Spectroscopic Detection of Carbon Monoxide in Two Late-type T Dwarfs

TL;DR

This study detects CO in late-type T dwarfs' atmospheres, indicating vertical mixing causes chemical disequilibrium, and measures atmospheric mixing timescales for the first time in such objects.

Contribution

It provides the first spectroscopic detection of CO in late T dwarfs and measures vertical mixing timescales, revealing a universal atmospheric process in these objects.

Findings

CO detected in late T dwarfs' atmospheres.

Vertical mixing drives CO out of chemical equilibrium.

Measured atmospheric mixing timescales in late T dwarfs.

Abstract

M band spectra of two late-type T dwarfs, 2MASS J09373487+2931409, and Gliese 570D, confirm evidence from photometry that photospheric 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 CO in Gliese 229B and existing M 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 of mid-to-late type T dwarfs. The M band spectra allow determinations of the time scale of vertical mixing in 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=925-975K, log g=5.20-5.47, log L/L_sun=-5.308 +/- 0.027. The age is 3-10 Gyr and the mass is in the range 45-69 M_Jup.