31st Annual Meeting of the DPS, October 1999
Session 2. Extra-solar Planets: Dwarfs and Disks
Contributed Oral Parallel Session, Monday, October 11, 1999, 9:00-10:00am, Sala Kursaal

## [2.03] The Cloudy Atmospheres of Brown Dwarfs: Lessons for Direct Detection of Extrasolar Giant Planets

M.S. Marley (New Mexico State Univ.), R. Freedman (NASA/Ames Research Center), J.I. Lunine (Univ. Ariz.)

Brown dwarfs are pathfinders to the era of direct detection and interpretation of extrasolar giant planets. As with our own giant planets, one of the more vexing issues in understanding these objects is the role of clouds.

Dusty condensate clouds play an increasingly important role in controlling the thermal emission spectra of brown dwarfs with effective temperatures warmer than about 1300 K. Atmospheres with Teff near 2000 K are perhaps the dustiest. Coincidentally this is the same effective temperature range where atmospheric carbon transitions from \rm CH4 to CO. At lower effective temperatures there are few major condensates until water clouds appear in atmospheres with Teff below about 500 K. Thus we can divide the atmospheres of brown dwarfs into three broad classes: (1) hot, CO-rich, dusty atmospheres (e.g. Kelu-1); (2) warm, exceptionally clear (flux from 100 bars can be detected for 30\,\rm MJ objects), \rm CH4-dominated atmospheres (e.g. Gliese 229B); and (3) cooler, \rm CH4-rich atmospheres with icy condensate clouds like Jupiter (e.g. \upsilon-And d, but no direct extrasolar detections, yet). The newly-coined L'' stellar classification spectral type roughly corresponds to class (1).

We have computed model spectra of extrasolar giant planets and brown dwarfs with 200 < Teff < 2000\,\rm K with and without clouds. It is becoming increasingly clear from this work and the growing sample of brown dwarf spectra that an adequate understanding of the first class of objects, and thus the abundant L-dwarfs, requires input from cloud microphysical models. We will present the signature of various cloud models and discuss the need for additional detailed microphysical modeling. Lessons for detecting and then interpreting the spectra of extrasolar giant planets will be considered.

Work supported by the NASA Planetary Atmospheres and MURED and NSF CAREER programs.