on Red Brown Dwarfs

Brown dwarfs are very small and dim objects made of hydrogen and helium. Often considered failed stars (or highly successful planets), they’re a bridge between stars (which fuse hydrogen into helium) and gas giant planets like Jupiter (which don’t).

Actually, because they’re defined as “objects smaller than stars that formed on their own in space”, brown dwarfs can be stranger than that.  Some are small enough to qualify as planets (if they were orbiting a star). Some are actually so cold they’re practically room temperature (300 Kelvin, or 80 F). Let that sink in: star formation in a vast nebula of gas and dust, producing an object so feeble it won’t even boil your coffee.

That’s why brown dwarfs are so neat to study: They have most (or all, as far as we know) of the properties of an extrasolar planet, without the pesky thermonuclear fireball right next to them. Sure, seeing a firefly in a gigantic field is tough, but it’s a lot easier when it’s not flitting around a bonfire.

This is what the BDNYC group has been working on. We like planets, but we want to know more than we can know right now.

Right now one of our main focuses is on red brown dwarfs. (Yes, that’s their actual name. Brown dwarfs give off most of their light in the infrared; RED brown dwarfs give off more light farther into the infrared (i.e. redder) than a brown dwarf with its properties ought to.


The prevailing theory is that it’s an extra layer of colder dust in the cloud layer. (Did I mention brown dwarfs have clouds? Well, Jupiter does, and the Sun doesn’t, so somewhere in there there’s gotta be a middling object that’s starlike but has clouds… and that’s another fun thing with brown dwarfs*). That dust layer eventually settles out of the atmosphere, falling down and letting us see deeper into the hotter part of the atmosphere. Thus, red brown dwarfs are younger than the normal brown dwarfs we’re used to, which is where my interest comes from.

One of the other members of the BDNYC group, Kay Hiranaka, has now managed to completely explain this extra far-infrared flux using clouds of mineral particles the size of cigarette smoke. It’s an impressive result, and she’s going to give more technical details in a presentation at the American Astronomical Society meeting next month.

I made this animation to illustrate her cloud of particles raining out of the atmosphere of a red L dwarf to accompany her thesis presentation. As you can see, once the particles are gone, the L dwarf is not quite as deep red as it once was.

Kay and I have tried to make the L dwarf look reasonably accurate, and the particle distribution is actually using the same distribution law as her theoretical calculations (which is one of her important innovations). The planet in orbit is just for show, though. The scene is so complex it took 3 times longer to render than my 25 parsec movie even though it’s a much shorter animation.

*The very coldest stars may actually have clouds.


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