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A. Schweitzer (Univ. of Georgia), J. E. Gizis (Univ. of Delaware), P. H. Hauschildt (Univ. of Georgia), F. Allard (CRAL), I. N. Reid (Univ. of Pennsylvania), J. D. Kirkpatrick (IPAC)
We present a spectroscopic study of L~dwarfs using our latest AMES-Cond and AMES-Dusty model atmospheres. These model atmospheres are calculated using the atmosphere code PHOENIX and include dust formation and dust absorption inside the photosphere. Early L~dwarfs turn out to have a dust filled photosphere and can be described with the AMES-Dusty models which leave the formed dust completely in the layer in which it forms. The late L~dwarfs turn out to have a dust free (rained out) photosphere and can be described with the AMES-Cond models which rain out the formed dust completely. Intermediate L~dwarfs, however, cannot be reproduced with either model yet and require unified models which consider the settling of dust to produce a self consistent dust stratification.
We derive effective temperatures of 1400~K to 2000~K for L8 to M9.5 dwarfs respectively. As expected for this field sample out of the 2MASS catalog, we derive high values for log(g) of 4.5 and higher.
This work was supported in part by NASA ATP grants NAG5-8425, NAG 5-3018 and LTSA grant NAG 5-3619. Some of the calculations were performed on the IBM SP2 of the UGA UCNS, on the IBM SP ``Blue Horizon'' of the San Diego Supercomputer Center, with support from the NSF, and on the IBM SP of the NERSC with support from the DoE. The observations were obtained at the W.~M. Keck Observatory, which is operated jointly by the University of California, the California Institute of Technology and NASA.