1996A&A...308L..29T

There is already some evidence of the dust formation in late M dwarfs. We show that observations of a transition object GD 165B can best be explained by a simple dusty model in which gas and dust coexist in a homogeneous mixture. The dusty atmosphere is highly opaque due to a large extinction of uniformly distributed dust, which produces noticeably observable effects in GD 165B. The spectrum of the genuine brown dwarf Gl 229B recently discovered, however, shows no evidence for dust. Instead, the observed broadband spectrum of Gl 229B between 0.8 and 10µm can be explained well by our dust-free model in which the formation of dust is artificially suppressed. We interpret this fact as evidence of the formation of dust clouds in cooler brown dwarfs. This cloudy atmosphere may be more transparent in the inter-cloud region than the gas-dust homogeneous atmosphere, so that the volatile gas such as CH₄ dominates the visible atmosphere. Thus limited observations now available suggest how dust forms in stellar through substellar photospheres; dust forms first as a gas-dust homogeneous mixture and then as a cloud.