Why wouldn't a brown dwarf grow in radius if it started to gain mass?
Why do you think it wouldn't? Here on PF it is bad form to just make a bald statement like that without citing where you are getting your informaiton. Are there particular conditions under which it wouldn't and some under which it would?Why wouldn't a brown dwarf grow in radius if it started to gain mass?
Tritium is short-lived and rare in nature.Adding mass to a degenerate body would cause it to shrink only so far as nothing else changes. For a brown dwarf, that something else would be hydrogen fusion. Once that occurs, the shrinkage would cease and it would expand due to the heat. A brown dwarf can remain degenerate, but initiate tritum fusion,at around 60 Jupiter masses [Mj].
So which do they and do they not initiate?At a mass around 13 Mj deuterium fusion is believed possible. Brown dwarfs in the 60-90 Mj] qualify as L spectral class, hot enough to initiate hydrogen fusion, but enough to trigger lithium fusion.
That's probably an arguable question of classification. Some white dwarfs are known to have cooled to 4000 K, yet they are called white dwarfs rather than red dwarfs.Brown dwarfs in the 13-60 Mj range are capable of deuterium fusion, but, not hydrogen or even lithium fusion. They are considered spectral class T or Y, depending upon temperature. Once the temperature falls below about 1000K it is no longer considered a brown dwarf, just an ordinary planet.