Rotational Velocities and Chromospheric Activity of M Dwarfs in the Hyades

Abstract

We have obtained high-resolution spectra of 52 K and M dwarf members of the Hyades open cluster. We have used these spectra to derive radial velocities, rotational velocities, and H? equivalent widths. Seven of the stars in our sample are double-lined spectroscopic binaries, and one is a spectroscopic triple. Of the apparently single stars, 13 have rotational velocities above our detection limit of 6 km s-1, with the maximum rotational velocity being about 25 km s-1. Given the relatively low mass for these stars, this modest spectroscopic rotational velocity of 25 km s-1 corresponds to a rotational period of order 17 hr, and thus the most rapidly rotating Hyades M dwarfs actually are quite rapid rotators. Somewhat surprisingly, the components of the spectroscopic binaries are, if anything, on average apparently more slowly rotating than the single stars. Forty-nine of the stars in our sample are M dwarfs. There is a good rotation-activity correlation for this sample, with stars with v sin i > 12 km s-1 having log (LX/LBol) ~ -3 and H? equivalent widths saturating at ~4 ?. There appear to be two branches of the rotation-activity relation as one approaches saturation, however?one set of stars approaches the saturation level at a very slow rotation rate (v sin i ? 6 km s-1), whereas the second set does not reach saturation until about v sin i = 12 km s-1. The components of the double-lined spectroscopic binaries are generally members of the first set since they are quite active but generally slowly rotating.