Cool Stars 14 - Submitted Abstract # 257
This version created on 05 October 2006

Dust Formation and Cloud Settling from Low Mass Stars to Planetary
Mass Brown Dwarfs


Derek Homeier, Institut fuer Astrophysik Goettingen
France Allard, Centre de Recherche Astronomique de Lyon
Peter Hauschildt, Hamburger Sternwarte
Hans-G. Ludwig, Observatoire de Paris-Meudon

We present a new grid of atmosphere models and theoretical spectra for
ultracool dwarfs, spanning the range from low-mass stars to the oldest
and coolest known field brown dwarfs.  A wealth of spectroscopic,
photometric and astrometric observations has revealed complex changes
of atmospheric chemistry accompanying the formation of condensates in
the visible photosphere.  While dust signatures are peaking in
mid-to-late L types, the depletion of condensible elements following
the formation and settling of dust clouds into larger optical depths
drives the transition from L to T over a very small range in effective
temperature, and might indicate the influence of additional
parameters.  Our models attempt, by including suitably simplified
descriptions of cloud physics and chemistry, to constrain the vast
parameter space of cloud configurations as much as possible.  These
simulations describe the settling of particle clouds by comparing
timescales of condensation, coagulation and gravitational
sedimentation to atmospheric mixing driven by convective instability.
Using a parameterised description of the latter based on results of
RHD simulations we reproduce well the behaviour of the cloud deck over
the BD temperature sequence.  In addition we find a strong dependence
of settling on gravity, thus explaining observed differences in cloud
opacity at constant effective temperature.  Stronger deviations from
the field BD relation between temperature and spectral type/IR colours
are predicted for very young/low gravity objects, recently
observationally confirmed for a number of low mass brown dwarfs and
planetary mass candidates such as 2MASS 1207334-393254B or HD 203030B.

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