Brown dwarfs don't have a high enough temperature to begin fusing hydrogen, but they can fuse deuterium or burn lithium, according to Wikipedia. Where do they get the deuterium from? Is there just enough of it about in any molecular cloud from which a brown dwarf might condense?
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The deuterium burned by brown dwarfs is mostly a remnant of the big bang. Big bang nucleosynthesis primarily created hydrogen-1 and helium-4. It also created small amounts of deuterium, tritium, and helium-3, and very small amounts beryllium-7. The tritium and beryllium-7 soon decayed to helium-3 and lithium-7.
David Hammen
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Most of the deuterium did come from the big bang. However some of it is produced as a by product. For example
$$p^+ + \gamma \to n^0 + e^+ +\nu $$
This induced reverse beta decay could create neutrons that can accelerate into other protons and create deuterium nuclei. This produces energy of roughly 2.2 Mev.
$$p^+ + n^0 \to d + 2.2\ mev $$
This is also a reaction which could create a very small amount of deuterium. On top of this some decay products could result in deuterium.
This heats up the brown dwarf maybe helping to create more deuterium creating reactions.
Roghan Arun
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