An Effective Temperature Scale for Late‐M and L Dwarfs, from Resonance Absorption Lines of Csiand Rbi

We present Keck HIRES spectra of 6 late-M dwarfs and 11 L dwarfs. Our goal isto assign effective temperatures to the objects using detailed atmosphericmodels and fine analysis of the alkali resonance absorption lines of CsI andRbI. These yield mutually consistent results (+-150 K) when we use``cleared-dust'' models, which account for the removal of refractory speciesfrom the molecular states but do not include dust opacities. We find a tendencyfor the RbI line to imply a slightly higher temperature, which we ascribe to anincomplete treatment of the overlying molecular opacities. The final effectivetemperatures we adopt are based on the CsI fits alone, though the RbI fitssupport the CsI temperature sequence. This work, in combination with resultsfrom the infrared, hints that dust in these atmospheres has settled out of thehigh atmosphere but is present in the deep photosphere. We also derive radialand rotational velocities for all the objects, finding that the previouslydiscovered trend of rapid rotation for very low mass objects is quitepervasive. To improve on our analysis, there is a clear need for bettermolecular line lists and a more detailed understanding of dust formation anddynamics.Comment: 53 pages, including 20 figures and 2 Tables; accepted in Ap