High-Resolution Spectroscopy of Ultracool M Dwarfs

Abstract

We present high-resolution echelle spectroscopy of 39 dwarfs with spectral types between M6.5 and L0.5. With one exception, those dwarfs were selected from the Two Micron All Sky Survey database using photometric criteria, (J-KS)>=1.1 and KS =2) for masses below 0.1 Msolar. A comparison between the rotational velocities and Hα fluxes shows no evidence for significant correlation. The mean activity level of the ultracool dwarfs lies almost a factor of 10 below that of early- and mid-type M dwarfs. The relative number of dwarfs with vsini<20 km s-1 with respect to greater than 20 km s-1 is independent of spectral type. Finally, velocity dispersions derived for our photometrically selected sample of ultracool dwarfs are significantly lower than those measured for nearby M dwarfs but show remarkable similarity to results for earlier type emission-line (dMe) dwarfs. The latter are generally assigned ages of less than ~3 Gyr.