THE ROTATION OF M-DWARFS

We attempt to measure rotational broadening of absorption lines in a sample of 47 nearby, late K- and M-type main-sequence stars. For each star, we measure nu-sin i from four lines by fitting them with artificially broadened lines from an extremely slowly rotating reference star of similar spectral type. We compare this convolution technique to a proper disk integration of specific-intensity line profiles and show that the derived nu-sin i values agree to within 0.5 km s-1. Rotation is detected above a threshold of 3 km s-1 in five stars: AD Leo, EQ Vir, YZ CMi, Gliese 369, and Gliese 569. The known rotation period for AD Leo permits an estimate of its inclination, 33-degrees < i < 44-degrees, and thus the polar region of one hemisphere is always in view. A similar analysis for EQ Vir yields a mutually consistent rotation period and nu-sin i, implying an aspect nearly equator-on. The presence of detectable rotation for GL 569A confirms that the system is indeed younger than about 1 Gyr. Thus its low luminosity companion remains an outstanding brown dwarf candidate. Upper limits for nu-sin i of 2-3 km s-1 are derived for the nondetections, all magnetically inactive M dwarfs. The average equatorial rotational velocity of M dwarfs in the solar neighborhood is less than 2 km s-1. This confirms M dwarfs as the slowest rotators on the main sequence and suggests that many lose about 98% of their angular momentum during their main sequence lifetimes. Zeeman broadening is shown to be negligible in optical lines of inactive M dwarfs.