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 goa l is to assign effective temperatures to the objects using detailed atmosph eric models and fine analysis of the alkali resonance absorption lines of C a I and Rb I. These yield mutually consistent results (+/- 50 K) when we us e "cleared-dust" models, which account for the removal of refractory specie s from the molecular states but do not include dust opacities. We find a te ndency for the Rb I line to imply a slightly higher temperature, which we a scribe 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 fit s support the Cs I temperature sequence. This work, in combination with res ults from the infrared, hints that dust in these atmospheres has settled ou t of the high atmosphere but is present in the deep photosphere. We also de rive 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 bet ter molecular line lists and a more detailed understanding of dust formatio n and dynamics.