High-resolution spectra of very low-mass stars

Abstract

We present the results of high-resolution (1–0.4 Å) optical spectroscopy of a sample of very low-mass stars. These data are used to examine the kinematics of the stars at the bottom of the hydrogen-burning main sequence. No evidence is found for a significant difference between the kinematics of the stars in our sample with I − K > 3.5 (M_bol ≳ 12.8) and those of more massive M dwarfs (M_bol ≈ 7–10). A spectral atlas at high (0.4-Å) resolution for M8–M9+ stars is provided, and the equivalent widths of Cs i, Rb i and Hα lines present in our spectra are examined. We analyse our data to search for the presence of rapid rotation, and find that the brown dwarf LP 944-20 is a member of the class of ‘inactive, rapid rotators’. Such objects seem to be common at and below the hydrogen-burning main sequence. It seems that in low-mass/low-temperature dwarf objects either the mechanism that heats the chromosphere, or the mechanism that generates magnetic fields, is greatly suppressed.

techniques: radial velocities, stars: activity, stars: chromospheres, stars: kinematics, stars: low-mass, brown dwarfs, stars: rotation