.
The Bayesian analysis of the PDFs of the total field strength leads to the same result for the importance of magnetic fields with respect to gravity that was discussed above: for lower densities (where clouds are predominately not self-gravitating), the mass-to-flux ratio is subcritical. At higher densities it is supercritical.
The statistical increase in field strengths with density, parameterized by the power law exponent κ, may be compared with theoretical predictions. The ambipolar diffusion theory has κ near zero at early stages when contraction of neutrals increases density but not field strengths as evolution proceeds, κ gradually increases to a maximum of 0.5, e.g., Mouschovias and Ciolek (1999). The Bayesian analysis value of κ ≈ 0.65 ± 0.05 does not agree with the ambipolar diffusion prediction. It does agree with the value κ = 2/3 found by Mestel (1966) for a spherical cloud with flux freezing. However, while spherical collapse does produce κ = 2/3, finding that clouds have κ near this value does not require that clouds be spherical. It only means that collapse is approximately self-similar. The Bayesian result does imply that magnetic fields in self-gravitating clouds are generally too weak to dominate gravity in a large fraction of molecular clouds. However, the Bayesian analysis is a statistical one that does not rule out ambipolar diffusion being dominant in a small proportion of molecular clouds.
Tritsis et al. (2015) have questioned the results of the Bayesian analysis described above on several grounds, including: (i) that the clouds are not observed to be spherical (ii) that the Bayesian analysis included both H I and molecular cloud data a non-Bayesian analysis by Tritsis et al. (2015) of molecular cloud detections only yielded κ ≈ 0.5 and, (iii) that they found inferred cloud densities in a separate literature search often differing from those used by Crutcher et al. (2010b), particularly higher CN cloud densities, and argued that the CN points in Figure 5 should move further right thus lowering κ. Collectively, these are open questions for which countervailing arguments and considerations exist both are important to our full understanding of the scientific interpretation of Zeeman observations. On (i) it can be argued that real clouds invariably have significantly non-spherical morphologies due to other forces such as bulk flows and turbulence. Regarding (ii), omitting clouds with Zeeman non-detections (and accordingly smaller inferred magnetic field strengths) in a non-Bayesian analysis can bias the estimation of κ downwards the subset of clouds with larger field strengths may well have a smaller κ than the total set. On the final point (iii), it is required to estimate the density of the Zeeman tracer as opposed to the highest density for each cloud. Further, high excitation lines of other molecular species may sample higher densities that the N = 1𠄰 CN transition due to excitation and astrochemical depletion. As current and future telescopes provide further data, as described in section 6, these questions will undoubtedly be further constrained.
4.2.3. Radial Dependence of Mass/Flux
Study of M/Φ such as that illustrated by Figure 4 compare different clouds. Also of interest is the variation of M/Φ within a cloud, for that can be indicative of the role of the magnetic field in the structure and evolution of a cloud. This is a very difficult observational task because spectral lines will generally be weaker away from cloud centers. However, Crutcher et al. (2009) reported such a study toward four dark clouds. Although determination of actual values of M/Φ requires knowledge of the unknown angle θ between the magnetic field vector and the line of sight, it is possible to map the variation from point to point within a cloud if one assumes that the magnetic field direction is the same at the various positions. This is a reasonable assumption if the magnetic field is strong and dominates turbulence, as in the standard ambipolar diffusion model of star formation. That model requires that M/Φ increase from envelope to core as collapse of neutrals through the magnetic field increases the mass but not (so much) the field strength in the core.
The Crutcher et al. (2009) result was that in all four clouds, M/Φ decreases from envelope to core—the opposite of the ambipolar diffusion prediction. This observational result agreed with results from a weak field, turbulence dominated simulation (Lunttila et al., 2009). The observed result could also be due to magnetic reconnection (Lazarian, 2005), since loss of magnetic flux due to turbulent reconnection will proceed more rapidly in envelopes that in cores, since in envelopes have larger spatial scales and in general stronger turbulence.
Mouschovias and Tassis (2009, 2010) reviewed the above results and conclusion, and argued that (1) motion of cores through surrounding more diffuse gas could lead to B in cores and their envelopes not being essentially parallel and (2) that since BLOS was not detected in the envelopes only upper limits should be considered. Crutcher et al. (2010a) discussed these arguments. The first point may have some validity, but observed correlation of BPOS directions in cores and surrounding gas argues against it. In any case, such a process would sometimes increase and sometimes decrease the observed radial dependence of M/Φ. Four clouds is not a large number, but all four did show the same result. On the second point, it is certainly true that at the 3σ upper-limit level, M/Φ constant or even decreasing slightly with radius is consistent with the data for each cloud individually, but the probability that this is true for all four clouds is ߣ × 10 𢄧 . None the less, clear observational evidence for the ambipolar diffusion theory was not provided by the results in Crutcher et al. (2009).
4.2.4. Models of Specific Clouds
Ambipolar diffusion models for specific clouds, B1 and L1544, have been produced for comparison with observational data including OH Zeeman detections (Crutcher et al., 1994 Ciolek and Basu, 2000). In both cases the models could agree with observations, but both required that the fields be mainly in the plane of the sky, since the field strengths required by the models were much larger than the line-of-sight strengths obtained from Zeeman observations. While this could be true for the very small sample of two, in the larger sample of dark clouds with OH Zeeman observations one might expect to find examples of the field lying mainly along the line of sight, such that very large BLOS would be found from Zeeman observations. Such large fields are not found.
Far Infrared and the Kuiper Astronomical Observatory
Far IR astronomy was in an early stage of development in the early 1970s, when we began work in this field at UC Berkeley. Our work progressed in a sequence of steps but always towards high sensitivity and high spectral resolution with Fabry-Perot interferometers.
We were fortunate to begin almost immediately with what seemed like an interesting and relatively simple measurement, even though a new spectrometer needed to be put together for it. Martin Harwit and his associates at Cornell had just published observations from high altitude rocket observations indicating a remarkably intense isotropic flux in the 0.4–1.3-mm wavelength range (Shivanandan et al 1968, Houck & Harwit 1969). The flux was about 25 times more than expected from a 2.7-K blackbody field. It appeared strong enough and at wavelengths where the atmosphere was transparent enough that we should be able to observe the radiation from a high altitude location on Earth. Mike Werner, a newly arrived postdoc at UC Berkeley, and John Mather, a graduate student interested in our research, were willing to try to measure this exciting but very puzzling radiation. Could it even be some intense spectral line? I asked Paul Richards, a fellow professor experienced with bolometers and far IR, for help to speed up the work, and fortunately he was also interested. These three put together both a tunable and a fixed Fabry-Perot interferometer with nickel mesh reflectors, an indium-antimonide bolometer detector, a chopper, and a focuser of 8-cm aperture (Mather et al 1971). The system was set up at a 12,500-ft altitude on White Mountain in eastern California, and spectra were taken in the 0.7–1.7-mm wavelength range with a resolving power of about 100. The radiation apparently detected by rocket flights didn't seem to be there! Rocket measurements are of course difficult, and this was not the only time that rocket measurements were to give misleading results in measuring the isotropic background radiation. The work clearly interested Paul Richards, who then moved into rocket measurements of the background radiation. John Mather became Paul's student, and eventually was to lead a spectacularly successful experiment, with the COBE satellite, to measure the background radiation and apparently really get it right.
The experiment on White Mountain helped us get started, and my own research group, including Mike Werner, continued development of systems to measure astronomical spectra in the far IR from airplanes and at wavelengths somewhat shorter than those of this initial ground-based experiment. Our first operating system used a bolometer detector and reflectors made of metal mesh structures deposited on quartz. At the time, unsupported mesh seemed insufficiently stable to withstand aircraft vibrations. Mike Werner and Bob McLaren, along with a student, Don Brandshaft, flew the system in NASA's Lear jet and were able to measure radiation of the Orion region between 60 and 100 μm, but with a resolution of only 4 μm (Brandshaft et al 1975). By 1976, NASA's much larger C141 plane was available with its larger telescope, 36 inches in diameter. With it, we could measure an ammonia rotational line in the Jovian atmosphere at 85-μm wavelength with a resolution of about 1 μm (Greenberg et al 1977).
The next major steps, carried out by student Dan Watson and postdoc John Storey, involved Fabry-Perot reflectors made of thin metal mesh tightly stretched on a circular frame. There were two Fabry-Perots in series, one fixed and one tunable. In addition, Kandiah Shivanandan of the NRL lent us a gallium-doped germanium detector. By 1978, this system had the sensitivity and resolution required to give a resolving power of 1000 and good measurements of the fine structure lines of OIII at 88 μm and OI at 63 μm (Storey et al 1979). The first far IR lines from outside the Solar System had already been detected by a group under Martin Harwit at Cornell (Ward et al 1975) and a French-European Space Agency group including Baluteau and Moorwood (Baluteau et al 1976). Harwit's group had used gratings to detect the OIII line the French-European Space Agency group had used a Michelson interferometer and achieved very high resolution. These were historical firsts the Cornell group obtained a clear-cut detection as early as 1975, but with a resolution of only 1.3 μm. The European group had excellent resolution, near 0.02 μm, but their spectral line detection was a bit marginal. We worked away at Fabry-Perot systems, believing them to be the most powerful simple systems for detecting and mapping lines. However, grating systems are certainly competitive an excellent recent grating system has been used by Ed Erickson of NASA Ames and others (Erickson et al 1995).
We were fortunate that Eugene Haller, a solid-state physicist at UC Berkeley, was making and doing research on doped germanium detectors. He provided us with gallium-doped germanium detectors, and by 1980, we could use one of Haller's antimony-doped germanium detectors, which sensitively detected photons of wavelength longer than the 120-μm limit of gallium-doped germanium. This allowed detection of rotational lines of CO in the Orion nebula (Watson et al 1980). These and OH lines (Storey et al 1981) gave good evidence of shocks in Orion, and they allowed determination of gas densities and temperatures.
During the following several years, the double Fabry-Perot system, using antimony- or gallium-doped germanium detectors and flying at about 41,000-ft altitudes in NASA's KAO, brought in much valuable information about atoms, ions, and molecules in our own and other galaxies. Under favorable conditions and at longer wavelengths, it achieved a resolving power of 30,000 and a sensitivity of 2 × 10 −15 watts/Hz 0.5 .
Reinhard Genzel had come to UC Berkeley in 1982 on a Miller postdoctoral fellowship. He worked with the far IR group and helped very much in extending the work begun by postdocs Storey and Crawford and graduate student Dan Watson. He was also soon appointed to the academic staff.
By the mid-1980s we took another step in development by putting three detectors in a row for more rapid mapping. These detectors could also be mechanically squeezed to extend their sensitivity somewhat towards wavelengths longer than their normal cutoff. Gordon Stacey, who did his thesis on far IR astronomy at Cornell with Martin Harwit, had come as a postdoc and was an important player in putting this system into operation.
Reinhard Genzel went back to Germany as a director of the Max Planck Institute at Garching in late 1986, but we continued to work closely together. The UC Berkeley and Garching groups jointly constructed the next far IR spectrometer, which first flew in the KAO in 1989. Its improvements included a choice of either two or three Fabry-Perots in series to allow either very high or medium-high spectral resolution and a 5-by-5 assembly of 25 detectors to provide rapid mapping along with good sensitivity and spectral resolution. Each detector had its own optics cone to put all the IR radiation in a given angular resolution element on its particular detector. This new system, dubbed FIFI (Far-IR Imaging Fabry-Perot Interferometer) (Poglitsch et al 1991), has been used in the KAO by the UC Berkeley–Garching groups and by guest observers to map many spectral lines in regions of our galaxy and in external galaxies. One of Genzel's students, Norbert Geis, came to work with the UC Berkeley group for some years on our joint projects with FIFI. Resolution of the 36-inch Kuiper telescope in the far IR is limited by diffraction to 30–50 arcsec. However, this is enough to significantly resolve features of a number of nearby galaxies.
NASA's grounding of the KAO in late 1995, in order to save funds to build a still better system, ended my own far IR observations. However, I am happy that this next step to a new system called SOFIA (Stratospheric Observatory Far Infrared Astronomy) is being taken. Among other advantages, it will carry a 2.4-m telescope with diffraction-limited angular resolution 2.8 times higher than that of the KAO and with sensitivity on small objects about an order of magnitude better. Genzel and his group at the Max Planck Institute have begun planning the next instrumental improvements towards still better spectral line measurement and mapping, and they will be one of the many groups that will keep SOFIA quite busy.
A Massive Rotating Disk in the Early Universe
Massive disk galaxies like the Milky Way are expected to form at late times in traditional models of galaxy formation, but recent numerical simulations suggest that such galaxies could form as early as a billion years after the Big Bang through the accretion of cold material and mergers. Observationally, it has been difficult to identify disk galaxies in emission at high redshift to discern between competing models of galaxy formation. In this contribution, the authors report imaging, with a resolution of
1.3 kiloparsecs, the 158-micrometre emission line from singly ionized carbon, the far-infrared dust continuum, and the near-ultraviolet continuum emission from a galaxy at a redshift of 4.2603, identified by detecting its absorption of quasar light. These observations show that the emission arises from gas inside a cold, dusty, rotating disk with a rotational velocity of
272 kilometers per second. The detection of emission from carbon monoxide in the galaxy yields a molecular mass that is consistent with the estimate from the ionized carbon emission of
72 billion M⊙. The existence of such a massive, rotationally supported, cold disk galaxy when the Universe was only 1.5 billion years old favors formation through either cold-mode accretion or mergers, although its large rotational velocity and large content of cold gas remain challenging to reproduce with most numerical simulations.
Figure caption: [Far left & left center] VLA CO contours, and ALMA contours of the [CII] and thermal dust emission from the z=4.3 Wolfe galaxy. [Right center & far right]: [CII] velocity field, and the rotation curve.
Publication: Marcel Neeleman (Max Planck Institute for Astronomy) et al., A Cold, Massive, Rotating Disk Galaxy 1.5 Billion Years after the Big Bang, Nature, 581, 269 (20 May 2020).
Molecular jets from low-mass young protostellar objects
Molecular jets are seen coming from the youngest protostars in the early phase of low-mass star formation. They are detected in CO, SiO, and SO at (sub)millimeter wavelengths down to the innermost regions, where their associated protostars and accretion disks are deeply embedded and where they are launched and collimated. They are not only the fossil records of accretion history of the protostars but also are expected to play an important role in facilitating the accretion process. Studying their physical properties (e.g., mass-loss rate, velocity, rotation, radius, wiggle, molecular content, shock formation, periodical variation, magnetic field, etc) allows us to probe not only the jet launching and collimation, but also the disk accretion and evolution, and potentially binary formation and planetary formation in the disks. Here, the recent exciting results obtained with high-spatial and high-velocity resolution observations of molecular jets in comparison to those obtained in the optical jets in the later phase of star formation are reviewed. Future observations of molecular jets with a large sample at high spatial and velocity resolution with ALMA are expected to lead to a breakthrough in our understanding of jets from young stars.
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