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Accreting brown dwarfs

  1. Aug 21, 2009 #1
    Hi All

    A puzzle of the very distant future. Imagine a brown dwarf, about 0.02 solar masses and 0.1 solar radii. It's moving through the galaxy and has cooled to near ambient so it can accrete mass via its gravity, almost like a giant ramscoop. As it gains mass what does that do to its orbit? It experiences the ISM as a headwind of 15 km/s, thus accreting from about 390 times its frontal area. I'm guessing the brown dwarf would slowly spiral inwards towards the core, unless it is involved in a scattering interaction with another star and is flung out of the galaxy. Alternatively it could collide with another brown dwarf or white dwarf and briefly (on this timescale that is) form a new star. I'm wondering if it might accrete enough mass to eventually ignite as a low mass star by itself or would pycnonuclear reactions transform it into helium before then? Or would it end up in the central black hole? Brown dwarfs close to the ignition mass might do so before the other possibilities claim them. The characteristic time of stellar collisions is ~1022 years and scattering will undo the galaxy in 1019-1020.
  2. jcsd
  3. Aug 22, 2009 #2


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    A brown dwarf could accrete enough mass to spontaneously ignite under its own gravity. This could take a very long time given the low average density of the interstallar medium. Stellar collisions are, however, extraordinarily rare events.
  4. Aug 22, 2009 #3
    Collisions are rare. That's why their characteristic time is ~1022 years, thus in a galaxy of 100 billion stars they happen ~1 every 100 billion years. But over 1022 years it happens with a certain inevitability. However the Galaxy is likely to relax long before then in ~1019-1020 years. About 90% escapes and 10% end up adding to the central Black Hole.

    Did you see how M87's CBH is now estimated at a mass of 6.4 billion solars! That means it's 18.9 billion kilometres in radius and a radially infalling mass would reach the central singularity in ~27.5 hours after crossing the event horizon! If 10% of the Milky Way disk ends up in the CBH then it'll mass about the same... what a monster accretion disk it'll form.

    I do wonder what intelligent life could do with such monsters. Feeding a black hole extracts ~5.7% of the rest mass of the infalling material as energy - could intelligent life, living in the wreck of the Milkomeda Galaxy use the CBH as their Last Redoubt against the cosmic night?
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