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Post Info TOPIC: White Dwarf GD 362
Blobrana


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RE: White Dwarf GD 362
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Title: The Survival of Water within Extrasolar Minor Planets
Authors: M. Jura, S. Xu (UCLA)
(Version v2)

We compute that extrasolar minor planets can retain much of their internal H_2O during their host star's red giant evolution. The eventual accretion of a water-rich body or bodies onto a helium white dwarf might supply an observable amount of atmospheric hydrogen, as seems likely for GD 362. More generally, if hydrogen pollution in helium white dwarfs typically results from accretion of large parent bodies rather than interstellar gas as previously supposed, then H_2O probably constitutes at least 10% of the aggregate mass of extrasolar minor planets. One observational test of this possibility is to examine the atmospheres of externally-polluted white dwarfs for oxygen in excess of that likely contributed by oxides such as SiO_2. The relatively high oxygen abundance previously reported in GD 378 plausibly but not uniquely can be explained by accretion of an H_2O-rich parent body or bodies. Future ultraviolet observations of white dwarf pollutions can serve to investigate the hypothesis that environments with liquid water that are suitable habitats for extremophiles are widespread in the Milky Way.

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Blobrana


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White Dwarf GD 362.kmz
Google Sky File

-- Edited by Blobrana at 17:07, 2008-01-12

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Blobrana


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Polluted Dead Star Indicates Planets Like Earth May Have Formed Around Other Stars.
The chemical fingerprint of a burned-out star indicates that Earth-like planets may not be rare in the universe and could give clues to what our solar system will look like when our sun dies and becomes a white dwarf star some five billion years from now.
Astronomers from UCLA report that a white dwarf star known as GD 362, which is surrounded by dusty rings similar to those of Saturn, has been contaminated by a large asteroid that left more than a dozen observable chemical elements in the white dwarf's atmosphere. Such an observation is unprecedented in astronomy. Was there some kind of violent interaction between the star and the asteroid?
The UCLA astronomers think that after about a billion years orbiting the white dwarf as part of an ancient planetary system, an asteroid got close enough to the star to be torn apart by its very strong gravitational force field. An Earth-sized but exceedingly dense white dwarf is the standard end state for most stars. This particular white dwarf, which is under investigation by the W.M. Keck Observatory in Hawaii, is located in the constellation Hercules, approximately 150 light-years, or 1,000 trillion miles, from Earth.

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Blobrana


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Title: Evidence for a merger of binary white dwarfs: the case of GD 362
Authors: E. Garcia-Berro, P. Loren-Aguilar, A.G. Pedemonte, J. Isern, P. Bergeron, P. Dufour, P. Brassard

GD 362 is a massive white dwarf with a spectrum suggesting a H-rich atmosphere which also shows very high abundances of Ca, Mg, Fe and other metals. However, for pure H-atmospheres the diffusion timescales are so short that very extreme assumptions have to be made to account for the observed abundances of metals. The most favoured hypothesis is that the metals are accreted from either a dusty disk or from an asteroid belt. Here we propose that the envelope of GD 362 is dominated by He, which at these effective temperatures is almost completely invisible in the spectrum. This assumption strongly alleviates the problem, since the diffusion timescales are much larger for He-dominated atmospheres. We also propose that the He-dominated atmosphere of GD 362 is likely to be the result of the merger of a binary white dwarf.

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Blobrana


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For the last two years, astronomers have suspected that a nearby white dwarf star called GD 362 was "snacking" on a shredded asteroid. Now, an analysis of chemical "crumbs" in the star's atmosphere conducted by NASA's Spitzer Space Telescope has confirmed this suspicion.

"This is a really fascinating system, that could offer clues to what our solar system may look like in approximately five billion years when our Sun becomes a white dwarf" - Dr. Michael Jura, of the University of California at Los Angeles (UCLA).

White dwarfs are essentially the glowing embers of stars that were once like our Sun. Sun-like stars spend most of their lives producing energy by fusing hydrogen atoms into "heavier" helium atoms. Our Sun is currently doing this.
Once the Sun-like star runs out of hydrogen, helium atoms will fuse to produce other heavier elements like carbon, which will eventually sink to the star's core. Meanwhile, the heat released during this helium fusion is so strong that the will star expand and vaporize all dust, rocks and planets that orbit nearby. At this stage, the star is called a "red giant." Ultimately, the red giant will shed its external layers, exposing a dense, hot core about the size of Earth, known as a "white dwarf."
Closely orbiting planets, asteroids, and dust are not expected to survive the red-giant phase of a Sun-like star's life, so astronomers were shocked to find so much dust around the white dwarf GD 362. According to Jura, GD 362 has been a white dwarf for approximately 900 million years -- so surrounding dust should have already been destroyed. He also notes that astronomers were surprised to find chemical elements heavier than hydrogen and helium in GD 362's atmosphere, because these elements should have already sunk to the star's core. When an abundance of heavy elements were first found in GD 362's atmosphere in 2004, scientists were not sure where they came from.
An explanation came in 2005, when two teams of astronomers independently found evidence for dust orbiting GD 362. Both groups argued that the elements in the atmosphere came from orbiting dust particles that rained onto star, and was vaporized by the white dwarf's intense heat. However, astronomers did not know where the dust came from.

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Title: Spitzer Observations of GD 362 and Other Metal-Rich White Dwarfs
Authors: J. Farihi, B. Zuckerman, E.E. Becklin, M. Jura

A Spitzer IRAC survey of 17 nearby metal-rich white dwarfs, nominally DAZ stars, reveals excess emission from only 3 targets: G29-38, GD 362 and G167-8. Observations of GD 362 with all three Spitzer instruments reveals a warm (~ 1000 K) dust continuum, very strong silicate emission, and the likely presence of cooler (~500 K) dust. While there is a general similarity between the mid-infrared spectral energy distributions of G29-38 and GD 362, the IRAC fluxes of G167-8 are so far unique among white dwarfs. However, further observations of G167-8 are required before the measured excess can be definitely associated with the white dwarf.

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Interacting White Dwarf
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New Spitzer Space Telescope observations of an unusual class of interacting binary stars detected excess amounts of infrared radiation, suggesting that these odd objects are surrounded by large disks of cool dust.

The results reported today in Washington, DC, at the 207th meeting of the American Astronomical Society (AAS) were produced by one of six teams of professional astronomers and high school teachers participating in a unique program co-sponsored by the Spitzer Science Centre and the National Optical Astronomy Observatory (NOAO).
The type of cataclysmic variable system being studied by the team consists of a highly magnetic white dwarf star (a “dead” remnant star formed from the core of a star like our Sun when it exhausts the available fuel to support nuclear fusion) and a very low mass, cool object similar to a brown dwarf. The two objects orbit so closely—about the distance from Earth to the Moon—that they make a complete revolution about each other in only 80-90 minutes. The white dwarf is Earth-sized but weighs about 60 percent of the mass of the Sun, while the companion star is Jupiter-sized but has about 40-50 times the mass of Jupiter.

The high mass of the white dwarf and the closeness of the companion result in mass exchange between the two stars. The gravitational influence of the white dwarf squeezes the companion star into a teardrop shape, and matter squirts from its pointed end toward the white dwarf, like water from the nozzle of a garden hose. This material eventually falls onto the white dwarf, causing tremendous heating of its atmosphere and the emission of a large amount of energy from X-rays to the far infrared.

A team of astronomers and teachers led by Steve B. Howell of NOAO observed four of these types of binaries with NASA’s Spitzer Space Telescope in an attempt to study the cool, low-mass object in the pair: EF Eridanus, V347 Pav, GG Leo and RX J0154.

To their surprise, excess infrared emission was discovered around all four. The team’s current best model for its origin is a large, cool circumbinary dust disk with a temperature of about 800-1,200 Kelvin (980-1,700 degrees Fahrenheit).

"Our explanation at this point is that the emission originates from a large, relatively cool disk of dust encircling the entire binary system. The discovery of dust disks around these old interacting binaries is very exciting. We have shown our initial results to a variety of specialists, and nobody yet has a better idea of what we are seeing" - Steve B. Howell.

Such circumbinary disks have been predicted on theoretical grounds and a few observational studies have attempted to find them, with mixed results. The disks may be the remains of the large “mass-loss” episode that occurred during the formation of the white dwarf. They also could be composed of material spewed from the binary in the form of strong winds (like a very dense version of our Sun’s solar wind), or material that was ejected during one or more previous nova explosions. Cyclotron emission due to the large magnetic field of the white dwarfs in these particular binaries cannot be eliminated completely as another potential source of at least part of the infrared emission.

“A number of ideas are on the table, as well the possibility of some still-unknown process. These objects are ripe for further study” - Steve B. Howell

Only two other white dwarfs (including one newly discovered) are known to be encircled by a dust disk—stars named G29-38 and GD362. Unlike the cataclysmic variables studied by Howell’s team, both of these are single white dwarfs, and the source of their dust disks is not known for certain. Dust disks made up of “left over” material from the star formation process are known to exist around very young stars and have been discovered around Sun-like stars as well. Some of these latter disks are known to harbour planetary-type objects, orbiting in cleared out “rings” within the disk.

"While we have no evidence for planetary objects in our disks, the possibility does exist. More work must be done to prove the infrared excess is from a disk and, if true, to discover its properties such as density and composition. We also would like to see if these disks exist in every interacting binary of this type or only in some. Their presence would greatly change our concept of the evolution of such systems" - Steve B. Howell.

These types of systems are important because they give astronomers insight into the accretion, or “mass transfer,” process that also plays a role in the formation of stars and planets, according to team member Donald W. Hoard, an astronomer at the Spitzer Science Centre in Pasadena, California.

“Cataclysmic variable accretion is one of the least complicated forms of mass transfer in the Universe. These systems are great to observe, because unlike accretion during the formation of stars and planets, or around supermassive black holes in far off galaxies, the process in cataclysmic variables happens on relatively short, human timescales” - Donald W. Hoard.

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Blobrana


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White Dwarf GD 362
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Astronomers have glimpsed dusty debris around an essentially dead star where gravity and radiation should have long ago removed any sign of dust. The discovery might provide insights into our own solar system’s eventual demise several billion years from now.
Solar systems may continue to exist around stars that have reached the end of their lifetimes, flared up and collapsed.
New evidence shows that asteroids and dust discs, and perhaps even planets, may circle white dwarf stars, the burned-out remnants of stars that have already undergone their all-consuming red-giant phase.
This suggests that, for our solar system too, there is a possibility of life after the presumed death of the inner planets – when the Sun expands to such a bloated size that it envelops the orbit of the Earth and beyond. But it may be a grinding sort of life.


Position(2000): RA = 17 31 34.3 Dec = +37 05 20

The new findings, to be published in the Astrophysical Journal, are based on high-resolution spectroscopic imaging of the white dwarf GD 362, made with the Gemini North, IRTF and Magellan telescopes on Mauna Kea, Hawaii. These observations showed an unexpected excess of infrared in the light of the star, as well as a huge abundance of calcium – the second-highest ever seen from a white dwarf.
The infrared excess around GD 362, which amounts to about 3% of the total stellar luminosity, can be explained by emission from a passive, flat, opaque dust disk that lies within the Roche radius of the white dwarf.

The calcium could only be explained by an influx of dust onto the white dwarf, and the infrared excess is best explained by a very thin, flat and highly opaque disc circling the star out to a radius of perhaps 1 million kilometres. The disc must be composed of dust that is continually being drawn onto the star's surface, producing its abundance of calcium, and other metals.

But there is a problem: Such a dust disc could only survive for a few centuries, yet this white dwarf has probably been cooling for five billion years, following its red giant phase. So there must be some process that is continually replenishing it.



After careful modelling, Eric Becklin at the University of California at Los Angeles (UCLA), US, and his co-authors concluded that "an asteroid, or possibly even a planet" strayed too deeply into the white dwarf's powerful gravitational field and was torn apart. Now, the remnants of that body are continually colliding, raining dust onto the star's surface.


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"Our best guess is that something similar to an asteroid or possibly even a planet around this long-dead star is being ground up and pulverized to feed the star with dust. The parallel to our own solar system’s eventual demise is chilling" - Eric Becklin.

And it may not be a rare process. If excess metals are a sign of dust accretion, "it would mean that metal-rich white dwarfs – and this is fully 25% of all white dwarfs – may have debris discs, and therefore planetary systems, around them", - Mukremin Kilic, graduate student at the University of Texas, US, who led the IRTF observing team.

"Planetary systems may be more numerous than we thought"

"We now have a window into how planetary systems like our own might behave billions of years from now" - Ben Zuckerman, UCLA professor of physics and astronomy, member of NASA's Astrobiology Institute, and a co-author on the Gemini-based paper.


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Observations and theory give strong evidence that all stars expel a large fraction of their mass throughout their lifetimes. The most intensive phase of this mass loss takes place during a stars late evolutionary phase. When a star nears the end of its life, its central region contains less fuel for hydrogen burning and the core region contracts until the temperature gets high enough for the nuclear fusion of helium to begin.
The structure of the star changes dramatically as hydrogen in the shell surrounding the core also begins burning. Depending on the stars initial mass on the main sequence (where a star spends most of its hydrogen-burning lifetime), it then enters an unstable period where variations in its temperature, radius and luminosity occur. This can result in structural changes in the star, such as the loss of its external layers.

For stars more than about eight times the mass of the Sun, the ultimate effect of these instabilities is a spectacular supernova explosion. Stars less than about eight solar masses enter a short phase (a few thousand years) that includes successive episodes of mass loss. This eventually leaves behind a stripped stellar core of carbon and oxygen mixed with a degenerate gas of electrons that determine the structure of the remnant. What’s left is called a white dwarf, an important end product of stellar evolution.

White dwarfs have an average diameter of about 10,000 kilometers, about the size of the Earth. However, their final masses are about half that of the Sun, which makes their density about a million times that of most common solid elements found on Earth. The properties of these stellar corpses are fascinating because of the curious nature of the degenerate electrons that provide the pressure to support them. The compressed electrons behave like a solid because of their high conductivity and incompressibility, but they are truly what is known as a degenerate gas.
This incompressible quality of white dwarfs has led some call them, "the largest diamonds in the universe"

The white dwarf phase of a star can last for billions of years, and during this time the object does not generate energy by thermonuclear reactions. The energy that radiates is sustained simply by cooling, just as a hot piece of iron metal emits radiation as it cools.

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