Program: GN-2016B-Q-46

We propose NIRI M' observations of nine 400K to 800K Y and late-T type brown dwarfs. The M' filter bandpass coincides with CO absorption that is a result of the rapid vertical mixing that takes place in these very cool atmospheres. The mixing changes the abundances of C and N species by more than an order of magnitude, and is a key feature of these atmospheres. Models which include mixing show that the shape of the 5um spectral region is sensitive to the mixing, and insensitive to variations in temperature, gravity or metallicity. Hence the proposed observations enable a clean measurement of the mixing efficiency, allowing in the future more confident exploration of cloud formation, and the impact of non-solar metallicity, as well as more accurate determinations of temperature and gravity, and hence mass and age, for these nearby planet-like bodies.