Key words :
extrasolar planets,
spectrum
,extrasolar planets
,dq
,white dwarfs
,hélium
,atmosphère
,carbon
,hydrogen
Just Plain Weird: An Odd Star Proves to Be a New Type of Object
5 Dec, 2007 10:35 am
On the night after Thanksgiving two years ago, I was at the Keck Telescope, trying to identify some very blue objects in the sky. I was looking for white dwarfs, which are the compressed ashes left behind when a low mass star ends its life. The technique I use, spectroscopy, involves splitting its light up into component colors. Different types of astronomical objects have vastly different spectra, depending on the objects temperature, chemical composition, distance, and other physics. So, we can learn a lot from a spectrum.
When I returned to Arizona, I showed the spectrum to my friend and collaborator, Jim Liebert. Jim knows far more about white dwarfs than any person I know -- if this was a white dwarf, he'd be able to tell me. He stared at it for a while, rummaged through some published papers of his, looked at other white dwarf spectra he had, and finally announced that he thought he knew what it was.
The star was a white dwarf, but a very rare kind called a "hot DQ" -- the "D" stands for white Dwarf, and the "Q" stands for carbon ("C" was already taken for another type of star). Jim and collaborators had discovered about a dozen similar white dwarfs out of nearly ten thousand known white dwarfs from the Sloan Digital Sky Survey (SDSS)-- very rare, indeed.
Otherwise, we knew little about this star. As far as we knew, all white dwarfs are carbon and oxygen ash covered by a thin but opaque atmosphere made of helium and (usually, but not always), hydrogen. If there is any hydrogen present in my hot DQ at all, we'd have seen it, because hydrogen has very distinct signatures in spectra of white dwarfs. But helium is exceptionally transparent, and so can be hidden if you mix in just a little bit of some other element. So, Jim and I proposed that this star, and all the other hot DQ white dwarfs, had helium atmospheres with a tiny bit of carbon pollution -- maybe 1 carbon atom for every 100 or 1000 atoms of helium. But we couldn't do much else, because nobody had ever studied what happens to carbon in the extreme atmospheres of white dwarfs -- pressure millions of times that of Earth's atmosphere, and temperatures of 30,000 degrees.
So, we published a paper on our star, and went on, hoping somebody would be able to help us figure more out some day.
Around the same time, an astronomy PhD student in Montreal named Patrick Dufour was finishing up his doctoral dissertation. Part of his thesis was the atmospheres of cool DQ white dwarfs -- also white dwarfs with traces of carbon in their atmospheres, but "only" 5,000 to 12,000 degrees -- much cooler than my oddball white dwarf and Jim Liebert's collection. Patrick found that, for the cool DQ white dwarfs, the idea that these are helium atmospheres with a tiny amount carbon pollution is correct.
So, after getting his PhD, Patrick came to Arizona to work with Jim Liebert. Once there, Patrick turned his white dwarf atmosphere models to the hot DQ stars. The problem was, they didn't work. According to Patrick's models, all of the hot DQs should show spectroscopic signatures of helium, if they are just hotter versions of the cool DQs (as we all thought). But we don't see helium.
One day, Patrick, on a whim, tried an atmosphere that had no helium, only carbon. This seemed silly, because we knew that all white dwarfs have helium (and maybe hydrogen) in them. But the atmospheric models looked almost exactly like the spectra of the hot DQ white dwarfs. More work by Patrick confirmed this. The result: Hot DQ white dwarfs have carbon-dominated atmospheres.
Patrick's paper on his discovery is in November 22nd issue of Nature, one of the most prestigious scientific journals. A longer and more thorough paper will soon be publish in our astrophysics journals showing that all of the hot DQs fit this model.
So, the hot DQs are an entirely new type of star. But we still don't know where they come from. Some scientists have propose that stars eight or nine time the mass of the sun can make white dwarfs with cores made out of oxygen, magnesium and neon ash surrounded by an atmosphere of carbon, and then perhaps some helium and hydrogen further out. If so, hot DQs might come from stars that were not quite massive enough to explode as supernovae but were close to the still ill-defined limit. Unfortunately, there is no way to know what the initial mass the stars were when they were still "alive" from the analysis of hot DQs in the solar neighbourhood.
So, now my star comes back into the picture. My hot DQ white dwarf is in a cluster of stars (Messier 35), and is the only hot DQ known in a star cluster. This gives us some valuable information, because all of the stars in a star cluster formed at the same time out of the same clouds of gas. So, we can figure out what the (now dead) parent star was like, and for my hot DQ, its parent star had to be at least five times more massive than the sun, maybe much more. There are lots of other white dwarfs in the star cluster, too, that I am currently studying. Our hope is that, with a little more study, we may be able to learn more about where my hot DQ white dwarf came from.
So, by a little bit of sheer luck, my "weird" star may be the key to understanding an entirely new group of stars! And it may prove that white dwarfs, once again, will play an important role in deepening our understanding of stellar evolution since, as Sir Arthur Eddington pointed out 85 years ago, science often progress most rapidly when confronted with exceptions to its theories.
Reference
1. Dufour P, Liebert J, Fontaine G, Behara N. "White dwarf stars with carbon atmospheres", Nature 450, 22 November 2007. Abstract available here.
Key words :
extrasolar planets,
spectrum
,extrasolar planets
,dq
,white dwarfs
,hélium
,atmosphère
,carbon
,hydrogen
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