Retrieval of atmospheric properties of cloudy L dwarfs

We present the first results from applying the spectral inversion technique in the cloudy L dwarf regime. Our new framework provides a flexible approach to modelling cloud opacity which can be built incrementally as the data requires, and improves upon previous retrieval experiments in the brown dwarf regime by allowing for scattering in two stream radiative transfer. Our first application of the tool to two mid-L dwarfs is able to closely reproduce their near-infrared spectra, far more closely that grid models for the targets Teff and gravity. By extrapolating our retrieved spectra we estimate for 2MASS J05002100+0330501 $T_{\rm eff} = 1835^{+22}_{-19}$K and $\log g = 5.2^{+0.08}_{-0.19}$ and for 2MASSW J2224438-015852 we find $T_{\rm eff} = 1735^{+16}_{-19}$K and $\log g = 5.28^{+0.07}_{-0.14}$, in close agreement with previous empirical estimates. Our best model for both objects includes an optically thick cloud deck which passes $\tau_{cloud} > 1$ (looking down) at a pressure of around 5 bar. This pressure is too high for silicate species to condense, and we argue that corundum and/or iron clouds are responsible for this cloud opacity. We also find the opacity consistent with a cloud of particles following a Hansen distribution dominated by sub-micron sized particles. Our retrieved profiles are cooler at depth, and warmer at altitude than the forward grid models that we compare, and we argue that some form of heating mechanism may be at work in the upper atmospheres of these L dwarfs. We also identify anomalously high CO abundance in both targets, which does not correlate with the warmth of our upper atmospheres or our choice of cloud model.