Friday, May 02, 2008

What if I'm wrong?

Yesterday I blogged about the new pulsating white dwarf my collaborators and I found. Or at least we think it is a pulsating white dwarf. We're pretty sure, but we aren't absolutely certain. How do we know that we may not know what we're looking at? And what else might this be? And why did we put out a press release if we might be wrong?

First, there are a couple of things we are pretty certain about. We know we are looking at a white dwarf, and we know that all we see of the white dwarf is carbon. We also know that the amount of light coming from the star is changing by a few percent every 7 minutes, and that variation is very repetitive and stable. We also know that theory predicts that this white dwarf should be varying, assuming we have its temperature and mass right. If this were all the information we had (and it is 90% of it!), there'd be no doubt in our minds that this is a pulsating white dwarf.

But there is one extra bit of information we have. As I said yesterday, the star is beating at two very precise frequencies, one octave apart. This is rare (but not unheard of) in pulsating white dwarfs, and it lets us average all of the few hundred individual pulsations we've observed into a single pulse. When you do that for other pulsating white dwarfs, you get something that looks like a shark fin: A shark-fin shaped pulsing white dwarf

The white dwarf rapidly gets brighter, gradually fades, and then stays faint or a bit before getting brighter again. Complicated but expected physics that I won't try to explain gets you this shape.

But for our pulsating white dwarf, the average pulse looks like this: The carbon atmosphere white dwarf pulse looks funny

To my eye, this looks more like the upside-down version of the normal pulse shape. In fact, at first we checked and re-checked our data to make sure that it wasn't backwards. But this is right, and it is totally unexpected.

There are objects in the universe that vary their light with this shape, but most of them we wouldn't mistake for our white dwarf. There is one type of object that worries us, though -- it's a star called AM Canes Venatici (which we give the nickname AM CVn, and pronounce it like "AM Can Van"). AM CVn is a big white dwarf that is slowly eating its neighbor star, which used to be a little white dwarf made of helium. When AM CVn was first discovered, it was thought to be a helium-atmosphere pulsating white dwarf. It took a lot of careful observations to learn the truth. And there are some eerie similarities between the story of the discovery of AM CVn and our discovery.

But, if our star is like AM CVn, it also must be different. Instead of a big white dwarf eating a little white dwarf made of helium, our star would have to be a big white dwarf eating an almost-as-big carbon white dwarf. Now, let's imagine that our white dwarfs were fish in the sea. A big fish has no problem eating a little fish, but two big fish usually don't try and eat each other. If they did, they'd get in a fight and rip each other apart.

In this case, the fish are white dwarfs, and the eating is done with gravity instead of teeth. And if our star were really two white dwarfs trying to eat each other, computers tell us that their mutual gravity would rip each other apart in a matter of seconds, and the debris would quickly fall back together into a single big white dwarf (or perhaps even explode as a supernova). And while the computer models may well be wrong, these same models get a lot of other things right, so we tend to trust them.

So, in short, we are pretty sure we have a pulsating white dwarf. We predicted it, looked for it, and found it. Nature would have to be pretty perverse to make our star yet another type of oddball object. Nature does sometimes play tricks on us (or, more often, we are a bit too clever for our own good and fool ourselves into believing something that is not true). And the ony evidence that this is not a pulsing white dwarf is the weird shape of the pulsing. It is possible to explain the weird pulsing within current theories, it's just that we've never seen it in a pulsing white dwarf before. I would estimate that 85% of the evidence says that this is a pulsing carbon-atmosphere white dwarf, and that's pretty good.

Why did we make a press release when we could be wrong? A couple of reasons. First, we are pretty sure we are right. I wouldn't bet the farm that we have found a pulsating white dwarf, but I'd be willing to bet a fair amount of money. Second, the other option -- two big white dwarfs trying to eat each other -- would be even more exciting. None of those are known to exist, and, as I said, they could explode as a supernova. Either way, we've found a very interesting object, and we wanted to share that excitement with people outside of our professional colleagues.

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