Thursday, May 01, 2008

Our new discovery

A color picture of a carbon-atmosphere white dwarf Image Credit: Sloan Digital Sky Survey

The faint blue dot in the center of the picture above is a white dwarf, the ashes left behind after a star used up all of its nuclear fuel and completed its life. Tens of thousands of white dwarfs are known (and tens of billions exist in our galaxy), but this particular one is unique among the known white dwarfs.

About a year ago, a collaborator of mine discovered that a small fraction of white dwarfs have carbon atmospheres. Most white dwarfs have atmospheres composed mainly of hydrogen, and almost all of the rest have atmospheres made up mostly of helium (and by mostly, we mean more than 99% of the atoms). The reason for most white dwarfs having these two atmospheres is that these are the two lightest elements in the Universe, and the gravity of white dwarfs is so high that it can separate the atoms in its atmosphere in a matter of days or weeks. So, if there is even the tiniest amount of hydrogen in an atmosphere, it will rise to the top and be the only thing we can see from Earth.

So, carbon atmosphere white dwarfs (which make up only about one in a thousand white dwarfs) tell us that these white dwarfs have somehow lost virtually all of their hydrogen and helium. This is strange and hard to explain. In fact, none of the guesses for the origins of the atmospheres of these white dwarfs are really that well-developed, and these guesses are all lacking in evidence. What we really would like is to get inside the star, and see its internal structures. That would tell us a lot about what sort of processes went on in the star before it became a white dwarf.

Amazingly, there is a way to look inside a star -- it's called asteroseismology. Some stars can ring, like a bell, as sound waves move around and through the star. And just like geologists can study the interior of the Earth by examining the "ringing" of the Earth due to events like earthquakes, we can use the waves on stars to probe their inner regions. Helioseismology, or the study of sound waves in the sun, has told us a lot about the sun and confirmed some of our most basic theories of the structure of normal stars.

So, one day I was wondering if these carbon-atmosphere stars might be susceptible to this "ringing," (in white dwarfs we call this ringing pulsating) and therefore able to be studied by asteroseismology. Luckily, one of my friends and colleagues here at the University of Texas, Mike Montgomery, is an expert in the theory of white dwarf pulsations. So, I asked him if these stars would pulsate. He went off, ran some simple models, and decided that they might pulsate under specific conditions (depending on the star's temperature).

It isn't too hard to find pulsating stars. The waves in the star act as though the atmosphere is sloshing around the surface of the star. When the gas piles up, it heats up a bit, and where it has left, the star cools down a bit. Where the gas is hotter, the star glows a little brighter, and where it is cooler, the star is a little fainter. So, as we watch the star from Earth, we'll see brighter and fainter spots moving in and out of view, and the star will appear to get a little brighter at some times, and a little fainter at other times. This brightening and fading happens on time scales around a few minutes. So, if the star is bright enough to take a picture of every 15 to 30 seconds or so, we can see the star actually changing brightness.

Mike and I then went and asked for telescope time to look at some carbon-atmosphere white dwarfs. Only one white dwarf appeared to meet the right conditions to pulsate, and the rest looked like they should be quiet. This made for a great test that scientists talk a lot about: we have a theory to test (if the stars pulsate when their temperatures are in the range that Mike calculated), a test sample that includes "control" stars (those that shouldn't be pulsating), and a experimental procedure (to look at them with a telescope). So we were given telescope time in February. Both Mike and I were busy on the days we were given (I was in England at a conference), so we got a grad student, Steven DeGennaro, to go run the telescope for us. Steven spent the first few nights looking at our control stars (we didn't tell him which one was likely to vary). That was quite boring, because they didn't do anything. Hours upon end, they looked identical from one picture to the next. This is what we expected, but we wanted to be sure.

Then, however, Steven went to the star we thought might vary. Within about 20 minutes, it became clear that the star was getting brighter and fainter every 7 minutes! Steven had discovered the first every pulsating carbon-atmosphere white dwarf. Here is a colorful plot of the brightness of the star:

Light curve of the pulsating carbon-atmosphere white dwarf

You can see that the star is getting brighter and fainter in a fairly regular fashion. In fact, the star is ringing at just two pure tones one octave apart. If we could hear sound across space, the star would be singing strongly at a slightly out-of-tune E-flat 16 octaves below middle C, with a softer overtone one octave higher. Alas, in space, no one can hear you sing.

This was exciting -- Mike had made a prediction, we went and looked and confirmed the prediction! We then wrote up a short paper on our discovery, which appears in today's edition of the Astrophysical Journal Letters.

Alas, the star has a very boring name: SDSS J1426+5752 (that's its nickname; its real name has about twice as many numbers). The white dwarf is about 800 light-years away in the constellation Ursa Major (the "Big Dipper"). Unfortunately, you need a really big telescope to see our star, as it is 200,000 times fainter than the faintest star you can see with your unaided eye. (For those of you who speak "magnitudes," astronomy's measure of brightness, it is magnitude 19.2.)

The other types of white dwarfs (hydrogen-atmosphere and helium-atmosphere) have been known to pulsate for over 25 years, so this marks the first new type of pulsating white dwarf in 25 years. For that reason, we decided to put out a press release announcing our discovery. So, you may very well read about this in the newspaper. Maybe. Really, it is hard to tell in advance what people find interesting and what they don't find interesting, so maybe no newspaper will pick up this story. But we put it out there, in the hopes that someone may enjoy the tale.

Tomorrow, I'll try and talk about why we may be completely wrong. I don't think we are, but we have to admit that possibility for now.

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