Did GD61 Eat a Planetessimal?

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The primary method by which astronomers hope to study exoplanet atmospheres is by detecting their absorption spectra as they transit their parent stars. However, another way would be to detect the signal of the atmospheric components in the atmosphere of a star that recently cannibalized a planet or other large body. White dwarfs offer an excellent class of stars on which to use this method since convection will pull heavy elements down more rapidly, leaving surfaces with near pristine hydrogen and helium photospheres. The presence of other elements would indicate recent accretion. This method has been used on several white dwarfs previously, but a new study reexamines data from a 2008 paper, adding their own data on the white dwarf GD61 to propose that the star isn’t just eating dust and small bodies, but a sizable one, likely containing water.

Data for the project were taken in 2009 using the SPITZER telescope. One of the first clues to the presence of a recent case of cannibalism was the presence of warm dust within the Roche limit of the star. This disc did not extend more than 26 stellar radii from the star, leading the team to suspect that this was not simply a large scale disc feeding the star with rocky materials, but an object that had fallen inwards to be tidally torn apart.

To support this, the new team used the Keck I telescope on Mauna Kea with the HIRES spectograph to analyze the spectrum. The findings from this confirmed the previous study that, in order of decreasing abundance, the star contained helium, hydrogen, oxygen, silicon, and iron. Based on the amount of material present in the spectrum and estimated convection rates for such stars, the team concluded that, if the disc were created by a single body, it would have been an asteroid at least 100 km in diameter. So why should the team expect that it was a single body as opposed to many smaller ones?

The key lies in the relative amount of detected elements. For GD61, oxygen was the most abundant element not typically present in white dwarf atmospheres. In fact, its presence far outweighed the other elements such that, even if all of it had been previously bound to the silicon, iron, carbon, and other trace elements, there would still be an inexplicable excess. This oxygen would necessarily have been combined into some molecule or have dissipated during the red giant phase. The only way the team could account for its presence would be to have it wrapped up in water (H2O) which, after disassociation, would allow the hydrogen to blend in the the expected hydrogen already present. Since water readily sublimates without sufficient pressures, the team notes that a large number of small bodies would be unable to bury the water deep enough to keep it from escaping previously, that the best explanation would be a large body which could shield water inside it during the previous red giant phase.

The evidence of water rich asteroids speaks to the formation of our own solar system because it provides a delivery mechanism for water to our planet beyond direct accretion. Water rich asteroids and comets would likely have supplemented our supply. Indeed, Ceres, the largest known asteroid in our solar system, is suspected to harbor as much as 25% of its mass in water.

Symbiotic Variable Star On the Verge of an Eruption?

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November 23rd, astronomers from the Asiago Novae and Symbiotic Stars collaboration announced recent changes in the symbiotic variable star, AX Persei, could indicate the onset of a rare eruption of this system. The last major eruption took place between 1988 and1992. In the (northern hemisphere) spring of 2009, AX Per underwent a short outburst that was the first time since 1992 this star had experienced a bright phase. Now AX Per is on the rise again. This has tempted astronomers to speculate that another major eruption could be in the making. 

Symbiotic variable stars are binary systems whose members are a hot compact white dwarf in a wide orbit around a cool giant star. The orbital periods of symbiotic variables are between 100 and 2000 days. Unlike dwarf novae, compact binaries whose periods are measured in hours, where mass is transferred directly via an accretion disk around the white dwarf, siphoned directly from the surface of the secondary, in symbiotic variables the pair orbit each other far enough away that the mass exchanged between them comes from the strong stellar wind blowing off the red giant. Both stars reside within a shared cloud of gas and dust called a common envelope.

When astronomers look at the spectra of these systems they see a very complex picture. They see the spectra of a hot compact object superimposed on the spectra of a cool giant star tangled up with the spectrum of the common envelope. The term “symbiotic” was coined in 1941 to describe stars with this combined spectrum.

Typically, these systems will remain quiescent or undergo slow, irregular changes in brightness for years at a time. Only occasionally do they undergo large outbursts of several magnitudes. These outbursts are believed to be caused either by abrupt changes in the accretion flow of gas onto the primary, or by the onset of thermonuclear burning of the material piled up on the surface of the white dwarf. Whatever the cause, these major eruptions are rare and unpredictable.

The AAVSO light curve of AX Persei from 1970 to November 2010. In the middle is the eruption of 1988-1992. The precursor outburst is the sudden narrow brightening left of the larger eruption. To the right of the light curve you can see the 2009 brightening event. Is this a precursor to a coming major eruption? Credit: AAVSO

AX Per underwent a short-duration flare about one year before the onset of the major 1988-1992 outburst. Now astronomers are tempted to speculate. Could the 2009 short outburst be a similar precursor type event? The present rise in brightness by AX Per might be the onset of a major outburst event similar to that in 1988-1992. The watch begins now, and professional and amateur variable star observers will be keeping a close eye on AX Per in the coming months.

Ranging from 8.5 to 13th magnitude, AX Persei is visible to anyone with an 8-inch telescope, and if it erupts to maximum it will be visible in binoculars. You can monitor this interesting star and report your observations to the American Association of Variable Star Observers (AAVSO). Charts with comparison stars of known brightness can be plotted and printed using the AAVSO’s Variable Star Chart Plotter, VSP.

The AAVSO comparison star chart for AX Persei

Do Puny White Dwarfs Make Wimpy Supernovae?

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Based on results from a radial velocity survey, Warren Brown, (Smithsonian Astrophysical Observatory) and his team have placed a few more pieces into the supernova puzzle.

Supernovae come in many flavors. There are Type Ia, the “standard candles” everyone has heard of; and there are Type Ib and Ic, which also involve binary systems. We also have Type II supernovae that are believed to be the core collapse of single, super-massive stars. There are also super-luminous supernovae, which may be the explosive conversion of a neutron star into a quark star, and finally the weak-kneed cousins of the bunch, the under-performing underluminous supernovae.

Underluminous supernovae are a rare type of supernova explosion 10–100 times less luminous than a normal SN Type Ia and eject only 20% as much matter. Brown and his team have been investigating the connection between underluminous supernovae and merging pairs of white dwarfs.

In the 1980s, on the basis of our theoretical understanding of stellar and binary evolution it was predicted that many close double white dwarfs would exist. However, it was not until 1988 that the first one was actually discovered.

The way to find close double white dwarfs is to take high resolution spectra of the H-alpha absorption line of a white dwarf at several different times and look for variation that is caused by the orbital motion of the white dwarf around an unseen (dimmer) companion. The first systematic searches were not very unsuccessful. Only one system was found. Then, during the 1990s, Tom Marsh and collaborators concentrated their search on low-mass white dwarfs, which, based on current theories, could _only_ be formed in a binary system. In this way a dozen more systems were found.

Extremely low mass (ELM) white dwarfs (WDs) with less than 0.3 solar masses are the remnants of stars that never ignited helium in their cores. The Universe is not old enough to have produce ELM WDs by single star evolution. Therefore, ELM WDs must undergo significant mass loss sometime in their evolution. Producing WDs with 0.2 solar masses most likely requires compact binary systems.

“These white dwarfs have gone through a dramatic weight loss program,” said Carlos Allende Prieto, an astronomer at the Instituto de Astrofisica de Canarias in Spain and a co-author of the study. “These stars are in such close orbits that tidal forces, like those swaying the oceans on Earth, led to huge mass losses.”

Observational data for ELM WDs is pretty hard to come by because of their rarity. For example, of the 9316 WDs identified in the Sloan Digital Sky Survey, less than 0.2% have masses below 0.3 solar.

Half of the pairs discovered by Brown and collaborators are merging and might explode as supernovae in 100 million years or more.

“We have tripled the number of known, merging white-dwarf systems,” said Smithsonian astronomer and co-author Mukremin Kilic. “Now, we can begin to understand how these systems form and what they may become in the near future.” Unlike normal white dwarfs made of carbon and oxygen, these are made almost entirely of helium.

“The rate at which our white dwarfs are merging is the same as the rate of under-luminous supernovae – about one every 2,000 years,” explained Brown. “While we can’t know for sure whether our merging white dwarfs will explode as under-luminous supernovae, the fact that the rates are the same is highly suggestive.”

At least 25% of these ELM WDs belong to the old thick disk and halo components of the Milky Way. This helps astronomers know where to look for underluminous SNe and where they are unlikely to find them, if the models are correct. If merging ELM WD systems are the progenitors of underluminous SNe, the next generation of surveys such as the Palomar Transient Factory, Pan-STARRS, Skymapper, and the Large Synoptic Survey Telescope should find them amongst the older populations of stars in both elliptical and spiral galaxies.

The papers announcing their find are available online at: http://arxiv.org/abs/1011.3047 and http://arxiv.org/abs/1011.3050.

Mitch’s Mystery Star, Curiouser and Curiouser

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“The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, ‘hmm… that’s funny…'” (Isaac Asimov)

A few short years ago, Zooite Hanny van Arkel discovered Hanny’s Voorwerp in an SDSS image of a galaxy (“What’s the blue stuff below? Anyone?”), and a new term entered astronomers’ lexicon (“voorwerpje”).

Very late last year, Zooite mitch too had a ‘that’s funny…’ moment, over a spectrum (yes, you read that right, a spectrum!).

Now neither Hanny nor mitch have PhDs in astronomy …

Mitch's Mystery Star (SDSS, Galaxy Zoo)


But I digress; what, exactly, did mitch discover? Judge for yourself; here’s the spectrum of the star in question (it goes by the instantly recognizable name 587739406764540066):
Spectrum of Mitch's Mysterious Star (SDSS)

“I asked a couple of white-dwarf aficionados, and neither recalls seeing any star with these features (nor does Jim Kaler, who wrote the book on stellar spectra),” Bill Keel, a Zooite Astronomer known as NGC3314 wrote, kicking off a flurry of forum posts, and a most interesting discussion!

“Can we rule out something along the line of sight, possibly a cold molecular cloud?” EigenState wrote; “If both stars are moving SE (towards the bottom left corner), could Mitch’s star (square) be affected by debris in the trail of the bright red star (triangle)? I am thinking of the trail left by Mira. So the spectrum would be white dwarf shining through cooled red star debris?” said Budgieye. NGC3314 continued “It can’t be like our current Oort Cloud since we don’t see local absorption from our own in front of lots of stars near the ecliptic plane. To show up this strongly, it would then have to be either much denser or physical much smaller. This just in – this may be the most extreme known example of a DZ white dwarf, which have surface metals. White dwarfs aren’t supposed to, because their intense surface gravity will generally sort atmospheric atoms by density, so this has been suggested (with some theoretical backing) to result from accretion either from circumstellar or interstellar material (so it could be at the star’s surface but representing material formerly in a surrounding disk). Watch this space…”

Then, two weeks after mitch’s discovery, Patrick Dufour, of the Université de Montréal, joined in “Hi everyone, I have known this objects for many years. I have done fits almost 5 years ago but just never took time to publish it. Will do it in the next few weeks. Meanwhile, enjoy this preliminary analysis… The abundances are very similar to G165-7, the magnetic DZ, but it’s a bit cooler (explaining the strength of the features).” Patrick, as you might have guessed from this, is an astronomer with specific expertise in white dwarfs; in fact the abstract to his PhD thesis begins with these words “The goal of this thesis is to accurately determine the atmospheric parameters of a large sample of cool helium-rich white dwarfs in order to improve our understanding of the spectral evolution of these objects. Specifically, we study stars showing traces of carbon (DQ spectral type) and metals (DZ spectral type) in their optical spectrum.”

Somehow yet another astronomer, Fergal Mullally heard about mitch’s mystery star and joined in too “Many other WDs with strong metal absorption lines are surrounded by a cloud of accretable material. This makes sense because the metals quickly sink below the surface (as mentioned by NGC3314). In some cases, metals are only visible for a few weeks before they are sink too deep to be seen. The disks are exciting, not only because they can be so young, but their composition suggests we might be looking at the remains of an asteroid belt (see http://arxiv.org/abs/0708.0198).” To which Patrick added “Mitch’s Mystery Star is a cool (~4000-5000 K) helium rich white dwarf with traces of metals (abundances similar to G165-7). The metals most probably originate from a tidally disrupted asteroid or minor planet that formed a disk around the star.”

So, mitch’s mystery star is just a rather weird kind of DZ star, and DZs are just unusual white dwarfs?

Yes … and no. “The asymmetrical line near 5000 is almost certainly MgH. As for the one at 6100, I am open to suggestion. I have never seen it anywhere else. For G165-7, the splitting is Zeeman. But the broadening is van der Waals by neutral helium. No splitting is observed in this star (and I took a really good spectra at MMT a few years ago to be sure).” Patrick again; so what is the mysterious asymmetrical line at 6100 Å?

Two more weeks passed, and a possible reason for Fergal’s interest emerges, in a post by NGC3314 “While we wait to see how Patrick’s new calculation shakes out, here’s an interesting new manuscript he was involved with, that points to likewise interesting things about the DZ stars. [] Wow. White-dwarf spectra as tombstones for planetary systems… wonder how the system stayed close enough to end up on the white-dwarf atmosphere all through the red-giant phases? The binary systems we can see look awfully far apart to have had helpful dynamical effects for this.” (in case you didn’t read up on Fergal, he’s very keen on exoplanets and ET).

Curiouser and curiouser

Then, in February, a tweet: “At campus observatory, seeing whether we can measure orbital motion between Mitch’s star and its K-dwarf companion.” The tale is becoming curiouser and curiouser (exoplanets in binary star systems? If life had evolved on a planet in orbit around the star which later went red giant then white dwarf, could it have somehow survived and landed on a planet in orbit around the K-dwarf companion?)

I’ll let NGC3314 have the final word: “This furnishes one more example of how the wide interest in Galaxy Zoo allows things once unthinkable – during the SDSS, the whole analysis plan never conceived that every bright galaxy in the survey, and every one of the million or so spectra would actually be examined by a human being.”

Oh, and the Asimov quote seems to be an urban myth (if any reader knows when, and where, Asimov actually said, or wrote, those words …).

Source: Galaxy Zoo Forum thread Mitch’s Mystery Star
Full caption for image at the top of this article (Credit: Bill Keel):
I had a look with the SARA 1m telescope in BVR filters last week, to check for obvious variability. Pending more exact measurements, it’s about as bright as it was in the SDSS images and the older Palomar plates. As SIMBAD shows, this is known as a star of fairly high proper motion (and that’s about all). You can see this when I register the original red-light Palomar photograph to the image from last week, a time span of almost 59 years. The attached picture compares red-light data from the original Palomar Schmidt sky survey in early 1951, the second-epoch Palomar survey around 1990, and SARA on Jan. 7, 2010. You can also see that the bright red star to the southeast has almost exactly the same (large) proper motion.

Supernova Simulations Point to White Dwarf Mergers

Type Ia supernovae, some of the most violent and luminous explosions in the Universe, have become a handy tool for astronomers to measure the size and expansion of the Universe itself. Because they explode with a rather specific peak luminosity, they can be used as “standard candles” to measure distances. New research presented at the American Astronomical Society meeting this week points to the increased likelihood that the mergers of the stars that create these explosions, white dwarfs, is more likely than previously thought, and could explain the properties of some Type Ia supernovae that are curiously less luminous than expected.

Research presented by Rüdiger Pakmor et al. from the Max-Planck Institute for Astrophysics in Garching, Germany simulated the merger of two white dwarfs in a binary system, and showed that these simulations match previously observed supernovae with odd characteristics, specifically that of 1991bg. That supernova, and others observed since, was curiously less luminous than should have been expected if it were a Type Ia supernovae.

Type Ia supernovae occur when there are two stars orbiting each other in a binary system. In one scenario, one of the stars becomes a white dwarf, a small but very, very dense star, and steals matter from the other, pushing itself over the Chandrasekhar limit – 1.4 times the mass of the Sun – and undergoing a thermonuclear explosion.

Another cause for these types of supernovae could be the merger of both the stars in the system. In the scenario analyzed by these researchers, both stars were white dwarfs of masses just under that of the Sun: .83-0.9 solar masses.

The researchers showed that as the system loses energy due to the emission of gravitational waves, the two white dwarfs approach each other. As they merge, part of the material in one of the stars crashes into the other and heats up the carbon and oxygen, creating a thermonuclear explosion seen in Type Ia supernovae.

You can watch an animation of the simulated merger courtesy of the Max-Planck Institute’s Supernova Research Group right here.

Observations of supernovae like 1991bg show them to burn a smaller amount of nickel 56, about 0.1 solar masses, than regular Type Ia supernovae, which typically burn 0.4-0.9 solar masses of nickel. This makes them less luminous, because the radiative decay of the nickel is one of the phenomenon that gives the luminous display of Type Ia supernovae its punch.

“With our detailed explosion simulations, we could predict observables that indeed closely match actual observations of Type Ia supernovae,” said Friedrich Röpke, a co-author of the paper.

Their simulations show that when the two white dwarfs merge, the density of the system is less than in typical Type Ia supernovae, and thus less nickel is produced. The researchers note in their paper that these types of white dwarf mergers could comprise between 2-11 percent of the Type Ia supernovae observed.

Understanding the mechanisms that create these fantastic explosions is a necessary step in getting a handle on both the extent of our Universe and its expansion, as well as the diversity of Type Ia supernovae themselves.

If you would like to learn more about their research and the details of their computer modeling, the paper is available on Arxiv here. Their results will also be published in the January 7, 2010 edition of Nature.

Source: AAS press release, Arxiv paper

Unusual Massive White Dwarf Stars Have Oxygen Atmospheres

White dwarfs are strange stars, but researchers recently discovered two of the strangest yet. However, these two oddballs are a missing link of sorts, between massive stars that end their lives as supernovae and small to medium sized stars that become white dwarfs. Somehow, these two once-massive stars avoided the core collapse of a supernova, and are the only two white dwarfs known to have oxygen-rich atmospheres. These so-called massive white dwarfs have been predicted, but never before observed.

The stars, named SDSS 0922+2928 and SDSS 1102+2054 are 400 and 220 light years from Earth. They are both remnants of massive stars that are at the end of their stellar evolution having consumed all the material they had available for nuclear fusion.

The low levels of carbon visible in their spectra indicate the stars have shed part of their outer layers and burned the carbon contained in their cores.

“These surface abundances of oxygen imply that these are white dwarfs displaying their bare oxygen-neon cores, and that they may have descended from the most massive progenitors stars in that class,” said astrophysicist Dr. Boris Gänsicke from the University of Warwick, lead author on a paper appearing in this week’s edition of Science Express.

Dr. Boris Gänsicke from the department of physics at the University of Warwick.
Dr. Boris Gänsicke from the department of physics at the University of Warwick.

Gänsicke told Universe Today that he and his team didn’t start out specifically looking for these previously theoretical stars. “I’ve been working with our research student Jonathan Girven on several projects on white dwarfs, and we came across a range of unusual looking objects — some we are still puzzling what they are. From a theoretical perspective, I was wondering if white dwarfs with oxygen-rich atmospheres exist, and combining both angles, we developed a specific search for these stars.”

In a search of Sloan Digital Sky Survey data, the astrophysicists did indeed discover two white dwarfs with large atmospheric oxygen abundances.

Almost all white dwarfs have hydrogen and/or helium envelopes that, while low in mass, are sufficiently thick to shield the core from direct view. Theoretical models predicted that if stars around 7 – 10 times the mass of our own Sun don’t end their lives as supernovae, the other option is that they will consume all of their hydrogen, helium and carbon, and end their lives as white dwarfs with very oxygen-rich cores.

Astrophysicists could then detect an extremely oxygen-rich spectrum from the surface of the white dwarf.
Most stellar models producing white dwarfs with such oxygen and neon cores also predict that a sufficiently thick carbon-rich layer should surround the core and avoid upward diffusion of large amounts of oxygen.

However, calculations also show that the thickness of this layer decreases the closer the progenitor star is to upper mass limit for stars ending their lives as white dwarfs. Hence one possibility for the formation of SDSS 0922+2928 and SDSS 1102+2054 is that they descended from the most massive stars avoiding core-collapse, in which case they would be expected to be very massive themselves. However current data is insufficient to provide any unambiguous measure of the masses of these two unusual stars.

What is the future for these massive white dwarfs? Gänsicke said the two stars will evolve very slowly. “Given that they are burnt-out stellar cores that do no longer undergo nuclear fusion, their destiny is to continue cooling and fading. This will be a very slow process, and any noticeable change in their appearance will take 10s to 100s million years.”

Lead image caption: Sloan Digital Sky Survey spectroscopy of this inconspicuous blue object — SDSS1102+2054 — reveals it to be an extremely rare stellar remnant: a white dwarf with an oxygen-rich atmosphere

Sources: Science, email interview with Gänsicke

Chandrasekhar Limit

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When a human puts on too much weight, there is an increased risk of heart attack; when a white dwarf star puts on too much weight (i.e. adds mass), there is the mother of all fatal heart attacks, a supernova explosion. The greatest mass a white dwarf star can have before it goes supernova is called the Chandrasekhar limit, after astrophysicist Subrahmanyan Chandrasekhar, who worked it out in the 1930s. Its value is approx 1.4 sols, or 1.4 times the mass of our Sun (the exact value depends somewhat on the white dwarf’s composition how fast it’s spinning, etc).

White dwarfs are the end of the road for most stars; once they have used up all their available hydrogen ‘fuel’, low mass stars shed their outermost shells to form planetary nebulae, leaving a high density core of carbon, oxygen, and nitrogen (that’s a summary, it’s actually a bit more complicated). The star can’t collapse further because of electron degeneracy pressure, a quantum effect that comes from the fact that electrons are fermions (technically, only two fermions can occupy a given energy state, one spin up and one spin down).

So what happens in the core of a massive star, one whose core weighs in at more than 1.4 sols? As long as the star is still ‘burning’ nuclear fuel – helium, then carbon etc, then neon, then … – the core will not collapse because it is very hot (electron degeneracy pressure won’t hold it up ’cause it’s too massive). But once the core gets to iron, no more burning is possible, and the core will collapse, spectacularly, producing a core collapse supernova.

There is a way a white dwarf can go out with a bang rather than a whimper; by getting a little help from a friend. If the white dwarf has a close binary companion, and if that companion is a giant star, some of the hydrogen in its outer shell may end up on the white dwarf’s surface (there are several ways this can happen). The white dwarf thus adds mass, and every so often the thin hydrogen envelope blows up, and we see a nova. One day, though, the extra mass may put it over the limit, the Chandrasekhar limit … the temperature in its center gets high enough that the carbon ‘ignites’, the ‘flame’ spreads throughout the star, and it becomes a special kind of supernova, a Ia supernova.

For more technical details of the Chandrasekhar limit, Richard Fitzpatrick of the University of Texas at Austin has an online Thermodynamics & Statistical Mechanics course, which includes a page on the Chandrasekhar limit.

Supernovae are very important to astronomy, so you won’t be surprised to learn that there are lots of Universe Today stories on the Chandrasekhar limit! Some examples: White Dwarf Theories Get More Proof, White Dwarf “Close” to Exploding as Supernova, and Colliding White Dwarfs Caused a Powerful Supernova.

Astronomy Cast Episode 90 (The Scientific Method) includes a look at how Chandrasekhar worked out the limit that now bears his name, and Where Do Stars Go When They Die? also covers this topic.

References:
Wikipedia
http://www.bluffton.edu/~bergerd/NSC_111/stars.html