ROTATION AND CHROMOSPHERIC ACTIVITY IN-FIELD M-DWARF

We have obtained high resolution spectra for a volume-limited sample o f 118 field M dwarfs. From these observations we derive projected rota tional velocities and fluxes in the H-alpha and H-beta lines(1). 8 sta rs are double-lined spectroscopic binaries with measured or probable p eriods short enough for rotation to be tidally synchronized with the o rbit, and another 11 are visual binaries where we cannot yet separate the lines of the two stars. Of the remaining 99 stars, 24 have rotatio nal velocities above our detection limit of similar to 2 km.s(-1), and some are quite fast rotators, including two with v sin i similar or e qual to 30 km.s(-1) and one with v sin i similar or equal to 50 km.s(- 1). Given the small radii of M dwarfs, these moderate rotational veloc ities correspond to rather short maximum rotational periods, of only 7 -8 hours. These three stars are good candidates for Doppler imaging. W e find that rotation is strongly correlated with both spectral type an d kinematic population: all stars with measurable rotation are later t han M3.5, and all but one have kinematic properties typical of the you ng disk, or intermediate between the young disk and the the old disk. We interpret this correlation as evidence for a spin-down timescale th at 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 hap pened for stars earlier than M3.5. The one star with measurable rotati on and a kinematics intermediate between old disk and population II ha s spectral type M6. It is probably an old star whose mass is low enoug h that it has retained significant rotation up to present, still consi stently with longer spin-down times for lower mass stars. We observe, on the other hand, no conspicuous change in the v sin i distribution o r activity pattern at the mass (M similar to 0.35 M.) below which star s remain fully convective down to the main sequence. These new data ar e consistent with a saturated correlation between rotation and activit y, similar to the one observed for younger or more massive stars: L-X/ L-bol and L-H alpha/L-bol both correlate with v sin i for v sin i less than or similar to 4-5 km.s(-1) and then saturate at respectively 10( -2.5) and 10(-3.5).