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Space Telescope Science Institute
Detectability of CO, PH3, AsH3, and GeH4 in the Atmosphere of Gl 229B

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.