The Sciences

Brown dwarf T party

Bad AstronomyBy Phil PlaitSep 20, 2006 2:14 AM


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Astronomers using the Spitzer Space Telescope have found a couple of brown dwarfs orbiting nearby stars. While brown dwarfs around other stars are seen fairly often, these ones are nifty for several reasons. One of them, HN Peg B, is interesting because it's fairly low mass, about 20 times Jupiter's mass. This makes it what's called a T dwarf. Those of you who've taken Astronomy 101 may remember that stars are classified according to their spectra, which corresponds roughly to their mass and temperature. The hottest, most massive stars are O stars, then, going lower mass and cooler, they are B, A, F, G, K, M. M stars are the lowest mass stars that can fuse hydrogen into helium into their cores. But brown dwarfs are cooler and lower mass. They get added to the end of the list above, as L and T dwarfs. T dwarfs are so cool that they can actually have methane in their atmospheres (in hotter stars, methane breaks down). HN Peg B orbits a star about 60 light years away, relatively close by as these things go. Since it's close, getting an image of it is easier (it appears brighter, and farther away from its parent star). The parent star is itself interesting, in that it's a solar analogue, a star like the Sun. For those keeping track at home, it's a G0, or slightly more massive and hotter than the Sun. No T dwarf has ever been seen orbiting a star like this. It's probably common (G stars make up about 10% of all stars, and some are bound to have brown dwarf companions), but T dwarfs are so faint compared to their parent stars that they are really hard to detect. This one can be seen in images because T dwarfs put out most of their light in the infrared, where G stars are relatively dim. That increases the contrast and makes the T dwarf easier to see. But the very neatest thing about the T dwarf is that it's young: HN Peg A, as it's called, is probably only about 300 million years old (this can be found by looking at a number of characteristics, including how fast it spins (younger stars tend to spin faster), how much lithium it has (lithium is destroyed easily in stars, so more lithium means the star is younger), magnetic activity (tied to spin, so young stars have stronger magnetic fields), and other properties). Assuming that HN Peg B formed together with A, which is likely, that means the brown dwarf is young, too. All other T dwarfs ever seen are much older, like a billion years or more, so seeing one that's this young is cool. Astronomers can learn more about T dwarfs! Learning more is always good. Always. The other T dwarf was found orbiting the star HD 3651 A, which is about 35 light years away. This one is cool because a planet has already been detected orbiting HD 3651 A. It's a Saturn-mass planet orbiting the star about as far out as Mercury orbits our Sun. The orbit of the planet is known to be highly elliptical, which is unusual for such a close-orbiting planet -- tides from the star tend to make the planets' orbits circular. It's been thought for a while that a massive object farther out might keep the inner planet orbit elliptical, and this is now confirmed. The T dwarf HD 3651 B orbits the star about 10 times farther out than Pluto orbits the Sun, which is a fair walk, but close enough that it can warp the inner planet's orbit. I studied brown dwarfs for a few years, and I'm pretty interested in them. I suppose I'd call myself an educated layman when it comes to these guys (as opposed to real experts like the ones who ran these studies), so I'm not fully up on what's what. But it's nice to see the field making so much progress, and as usual I'm pretty eager to find out what will happen next!

Spitzer Space Telescope images of the two T dwarfs: HD 3651 B on the left, and HN Peg B on the right. Those pictures may look a lot alike, but they are indeed of two different stars; compare the background stars.

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