Underabundance of NH_3 in the brown dwarf Gliese 229B

Details Underabundance of NH_3 in the brown dwarf Gliese 229B Underabundance of NH_3 in the brown dwarf Gliese 229B

Sep 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999dps....31.0905s&link_type=abstract

American Astronomical Society, DPS meeting #31, #09.05

Astronomy and Astrophysics

Astronomy

Scientific paper

The presence of CH_4 in the atmosphere of Gl 229B and the near absence of CO indicate that its effective temperature is less than ~ 1000K. Atmosphere models reveal that at such a low temperatures, nitrogen should be in the form of N_2 and NH_3, with the latter favored at lower temperatures and at higher pressures. While N_2 is spectroscopically invisible, NH_3 has several strong absorption bands, most notably near 2mu m and in the 10mu m region. We present a high-resolution, high signal-to-noise K band spectrum of Gl 229B which we fit with synthetic spectra to constrain the effective temperature, the surface gravity, the metallicity and the NH_3 abundance of Gl 229B. We find that the atmosphere of Gl 229B has a subsolar H_2O abundance. NH_3 is detected in the spectrum but with a NH_3/H_2O ratio of about 25% of the value expected from chemical equilibrium. This suggests that the NH_3 to N_2 chemical equilibrium is quenched at a non-equilibrium value by dynamical processes in the atmosphere. Finally, we discuss how observations in the 10mu m region can provide a better constraint on the NH_3/H_2O ratio. This work has been supported in part by NASA grant NAG5-4988.

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