High Resolution Spectra of L Type Stars and Brown Dwarfs

Summary

The first brown dwarfs were confirmed only three years ago. Already, however, a library of echelle spectra of objects of a variety of temperatures has been accumulated. This process has been greatly aided by the discovery of relatively nearby free-floating brown dwarfs and companions to M dwarfs. Their spectra show the rapidly increasing importance of dust formation in the atmosphere, and its concomitant decrease of the TiO molecular features which define the M spectral class. This has lead to the proposal of a new spectral class, L, for cooler objects. The primary atomic features visible in red spectra of L and late M stars are resonance lines of alkali metals (Na, K, Rb, Cs, and sometimes Li). Here we present a sample of line profiles from mid-M to mid-L objects, which include both very low mass stars and confirmed brown dwarfs. We compare the line profiles in the alkali lines to very recent models which include effects of dust formation. We show that the models can already make a reasonable representation of the observations, and begin to set a temperature scale for these new very cool objects. There are certainly issues remaining to be addressed, however.

Introduction

Since the announcement of the first brown dwarfs in 1995, the field has been moving very quickly. The number of known brown dwarfs has increased rapidly, along with a growing collection of stars at the bottom of the main sequence.