Dust in the Photospheric Environment: Unified Cloudy Models of M, L, and T Dwarfs

We address the problem of how dust forms and how it could be sustained in thestatic photospheres of cool dwarfs for a long time. In the cool and dense gas,dust forms easily at the condensation temperature, T_cond, and the dust can bein detailed balance with the ambient gas so long as it remains smaller than thecritical radius, r_cr. However, dust will grow larger and segregate from thegas when it will be larger than r_cr somewhere at the lower temperature, whichwe refer to as the critical temperature, T_cr. Then, the large dust grains willprecipitate below the photosphere and only the small dust grains in the regionof T_cr < T < T_cond can be sustained in the photosphere. Thus a dust cloud isformed. Incorporating the dust cloud, non-grey model photo- spheres inradiative-convective equilibrium are extended to T_eff as low as 800K. Observedcolors and spectra of cool dwarfs can consistently be accounted for by a singlegrid of our cloudy models. This fact in turn can be regarded as supportingevidence for our basic assumption on the cloud formation.Comment: 50 pages with 14 postscript figures, to be published in Astrophys.