1998A&A...331..581D

We have obtained high resolution spectra for a volume-limited sample of 118 field M dwarfs. From these observations we derive projected rotational velocities and fluxes in the H_α and H_β lines. 8 stars are double-lined spectroscopic binaries with measured or probable periods short enough for rotation to be tidally synchronized with the orbit, and another 11 are visual binaries where we cannot yet separate the lines of the two stars. Of the remaining 99 stars, 24 have rotational velocities above our detection limit of ∼2km/s, and some are quite fast rotators, including two with vsini≃30km/s and one with vsini≃50km/s. Given the small radii of M dwarfs, these moderate rotational velocities correspond to rather short maximum rotational periods, of only 7-8 hours. These three stars are good candidates for Doppler imaging. We find that rotation is strongly correlated with both spectral type and kinematic population: all stars with measurable rotation are later than M3.5, and all but one have kinematic properties typical of the young disk, or intermediate between the young disk and the the old disk. We interpret this correlation as evidence for a spin-down timescale that increases with decreasing mass. At the age of the old disk or halo, all stars earlier than M5-M6 (0.1-0.15M_☉) have spun-down to below our detection limit, while at the age of the young disk this has only happened for stars earlier than M3.5. The one star with measurable rotation and a kinematics intermediate between old disk and population II has spectral type M6. It is probably an old star whose mass is low enough that it has retained significant rotation up to present, still consistently with longer spin-down times for lower mass stars. We observe, on the other hand, no conspicuous change in the vsini distribution or activity pattern at the mass (M∼0.35M_☉) below which stars remain fully convective down to the main sequence. These new data are consistent with a saturated correlation between rotation and activity, similar to the one observed for younger or more massive stars: L_X/L_bol and L_Hα/L_bol both correlate with vsini for vsini≲-5km/s and then saturate at respectively 10^–2.5 and 10^–3.5.