An Effective Temperature Scale for Late‐M and L Dwarfs, from Resonance Absorption Lines of Cs<scp>i</scp>and Rb<scp>i</scp>

Abstract

We present Keck HIRES spectra of 6 late-M dwarfs and 11 L dwarfs. Our goal is\nto assign effective temperatures to the objects using detailed atmospheric\nmodels and fine analysis of the alkali resonance absorption lines of CsI and\nRbI. These yield mutually consistent results (+-150 K) when we use\n``cleared-dust'' models, which account for the removal of refractory species\nfrom the molecular states but do not include dust opacities. We find a tendency\nfor the RbI line to imply a slightly higher temperature, which we ascribe to an\nincomplete treatment of the overlying molecular opacities. The final effective\ntemperatures we adopt are based on the CsI fits alone, though the RbI fits\nsupport the CsI temperature sequence. This work, in combination with results\nfrom the infrared, hints that dust in these atmospheres has settled out of the\nhigh atmosphere but is present in the deep photosphere. We also derive radial\nand rotational velocities for all the objects, finding that the previously\ndiscovered trend of rapid rotation for very low mass objects is quite\npervasive. To improve on our analysis, there is a clear need for better\nmolecular line lists and a more detailed understanding of dust formation and\ndynamics.\n