A systematic study of departures from chemical equilibrium in the atmospheres of substellar mass objects

We present a systematic study of the spectral consequences of departures from chemical equilibrium in the atmospheres of L and T dwarfs, and for even cooler dwarfs. The temperature/pressure profiles of the nonequilibrium models are fully consistent with the nonequilibrium chemistry. Our grid of nonequilibrium models includes spectra for effective temperatures from 200 to 1800 K, three surface gravities, four possible values of the coefficient of eddy diffusion in the radiative zone, and three different CO/CH4 chemical reaction prescriptions. We also provide clear and cloudy model variants. We find, in keeping with previous studies, that there are essentially only two spectral regions where the effects of departures from chemical equilibrium can influence the predicted spectrum. These are in the M(similar to 4-5 mu m) and N (8-14 mu m) bands due to CO and NH3, respectively. The overabundance of CO translates into flux suppressions of at most similar to 40% between effective temperatures of 600 and 1800 K. The effect is largest around T-eff approximate to 1100 K. The underabundance of ammonia translates into flux enhancements of no more than similar to 20% for the T-eff range from 300 to 1800 K, with the largest effects at the lowest values of T-eff. The magnitude of the departure from chemical equilibrium increases with decreasing gravity, with increasing eddy diffusion coefficient, and with decreasing speed of the CO/CH4 reaction. Although these effects are modest, they lead to better fits with the measured T dwarf spectra. Furthermore, the suppression in the M band due to nonequilibrium enhancements in the CO abundance disappears below similar to 500 K, and is only partial above similar to 500 K, preserving the M-band flux as a useful diagnostic of cool atmospheres and maintaining its importance for searches for the cooler brown dwarfs beyond the T dwarfs.