Dust Formation in Brown Dwarfs

M. Lüttke, Ch. Helling, M. John, K.S. Jeong, P. Woitke, and E. Sedlmayr

Astronomische Gesellschaft Abstract Series, Vol. 17., (2000)

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Abstract:

Brown Dwarfs provide favorable conditions for the gas-solid phase transition since in its atmosphere low temperatures with high densities are combined. Observations of unexpectedly weak molecular absorption bands indicate the existence of dust in their atmospheres.

Considering stability arguments, high temperature compounds like CaTiO3, Fe2-xSixO4 or Al2O3 are expected to form first. Another argument for the formation of heterogeneous dust is given by the wide variety of molecular species of comparable abundances in an oxygen-rich gas. The description of the formation of such heterogeneous particles, however, is still a matter of debate since the nominal molecules are only present in negligible amounts or even completely absent in the gas phase. Furthermore, the formation of solid particles has to proceed via the formation of seed particles which is followed by the growth towards macroscopic sizes. Thereby, a large supersaturation has to be achieved which results in a considerable gap between typical nucleation temperatures and the sublimation temperatures of plane solid. Promising
astrophysical seed candidates are (TiO2)N and (Fe)N clusters which appear in appropriate amounts in oxygen-rich gases; Fe seeds maybe even more favorable in the densest regions of the atmospheres (n<H> > 1019cm-3).

In this presentation, the efficiency of nucleation and the stability of solid compounds are examined for a typical atmosphere of a Brown Dwarf in comparison to M dwarfs and Jupiter-like planets. We additionally present first results of numerical calculations for Brown Dwarf atmospheres including a detailed time-dependent, phase non-equilibrium description of the formation of core-mantel grains. Dust properties like the amount and the sizes of the solid particles are thereby a result of the calculation.

Keywords:

Stars: brown dwarfs -- Stars: oxygen