Astro Seminar: Brown dwarf basics

08Feb07

The first meeting of 8.972, my brown dwarf/exoplanet seminar, was today. It’s jointly taught by Profs. Adam Burgasser and Josh Winn, split roughly so that Adam takes the dwarfs and Josh takes the planets. The class is technically a graduate seminar, but about half the students are undergrads. I know a handful of them: a few astro geeks, a few grad students of profs I know, a few fans of Josh or Adam. And Anna, my new partner in Junior Lab.

There are, Josh tells us, a couple of objectives to this seminar. One, obviously, is to give us a fairly detailed understanding of a subfield of astrophysics. We should someday be able to scan astro-ph for brown dwarf or exoplanet papers and read them and know what’s going on. But the other objective has more to do with training us in the Ways of Physicists. To that end, we’re expected to discuss things and review literature and finally produce a nice little term paper on some relevant topic. And Josh and Adam will hammer us into shape.

Now, world, I can’t help you with the how-to-be-a-physicist thing, because I don’t have any extra academia cred. But if you’re interested, I’ll give you a writeup of the lectures and discussions in seminar. Today’s program: fiddling with the projector.. followed by an overview of the historical and physical features of each kind of not-really-a-star. Good stuff! Okay, world, here we go:

First up: Adam & the [500] Dwarfs.

Stars are classified by temperature, on a scale that runs O-B-A-F-G-K-M-L-T, highest to lowest. The Sun, for instance, is a type G star. The L’s and T’s are the brown dwarfs; they’re the very coldest variety of starry thing. Brown dwarfs are formed like stars, but differ in that they can’t fuse hydrogen in their cores. Like stars, they’re self-luminous, and can be found in isolation. They exhibit other characteristically “stellar” behaviors, too, such as circumstellar disks and binary companions. On the other hand, they have low-temperature atmospheres, like planets; they have clouds, like planets; their masses are on the order of 10 Jupiters, and their radius only around 1 Jupiter. So they’re sneaky, double-crossing little beasts. They’re like teenagers, or mermaids, or the goop you make out of cornstarch and water: not really one thing and not really the other.

So the big question is: What separates a planet from a little tiny star? And that is what we’ll be wondering allll semester.

Anyway, here’s another question: Why are these things cool? Well, for starters, there seem to be a damn lot of them, and they have been there all this time, and NOBODY KNEW. Says Adam, what if we suddenly found a hidden species of people, 6 billion strong, living right alongside us? When you put it like that, yeah, it’s a little creepy. On a more practical note, though, brown dwarfs are useful sources of data on low-temperature atmospheres, cloud behavior, EXXXTREME climatology…all of those planet-type features. In the aggregate, they tell us about galactic processes: What kind of conditions did they need to form? How are they distributed throughout the galaxy? What do they do to galactic dynamics? Again, answers may or may not arrive in the coming weeks. But first, here’s a short historical overview.

Back in the day a.k.a. 1963, some people started batting around a conjecture. They noticed that there are some fairly strict lower bounds on the temperature and pressure you’d need to fuse hydrogen nuclei in the center of a star. A star like the Sun is massive enough that gravity pulls it strongly together and maintains that pressure. It doesn’t keep collapsing forever, though, because at some point the inward force of gravity is balanced by the outward push of all that fusion – it’s a bit like a well-behaved explosion. But there’s nothing stopping the formation of an arbitrarily smaller star. If a star doesn’t start life with much mass at all, it never gets to the point where fusion can take over. It still doesn’t collapse forever, because eventually the electrons just refuse to get any closer to each other. So we’re left with this ball of STUFF that’s not very good at glowing. And as time goes on, they just become colder, and fainter…and colder…and fainter…

At this point in history, everyone started saying, “ZOMG these things are dark and massive and that means they are DARK MATTER.” And because they couldn’t go around calling them “things”, a bunch of names were proposed – planetar, black dwarf, failed star, substar – but none of them really caught on. Finally, at some point, Dr. Jill Tarter, who is now famous for being an important SETI person, said, “We don’t know what color they are, so let’s call them brown dwarfs.” I don’t understand the logic there, but the name stuck. Later, when people actually found the things and gave them a spectroscopic once-over, they turned out to be PURPLE. (Wait, if hot stars are white and HOTT stars are blue, why are cold stars purple? It’s just an artifact of absorption features, that’s why.)

In 1985, people started to actually observe the things…or so they thought. The first one found was called VB 8B, and caused a big ruckus and an entire conference, but turned out not to exist. In 1988, another funny object was found.It was a companion to a white dwarf, but it had a bizarre spectrum, and nobody really knew what to do with it. And so on, and so forth. The big year was 1995, when people found Gliese 229B. Its spectrum showed that it was clearly not a planet, and other people confirmed that it was actually there, and a proud article was sent off to Nature. Somehow, though, that issue of Nature has a goofball octopus on the cover. Come on, Nature, a brown dwarf can totally take an octopus!

After that, people started finding brown dwarfs with relative ease. The problem wasn’t really with the observers, but with the technology necessary to detect in the near-IR and IR band. Today, there are about 500 known brown dwarfs. Here are some other relevant numbers:

• Maximum Mass: 0.07 Sun-masses, or 75 Jupiter-masses;
• Radius: 0.1 Sun-radii;
• Minimum Luminosity: 10^-6 Sun-luminosities;
• Lowest Temperature: 700 K;
• Central Pressure: 10^11 bars;
• Spectral Classes: 3;
• Ratio of # BDs to # Stars: 1;
• Percent of Dark Matter they make up: less than 2.

This is a looong post. I’m going to split it here and put the exoplanet stuff on its own.

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