Brown Dwarfs Come in All Sizes

[mikeemmert]

While the car was getting lubed this morning, I went into the public library. It was a different branch than the one I escaped the cold in during the homeless horror.<br /><br />But they did have the print version of Scientific American for January of 2006. They had a nice article on brown dwarfs by Subhanjoy Mohanty and Ray Jayawardhana, so I photocopied it. It's not online, to my knowlege, so the inevitable typos quoting it are my bad. I also changed the order of some paragraphs because I felt it was clearer that way, my bad.<font color="yellow"><br /><br />"What is a planet? It seems such a simple question, but the answer keeps getting more and more confused. On the one hand, the line between planets and lesser bodies is notoriously hazy...Less well known, though, is the muddle at the upper end of the planetary scale: the blurring of the divide between planets and stars...<br /><br />These are the brown dwarfs. They span a mass range of 12 to 75 Jupiters: too light to aattain the high central temperatures required to fuse ordinary hydrogen nuclei but heavy enough to fuse deuterium, a less common isotope of hydrogen. Newly formed brown dwarfs shine like feeble stars but quickly exhaust their deuterium supply...<br /><br />Do they form basically like planets or basically like stars? According to the most popular model, a gas giant planet stars with the gradual agglomeation of dusto debris inot larger and larger bodies. Once such an object reaches a few Earth masses in size, it undergoes ruanway growth...the disk gas also dissipates by either accreting onto the central star or getting blown<br /> out of the system altogether. So the amount of gas available to build a gas giant planet diminishes with time, limiting the heaviest object that can arise to a mass of about 10 to 15 Jupiters...<br /><br />...stars form within so-called molecular clouds; vast agglomerations of cold gas and dust, each with enough material to form scores of suns. Within such a cloud some reg</font>

 

[Philotas]

Most interesting; thanks for bringing it up.<br /><br />When it comes to definitions, it would be interesting to see if 'planetary' brown dwarfs overlap in mass with 'solar' brown dwarfs. We would need two different categories, or not even call both classes for brown dwarfs. <div class="Discussion_UserSignature"> </div>

 

[mikeemmert]

This might or might not be a nitpicking factor, but I think (I'm not sure) that beryllium-7 fuses easier than deuterium. But the supply of fuel in that case might range from a few days to a few millenia. That might have a significant effect on the evolution of these objects, though.

 

[mlorrey]

Another question is whether a brown dwarf could eventually pick up enough material to stellate. For instance, in our current investigation, imagine a relatively old brown dwarf orbiting another star, in and out of its oort cloud, gradually absorbing enough cometary material to either maintain a deuterium fusion process, or even stellate to plain hydrogen fusion as a red dwarf. I would expect sampling the red dwarf population may help in this: we should see a sub-group of red dwarfs that are small and have a very high metallicity compared to 'original' red dwarves. <br /><br />Of the un-parallaxed red dwarf population, this may help narrow the search down, by starting with the high metallicity small red dwarfs.

 

[mikeemmert]

I've wondered that too, but more in conjunction with brown dwarfs entering molecular clouds. I read somewhere (Asimov?) that passage though a molecular cloud should be expected about every two billion years for a typical star in our galaxy.<br /><br />Molecular clouds range from about 50,000 to 1,000,000 solar masses. But there are a few hundred much smaller cores within those. They are of varying densities. Due partly to the fact that I was editing and also the writers of the article wanted to keep it from being too long, it was not expressed above that cores are a part of the centers of molecular clouds. There's a complex process behind that of rotation and fragmentation.<br /><br />If stars or brown dwarfs can be ejected from these clouds, they can also be slowed down enough to capture significant mass, I think (I haven't modeled it). A star undergoing fusion wards off material from a molecular cloud by light pressure and stellar wind, this I got from elsewhere (print somewhere). But a dead brown dwarf should be able to pick up gas.<br /><br />There's probably not enough solid material, asteroids or KBO's or even Jupiters (certainly not in our solar system) to fire up a brown dwarf.