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[23.06] Lithium chemistry in brown dwarfs and extrasolar gas giant planets

K Lodders (Washington Univ.)

Lithium chemistry (and that of all other naturally occurring elements) in brown dwarfs and gas giant planet atmospheres is being investigated by thermochemical equilibrium calculations, using the CONDOR code (Fegley and Lodders, 1994, Icarus 110, 117). The calculations are done for solar composition material over a wide pressure (P) and temperature (T) range appropriate for model atmospheres of Gl 229B, other brown dwarfs and putative extrasolar gas giant planets. Monatomic Li is the major Li gas at high T (but below the Li-Li+ boundary). As T decreases, Li (g) reacts to form LiCl. The abundances of LiH and LiOH also increase with decreasing T, but they are always less important than LiCl. The Li-LiCl boundary (equal abundance of these two gases) is given by the equation: 10,000/T \approx 6.565-0.3574 log P (bar). Between the Li-LiCl boundary and the Li₂S condensation line, which removes all Li from the gas at even lower T, monatomic Li is still present but at lower abundances than LiCl. The presence or absence of detectable monatomic Li in cool, low-mass objects is often used as a discriminant of whether or not the object has reached the stellar mass limit of 0.07-0.08 solar masses (Li absent) or has a lower substellar mass (Li present). However, this work shows that the Li-test for brown dwarf candidates suggested by Rebollo and coworkers (1992, ApJ. 389, L83) may only apply to objects with sufficiently hot atmospheres, where Li is dominantly in its monatomic form. In the coolest brown dwarfs and in gas giant planets low atomic Li abundances or the absence of Li occurs because Li forms LiCl gas or condenses out as Li₂S. Supported by NAG5-6366 from the NASA Planetary Atmospheres Program.