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How can white dwarf form Oxygen ? (Temperature problem)

How can white dwarf form Oxygen ? (Temperature problem)


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I've got a question about white dwarfs and oxygen.

I read in a book that a temperature of 100 million degrees is required to fuse Helium in the core of a red giant. The Helium fuses into Carbon by the triple-alpha process.

It's also written that, after a temperature of 350 million degrees is reached, the core of a red giant ceases to be degenerate. So, the core is able to expand and its temperature is controlled. The core won't reach a temperature of more than 350 million.

But my teacher has said that a temperature of 600 million degrees is needed to form Oxygen from Carbon: Carbon fuses into Neon and by photodisintegration gives Oxygen.

So, how can there be Oxygen in white dwarfs, if we don't reach a temperature higher than 350 million degrees? By what process can oxygen be formed in a red giant?


Some oxygen is produced during CNO cycle processing of hydrogen, starting with carbon nuclei. Oxygen is also produced by alpha capture onto carbon nuclei at temperatures well below 350 million K.

Both of these occur in and around the cores of low mass stars before they become white dwarfs. Neon production is not required.


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Observations shed more light on the atmosphere of white dwarf GD 424

Average spectrum of the white dwarf GD 424 obtained with WHT/ISIS on August 26, 2017. Credit: Izquierdo et al., 2020.

Astronomers have performed spectroscopic observations of a newly detected white dwarf star known as GD 424. Results of the observational campaign provide more insights into the atmosphere of this object. The study was presented in a paper published December 23 on arXiv.org.

White dwarfs are remaining compact cores of low-mass stars that have exhausted their nuclear fuel. Although their atmospheres are mainly composed of hydrogen or helium, between 25 and 50 percent of all known white dwarfs show traces of metals in their spectra. It is assumed that these metals originate in the accretion of tidally disrupted planetary bodies. Spectroscopic observations of metal-polluted white dwarfs could be an essential tool to measure the bulk compositions of the parent bodies.

Hence, a team of astronomers led by Paula Izquierdo of the University of La Laguna, Spain, conducted spectroscopic observations of GD 424—a metal-polluted, helium-atmosphere white dwarf of spectral type DB with a large amount of trace hydrogen. For this purpose, they employed the Intermediate dispersion Spectrograph and Imaging System (ISIS) mounted on the 4.2-m William Herschel Telescope (WHT) and the High-Resolution Echelle Spectrometer (HIRES) of the 10-m Keck I telescope.

"We presented the discovery and chemical abundances analysis of GD 424, a metal-polluted DBA white dwarf with one of the largest amounts of trace hydrogen measured so far among white dwarfs with similar temperatures," the scientists wrote in the paper.

The researchers used a hybrid method to fit synthetic spectra, survey photometry and data from the ESA's Gaia DR2 parallax to the obtained WHT optical spectrum, which allowed them to determine the photospheric parameters of GD 424. It was found the white dwarf has an effective temperature of about 16,560 K, mass of around 0.01 solar masses, radius of around 0.0109 solar radii and cooling age estimated to be approximately 215 million years.

Analyzing the spectra from WHT and Keck, the team identified 11 metals in the atmosphere of GD 424, namely oxygen, sodium, manganese, chromium, nickel, silicon, iron, magnesium, titanium, calcium and aluminium. The astronomers assumed that the presence of these elements is due to the accretion of a planetary body onto the white dwarf.

They added that GD 424 is most likely accreting dry, rocky debris in either the increasing or steady state. The photometric results also allowed the researchers to estimate the parent body composition.

"The estimated composition of the parent body is consistent with both CI chondrites and the bulk Earth. (. ) The composition of the parent body did not reveal an oxygen excess. This suggests that the large amount of trace hydrogen is probably the result of the earlier accretion of water-rich planetesimals," the authors of the paper concluded.

Further observations of GD 424, focused on measuring abundances of volatile elements, are required in order to get more insights into the nature of the parent object.


White dwarf with almost pure oxygen atmosphere discovered

Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint pinprick of light to the lower left of the much brighter Sirius A. Image: NASA, ESA

A trio of researchers, two with the Federal University of Rio Grande do Sul and the other with Universität Kiel has discovered something very unique—a white dwarf with an atmosphere that is made almost completely of oxygen. In their paper published in the journal Science, Kepler de Souza Oliveira, Detlev Koester and Gustavo Ourique describe how they came to discover the oddity and offer some ideas on how it might have come to exist. Boris Gänsicke with the University of Warwick offers an essay on the work by the team in the same journal issue.
White dwarfs come about, scientists believe, when a relatively 'small' star runs out of fuel, losing its outer layer as the star shrinks down due to gravity—the stronger gravitational force then usually causes the heaviest elements to be drawn towards the core pushing the lighter ones, such as helium and hydrogen to the surface. But this new white dwarf is different, the researchers report, instead of the usual mix of light elements at the surface, there is almost nothing but pure oxygen. Nicknamed Dox, the star is the first ever of any kind to be observed to have a nearly pure oxygen outer layer.

Such a phenomenon has been predicted before, but most in the field never believed that such a star would ever be observed, thus it came as quite a surprise to team member Gustavo Ourique as he poured over thousands of simple graphs made from data generated by the New Mexico observatory. It was not until further tests were run that it was confirmed that the unique graph he had found turned out to represent data from the strange white dwarf.

Though it is impossible at this point to say with any certainty what caused the unique star formation, the researchers believe it is likely tied to an earlier event—they believe that Dox may once have been one of a pair of stars forming a binary system, and as the other star ran out of fuel it would have become a red giant, which would perhaps have been able to interact with its partner directly. The outcome, the researchers suggest could have laid the groundwork for a later explosion that caused Dox to lose its other lighter elements, leaving mostly pure oxygen in its outer layer.

Researchers have discovered a white dwarf star with an atmosphere dominated by oxygen, a type of white dwarf that has been theorized to exist but not identified to date. The finding could challenge the textbook wisdom of single stellar evolution, and provide a critical link to some types of supernovae discovered over the past decade. As relatively small stars (those less than ten times the mass of our sun) near the end of their lives, they throw off their outer layers and become white dwarf stars, which are very dense. The high gravity that occurs under such density causes the lighter elements, such as hydrogen or helium, to float to the surface of the star, masking the heavier elements below. While combing through data from the Sloan Digital Sky Survey (SDSS), Souza Oliveira Kepler et al. identified SDSS J124043.01+671034.68, a white dwarf with its outer layer of light elements stripped away, revealing a nearly pure layer of oxygen. Several different theories have predicted that the outer layer of a white dwarf can be stripped, but identification of SDSS J124043.01+671034.68 provides the first evidence of this phenomenon. One possibility is that interactions with a nearby companion in a binary star caused SDSS J124043.01+671034.68 to bare its oxygen envelope. Another possibility is that a massive pulse of burning carbon from the center of the star, emulating outwards, eliminated the lighter elements. A Perspective by Boris Gänsicke provides further context.


The Very Hungry White Dwarf

White dwarfs are the remnants of a once vibrant main sequence star, like our Sun. But, what happens to the planets that were in that solar system after the star dies? In recent years, astronomers have observed unexpected metals (elements heavier than hydrogen and helium) in the spectra of white dwarfs, indicating the destruction and accretion of a planet into the former star’s photosphere.

Without these metals, white dwarf spectra usually show mostly hydrogen, helium, carbon, or oxygen, since the outer layers of the star were lost when they were blown off into a planetary nebula at the end of the star’s life, revealing the stellar core. There are many types of white dwarfs (based on which of these elements show up in their spectra), one of which is the helium-dominated white dwarf. If we observe a helium white dwarf with extra hydrogen in its spectrum, the natural question then is—where did that hydrogen come from? Astronomers consider three possibilities: hydrogen that’s simply left over from earlier in the star’s life, hydrogen accreted from an external source (like the interstellar medium or a planet), or hydrogen dredged-up from the deeper layers of the star.

Previous research surveyed a number of white dwarfs and found that hydrogen was almost twice as common in stars with metal pollution than those without, indicating that planets could be the reason for the excess hydrogen. A different study confirmed that planet accretion contributes to hydrogen in metal-polluted white dwarfs by looking at excesses of oxygen in their spectra. Oxygen is interpreted as coming from the accretion of water, so that means there would be some hydrogen accreted, too!

Today’s paper takes a detailed look at GD 424, a metal-polluted, helium white dwarf (with a large amount of hydrogen!). By studying the spectrum of this star in detail, the authors want to figure out the history of the accreting white dwarf and the composition of the destroyed planet.

First things first: know your star!

Using observations of spectra and photometry from the William Herschel Telescope at the Observatorio del Roque de los Muchachos on La Palma, Spain, the authors observed GD 424 and found that it had “a helium-dominated photosphere with presence of hydrogen and a number of much narrower metallic absorption lines of oxygen, magnesium, silicon, and calcium.” They first needed to use models to understand properties of the white dwarf’s photosphere, like temperature and surface gravity. Once these were determined, they were able to determine abundances for the different metals observed (see Figure 1 for GD 424’s spectrum, showing absorption lines for different elements).

Figure 1: Spectrum of GD 424, showing strong absorption of helium and hydrogen and other absorption features such as oxygen, magnesium, silicon, and calcium. (Figure 1 from the paper.)

So, what’s going on with that accretion then?

To come up with the composition of the accreted planet, we’d have to make some assumptions about when the accretion started and how fast the accretion is happening. The authors use a simple model for accretion, which happens in three stages: increasing accretion (which leads to a linear increase in metal abundance), steady state accretion (where the metals reach equilibrium in the photosphere), and decreasing accretion (where metals exponentially decay due to diffusion and sinking into the white dwarf). All of this comes down to one takeaway: metal abundance depends on the balance between accretion (how much is going into the photosphere) and diffusion/sinking (how much is going out of the photosphere).

Unfortunately, there are no obvious clues to which of these three states the star is in, so the authors tested all three. Both the increasing and steady states seem possible, and would imply a planet similar to the composition of Earth but with a bit too much calcium (see Figure 2 for comparisons of metallicity!). The accretion rate is among the highest observed for white dwarfs, and it’s already accreted at least the mass of the solar system asteroid 10 Hygeia!

Figure 2: Abundances of different metals (relative to silicon) in the accreting planetary body near GD 424, normalized to their bulk Earth abundances. The darker blue points refer to the different models “ss” (circles) refers to the steady state accretion assumption, “is” (triangles) to the increasing state, and “ds” (squares) to the decreasing state. Abundances from parts of the Earth are shown for comparison (hollow circles and triangles), as are other white dwarfs (light pink and blue dots). (Figure 8 from the paper.)

Another interesting piece of the puzzle is figuring out how much water was on this Earth-like planet. The authors come up with the water content by looking at excess oxygen that can’t be accounted for with other common minerals, such as magnesium oxide (MgO) or titanium dioxide (TiO2). Turns out, GD 424 is currently accreting dry, rocky planetary debris.

Where did its excess hydrogen come from then?!

If the planet that’s currently being accreted didn’t have water, then where’s all the excess hydrogen from? Well, for hydrogen to show up in the spectrum it had to be accreted recently, after the white dwarf had sufficiently cooled to keep the hydrogen around in the photosphere. This means the hydrogen likely had to come from a previous accretion episode, where a different water-rich planet was destroyed, and the oxygen from that episode is gone simply because it diffuses out on a shorter timescale.

Now that we know all the pieces, we can put the whole story together: Since transitioning off the main sequence, GD 424 has eaten a whole water-rich planet, and now it’s accreting dry, rocky debris from another planet. Who knows if there’s anything left on the menu, but one thing is for certain: GD 424 is a very hungry white dwarf.


White dwarf atmospheres might contain the pulverized crusts of their dead planets

Credit: Dr. Mark A. Garlick

Astronomers have developed a new technique to search for exoplanets—by looking for their crushed up bones in the atmospheres of white dwarfs. And it's working.

The search for planets outside the solar system, known as exoplanets, has one significant limitation: We can only find exoplanets that exist right now. But our universe has been hanging around for over 13 billion years, and many generations of planetary systems have come and gone in that vast expanse of cosmic time.

Unfortunately, when stars die, they usually take their planets with them. Especially the most massive stars, which die as supernovae—those deaths usually obliterate any orbiting planet completely. But even when less massive stars like the sun die, it's generally bad news for their planets.

But as a new research paper has pointed out, that doesn't remove all evidence of the planetary system off the galactic map. If any planets (or remnant cores of planets) survive, they can occasionally gravitationally scatter off of each other. This doesn't usually happen in stable systems, but in the death throes of a star anything is possible (gravitationally speaking).

Some of those scattered objects can head inward to the white dwarf, the leftover core of the parent star. That white dwarf is made of almost completely pure carbon and oxygen, surrounded by a dense but thin shell of hydrogen and helium. Naturally, any object passing too close will get torn to shreds by the extreme gravity of the white dwarf, with the debris making its way to the surface to mix and mingle with the hydrogen and helium.

Once there, any elements in the destroyed object, like lithium and calcium, can release their own light, giving a spectral fingerprint that astronomers can potentially spot. Most white dwarfs are too hot, though, and that light outshines any contamination. But the recent Gaia mission was able to map dozens of old, cool white dwarfs, and astronomers have detected the distinct signature of crushed up planets in their atmospheres.

The astronomers found that the abundance of enriched elements matches what we know from our own solar system, indicating that planetary systems like ours have been in the universe for a very, very long time.


White Dwarfs and Planetary Nebulae

After a star like the sun exhausts its nuclear fuels, it loses its outer layers as a "planetary nebula" and leaves behind the remnant "white dwarf" core. The white dwarfs are extremely small stars -- they are the bare remnant cores of stars after they have gone through all of their lifetimes. (from Imagine the Universe,http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html)
We show the evolution of a star like the sun on the H-R diagram (to left) and as it might appear if we were watching (to right) (from Jake Simon and Charles Hansen, http://rainman.astro.uiuc.edu/ddr/stellar/index.html). It is pretty boring because the main sequence lifetime is so long (a good thing for us!). Eventually the star becomes a red giant - its luminosity goes way up, it swells, and its temperature drops. That does not last long - quickly it expels its outer layers and its core shrinks to a white dwarf. We see it briefly as a planetary nebula, but the gas dissipates and we end with an isolated white dwarf that cools as it loses its stored energy. The bar to the lower left shows the age of the star.

An early step in the mass loss and transformation to planetary nebula is when tiny dust grains form near the surface of the star and the pressure of the light photons on them expels them, carrying some of the gas along. Here is a simulation of how it would look - the diameter of the field is about 10 times the size of the orbit of the earth. (from Peter Woitke, http://www.strw.leidenuniv.nl/

When only the white dwarf core is left of the star, it lights up the material that was expelled because the white dwarf is very hot. Here is a very young planetary nebula, where a lot of the structure of the just-ejected material is still present. The visible light image to the left shows shadowing by a disk, waves from pulsed mass loss, and "searchlights" of light. The infrared image on the right shows molecular hydrogen (red) and the denser material that shapes the visible searchlights.
Older planetary nebulae take many wonderful and beautiful forms. However, they all are created through similar processes. This one is called the Cat's Eye, (Vicent Peris, HST, STScI)
From Doppler shift velocities, most planetary nebulae have ages around

The Nature of White Dwarfs

Once nuclear reactions cease, the stellar remnant has no means of counteracting the force of gravity, and the interior of the star collapses. It doesn't collapse forever because a new force develops that can resist gravity. This force is electron pressure. The material in a white dwarf has been compressed so much by gravity that all the electrons have been stripped away from all of the atomic nuclei. The electrons form a gas. The electrons are squeezed together by gravity, but as described by quantum mechanics, the electrons eventually resist being squeezed together any further. Thus happens when they become degenerate:

Degenerate matter : No two electrons can have exactly the same energy, spin, position (according to quantum mechanics) so when electrons are compressed enough, they fill up all of the available energy states. Such dense matter is called degenerate.

A white dwarf has a diameter similar to the Earth's and a density such that a teaspoonful weighs a ton!

Models of white dwarfs can be calculated using the laws of quantum mechanics --

When the mass exceeds 1.4 M, electron degeneracy is no longer strong enough to resist the pull of gravity and the white dwarf abruptly collapses into a neutron star. (animation by G. Rieke)

The sun will end its life as a white dwarf.

Neutron Stars: The Fate of Stars with M>1.4M

Massive stars can lose mass even more dramatically than for planetary nebulae:

Further collapse is prevented by pressure from the "neutron gas," which behaves as degenerate matter in a fashion analogous to the behavior of electrons in white dwarfs.

In the late 1960s, radio astronomers discovered a type of radio source that they dubbed "pulsars"

1) typically do not have visible counterparts

2) their radio output varies in a precise, repetitive pattern

3) they are found in or near supernova remnants

Pulsars were considered extremely remarkable when first discovered because of the extremely short periods of their variations (ranging from a fractions of a second to a few seconds vastly different from the 10-1000 day periods for red giant/supergiant variables), and because of the extreme constancy of their periods --- they are extremely accurate clocks. A normal star could not vary so quickly and regularly, because even at the speed of light it could not communicate across its diameter fast enough! Neutron stars were the only objects predicted to exist that could have the behavior of pulsars.The concept is strengthened by noting that if you imagine shrinking a normal star with its

20 day rotation period, it would speed up to pulsar-like time scale if shrunk to a size only 10 km in diameter.

Later it was discovered that all pulsars are slowing down, but slowing down so gradually that the change can be detected only be using the most accurate atomic clocks. Further confirmation comes from the equivalence of the amount of energy in the escaping beam of particles and light and the energy loss indicated by the rate of slowing down.


Author information

Affiliations

Department of Physics, The University of Warwick, Coventry, UK

M. A. Hollands, P.-E. Tremblay, B. T. Gänsicke, P. Chote, N. P. Gentile-Fusillo, M. J. Hoskin, T. R. Marsh & D. Steeghs

Facultad de Ciencias Astrónomicas y Geofísicas, Universidad Nacional de La Plata, La Plata, Argentina

M. E. Camisassa & A. H. Córsico

Instituto de Astrofísica de La Plata, UNLP-CONICET, La Plata, Argentina

M. E. Camisassa & A. H. Córsico

Institut für Theoretische Physik und Astrophysik, University of Kiel, Kiel, Germany

Department of Physics, Faculty of Science, Naresuan University, Phitsanulok, Thailand

Department of Physics and Astronomy, University of Sheffield, Sheffield, UK

Instituto de Astrofísica de Canarias, Tenerife, Spain

European Southern Observatory, Garching, Germany

Departamento de Astrofísca, Universidad de La Laguna, La Laguna, Spain

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Contributions

M.A.H., P.-E.T. and B.T.G. led the project, including the interpretation of WD J0551+4135. M.E.C. calculated the interior CO/ONe-core models. D.K. calculated the envelope models and advised M.A.H. on atmospheric modelling. N.P.G.-F. acquired the initial Liverpool Telescope lightcurve. A.A., V.S.D. and T.R.M. acquired the TNT lightcurves. P.C. calibrated the Liverpool Telescope and TNT lightcurves and their amplitude spectra. A.H.C. calculated the pulsation properties of WD J0551+4135 from the CO/ONe interior models. M.J.H. and P.I. acquired the WHT spectroscopic data of WD J0551+4135. D.S. acquired and calibrated the Swift photometry of WD J0551+4135.

Corresponding author


White Dwarfs and the Chandrasekhar Limit

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A white dwarf is the remnant of a main-sequence star of mass (less than about four times the mass of the Sun) that has exhausted its hydrogen fuel by fusion into helium. Such a star will first expand to form a red giant as it fuses helium in its core to carbon and oxygen by triple-alpha processes. After the star sheds its outer layers, ejecting a planetary nebula, the remnant will be composed mainly of carbon and oxygen, incapable of further fusion reactions. The surface temperature initially lies in the range 8000 to 40,000 K, which implies a white color, hence the designation white dwarf. The gravitational field of the white dwarf causes a collapse to a body about the size of the Earth. Thereby a mass comparable to the Sun's, , is compressed to a radius comparable to that of the Earth, . Further collapse is resisted by the electrons of the carbon and oxygen atoms, which form a degenerate electron gas following a Fermi&ndashDirac distribution. The outward pressure of the electrons, countering the gravitational compression, is thus a purely quantum-mechanical effect, which can be attributed to the exclusion principle.

Among the first identified white dwarfs, in 1915, is Sirius B, the companion to Sirius.

S. Chandrasekhar proposed in 1931 that in a stellar remnant with mass greater than approximately 1.44 , known as the Chandrasekhar limit, gravitation overcomes the electron degeneracy pressure and the white dwarf collapses into a fraction of its volume to form a neutron star. This is associated with the electrons near the Fermi level becoming ultra-relativistic, with energies approaching the electron rest energy . A neutron star is also a degenerate fermionic quantum system of neutrons, into which the carbon and oxygen nuclei collapse. In a neutron star, a stellar mass is compressed to a radius of the order of 10 km. Some neutron stars can emit beams of electromagnetic radiation, which makes them detectable as pulsars.

It is sometimes said that white dwarfs have densities of the order of tonnes per teaspoon, while neutron stars have densities of billions of tonnes per teaspoon (1 tonne, or metric ton, equals 1000 kg).

If the remnant star has a mass exceeding the Tolman&ndashOppenheimer&ndashVolkoff limit of around , the combination of degeneracy pressure and nuclear forces becomes insufficient to support the neutron star and it continues collapsing to form a black hole.

Contributed by: S. M. Blinder (April 2020)
Open content licensed under CC BY-NC-SA