Spectroscopic detection of carbon monoxide in two late-type t dwarfs

M-band spectra of two late-type T dwarfs, 2MASS J09373487+2931409, and Gliese 570D, confirm evidence from photometry that photospheric carbon monoxide (CO) is present at abundance levels far in excess of those predicted from chemical equilibrium. These new and unambiguous detections of CO, together with an earlier spectroscopic detection of COinGliese 229B and existingM-band photometry of a large selection of T dwarfs, suggest that vertical mixing in the photosphere drives the CO abundance out of chemical equilibrium and is a common, and likely universal feature ofmid-to-late-type T dwarfs. TheM-band spectra allowdeterminations of the timescale of vertical mixing in the radiative region of the atmosphere of each object, the first such measurements of this important parameter in late T dwarfs. A detailed analysis of the spectral energy distribution of 2MASS J09373487+2931409 results in the following values for metallicity, temperature, surface gravity, and luminosity: [M/H] ∼ -0.3, T_eff = 925-975 K, log g = 5.20-5.47, and log L/L⊙ = -5.308±0.027. The age is 3-10 Gyr and the mass is in the range 45-69M_Jup.