We have analyzed the rotational broadening and chromospheric activity in a sample of 123 M-dwarfs, usingspectra taken at the W.M. Keck Observatory as part of the California Planet Search program. We find thatonly seven of these stars are rotating more rapidly than our detection threshold of v sin i 2.5 km s" .Rotation appears to be more common in stars later than M3 than in the M0–M2.5 mass range: we estimatethat less than 10 of early-M stars are detectably rotating, whereas roughly a third of those later than M4show signs of rotation. These findings lend support to the view that rotational braking becomes less effectivein fully convective stars. By measuring the equivalent widths of the CaII H and K lines for the stars in oursample, and converting these to approximate Lca/Lbolmeasurements, we also provide constraints on the connectionbetween rotation and magnetic activity. Measurable rotation is a sufficient, but not necessary condition for activityin our sample: all the detectable rotators show strong Ca ii emission, but so too do a small number of non-rotating stars, which we presume may lie at high inclination angles relative to our line of sight. Our data areconsistent with a "saturation-type" rotation–activity relationship, with activity roughly independent of rotationabove a threshold velocity of less than 6 km s~(-1) . We also find weak evidence for a "gap" in Lca/Lbolbetweena highly active population of stars, which typically are detected as rotators, and another much less active group.
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