AAS 202nd Meeting, May 2003
Session 32 Stellar Atmospheres and Circumstellar Material
Poster, Tuesday, May 27, 2003, 10:00am-6:30pm, West Exhbit Hall

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[32.01] Spectral Analysis of Ultracool Dwarfs

D. Homeier (U. Georgia), P.H. Hauschildt (Hamburger Sternwarte), F. Allard (CRAL Lyon)

Improvements of opacity data for the molecular bands governing the spectral energy distribution of the coolest brown dwarfs have brought about major advances to the modelling of substellar atmospheres. At this point uncertainties in the lines strengths for the most important absorbers in the infrared, H2O and CH4, are becoming secondary to uncertainties in the temperature structure, e.\,g.\ related to the distribution of dust, for objects in the T dwarf temperature range. Theoretical spectra generated with these models can set constraints on the temperatures and gravities of ultracool dwarfs independent from age determinations and cooling models. Thus, a quantitative analysis of infrared spectra can serve as a test for evolutionary models and help to identify brown dwarfs in the planetary mass range (PMOs). We present the latest PHOENIX models for effective temperatures 700\,K\lesssim T\rm eff\lesssim1400\,K, discussing the effects of metallicity and gravity on the near infrared absorption bands. The accuracy currently attainable in determining these parameters from low resolution spectral fits or infrared colors alone, and using the additional information available from absolute-flux calibrated spectra, is assessed.

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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.