Wednesday, November 21, 2007

"Just plain weird" -- An odd star proves to be a new type of object

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 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.

I'd been working on this project for quite a while, and I could easily identify the spectra of every object we were looking at. Most were white dwarfs, some where quasars (black holes halfway across the universe scarfing down tremendous amounts of matter), and the rest were other types of stars in our Milky Way galaxy. But there was one object that mystified me -- I'd never seen a spectrum like it before. I spent a lot of time trying to identify the spectrum, before finally giving up and writing "Just plain weird" in the comment section of my observing logs.

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 -- it 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 (I still don't know why, other than "C" is taken for another type of star). Jim had discovered about a dozen similar white dwarfs out of nearly ten thousand known white dwarfs -- 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 50,000 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. His topic was the atmospheres of cool DQ white dwarfs -- also white dwarfs with traces of carbon in their atmospheres, but "only" 10,000 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 atmospheres.

Patrick's paper on his discovery is in tomorrow's issue of Nature, the most prestigious scientific journal. He's busy writing a longer and more thorough paper to publish in our astrophysics journals showing that all of the hot DQs fit this model.

To an astronomer, this is completely weird. The universe is full of hydrogen and helium; almost all objects are composed mostly of hydrogen and helium. Jupiter and Saturn are mostly made of hydrogen and helium, as are Neptune and Uranus. The Earth's gravity is too weak to hold on to hydrogen and helium, which is the only reason that the Earth and all the other rocky planets aren't mostly made of those elements.

So, the hot DQs are an entirely new type of star. But we still don't know where they come from. It may be that stars eight or nine time the mass of the sun can make white dwarfs with cores made out of oxygen and neon ash surrounded by an atmosphere of carbon, and then perhaps some helium and hydrogen further out. If this is right, why don't we see the hydrogen and helium? Or maybe these stars are normal helium-atmosphere white dwarfs that somehow re-start nuclear fusion of their atmospheres (since helium fuses to make carbon). This seems contrived.

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! But now the real work begins -- we need a lot more information and analysis. I'll be sure to let you know what we learn!

To read press stories on Patrick Dufour's discovery, try this Reuters story and's article. Just to make it clear, my hot DQ star is not covered in Patrick's first couple of articles; we need a little more data to understand them first.

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