Modeling the Formation of Clouds in Brown Dwarf Atmospheres

Authors:
  • CCCurtis S. Cooper
    Curtis S. Cooper
  • DSDavid Sudarsky
    David Sudarsky
  • JMJohn A. Milsom
    John A. Milsom
  • JLJonathan I. Lunine
    Jonathan I. Lunine
  • ABAdam Burrows
    Adam Burrows
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Abstract
Because the opacity of clouds in substellar mass object (SMO) atmospheres depends on the composition and distribution of particle sizes within the cloud, a credible cloud model is essential for accurately modeling SMO spectra and colors. We present a one-dimensional model of cloud particle formation and subsequent growth based on a consideration of basic cloud microphysics. We apply this microphysical cloud model to a set of synthetic brown dwarf atmospheres spanning a broad range of surface gravities and effective temperatures (g = 1.78e3 -- 3e5 cm/s^2 and T_eff = 600 -- 1600 K to obtain plausible particle sizes for several abundant species (Fe, Mg2SiO4, and Ca2Al2SiO7). The particle sizes we have thus computed range from ~5 microns to over 300 microns in radius over the full range of atmospheric conditions considered. We show that modal particle sizes decrease significantly with increasing brown dwarf surface gravity. We also find that brown dwarfs with higher effective temperatures have characteristically larger cloud particles than those with lower effective temperatures. We thus conclude that it is unrealistic when modeling SMO spectra to apply a single particle size distribution to the entire class of objects.