Keith S. Noll
Space Telescope Science Institute, Baltimore MD
Mark S. Marley
New Mexico State University, Las Cruces NM
Physical conditions in the observable atmosphere of the brown dwarf
Gl 229B are intermediate between those found in a giant planet atmosphere,
such as Jupiter's, and a stellar atmosphere. In one important respect, however,
Gl 229B shares kinship with giant planet atmospheres: temperatures in the
observable atmosphere are cool enough that the gas is essentially completely
molecular. The 4.5 to 5.3 µm spectrum window has proved to be the most
productive portion of Jupiter's and Saturn's spectra for identification of
heavy-element containing gases and includes strong vibration-rotation bands
of arsine (AsH3), germane (GeH4), and phosphine (PH3). The carbon monoxide
(CO) 1-0 fundamental also falls in this spectrum interval. Thermochemical
equilibrium calculations predict all three hydrides as the major reservoirs for
arsenic, germanium, and phosphorous in Gl 229B's upper atmosphere
( K) assuming modest convective motions take place.
The situation for carbon monoxide is less certain; Jupiter-like abundances of
1 part per billion up to much larger abundances of 10 parts per million are
possible for CO. In this work, we show calculated spectra using a model
atmosphere for Gl 229B and high-resolution, line-by-line radiative transfer
code including molecular hydrogen collision-induced opacity and molecular
line absorption from H2O, CO, PH3, GeH4, and AsH3. We evaluate the prospects
for detection of each of these molecular species with existing and potential
infrared spectrometers.