Abstract
The chemical species containing carbon, nitrogen, and oxygen in atmospheres
of giant planets, brown dwarfs (T and L dwarfs), and low-mass stars
(M dwarfs) are identified as part of a comprehensive set of thermochemical
equilibrium and kinetic calculations for all elements. The calculations
cover a wide temperature and pressure range in the upper portions
of giant planetary and T-, L-, and M-dwarf atmospheres. Emphasis
is placed on the major gases CH4, CO, NH3, N-2, and H2O but other
less abundant gases are included. The results presented are independent
of particular model atmospheres, and can be used to constrain model
atmosphere temperatures and pressures from observations of different
gases. The influence of metallicity on the speciation of these key
elements under pressure-temperature (P-T) conditions relevant to
low-mass object atmospheres is discussed. The results of the thermochemical
equilibrium computations indicate that several compounds may be useful
to establish temperature or pressure scales for giant planet, brown
dwarf, or dwarf star atmospheres. We find that ethane and methanol
abundance are useful temperature probes in giant planets and methane
dwarfs such as Gl 229B, and that CO2 can serve as a temperature probe
in more massive objects. Imidogen (NH) abundances are a unique pressure-independent
temperature probe for all objects. Total pressure probes for warmer
brown dwarfs and M dwarfs are HCN, HCNO, and CH2O. No temperature-independent
probes for the total pressure in giant planets or T-dwarf atmospheres
are identified among the more abundant C, N, and 0 bearing gases
investigated here. (C) 2002 Elsevier Science (USA).
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