An effective temperature scale for late-M and L dwarfs, from resonance absorption lines of CSI and RB I

We present Keck HIRES spectra of six late-M dwarfs and II L dwarfs. Our goal is to assign effective temperatures to the objects using detailed atmospheric models and fine analysis of the alkali resonance absorption lines of Ca I and Rb I. These yield mutually consistent results (+/- 50 K) when we use cleared-dust models, which account for the removal of refractory species from the molecular states but do not include dust opacities. We find a tendency for the Rb I line to imply a slightly higher temperature, which we ascribe to an incomplete treatment of the overlying molecular opacities. The final T-eff we adopt are based on the Cs I fits alone, though the Rb I fits support the Cs I temperature sequence. This work, in combination with results from the infrared, hints that dust in these atmospheres has settled out of the high atmosphere but is present in the deep photosphere. We also derive radial and rotational velocities for all the objects, finding that the previously discovered trend of rapid rotation for very low mass objects is quite pervasive. To improve on our analysis, there is a clear need for better molecular line lists and a more detailed understanding of dust formation and dynamics.