Rotational velocities and chromospheric/coronal activity of low-mass stars in the young open clusters IC 2391 and IC 2602

We have obtained high-resolution, moderate signal-to-noise ratio spectra for approximately 80 candidate low-mass members of the nearby, very young open clusters IC 2391 and IC 2602. Most of the stars observed are confirmed as cluster members based on a combination of photometric and spectroscopic criteria. We provide radial velocities, rotational velocities, and H alpha equivalent widths for these stars. From comparison to theoretical pre-main-sequence (PMS) evolutionary isochrones from D'Antona and Mazzitelli, we derive an estimated age of the two clusters of similar to 25 Myr. By contrast, the usually quoted upper main-sequence turnoff age for the clusters is similar to 35 Myr. We do not believe that this provides evidence for noncoeval star formation within these clusters, but rather that the best age estimate for them given the uncertainties is similar to 30 +/- 5 Myr. In principle, the scatter of stars about the PMS isochrone provides a measure of the age spread among the low-mass stars in these clusters; however, with the data presently available, we are able to derive only a relatively uninteresting upper limit for an age spread of order 20 Myr. We compare the rotational velocity distribution for IC 2391/2602 to that observed for the Pleiades. For the G dwarfs in the IC clusters, we resolve rotation in all but one of the probable cluster members, and thus except for inclination effects, our data provide the complete distribution of rotational velocities for solar mass stars on their arrival on the ZAMS. The projected rotational velocities (v sin i) of the G dwarfs in the two IC clusters span the range from similar to 8 to similar to 200 km s(-1). Comparison of the distribution of rotational velocities for the G dwarfs of the Pleiades and the IC clusters indicates that both the slow and the rapid rotators lose of order half their angular momentum during the first similar to 35 Myr on the main sequence if they rotate as solid bodies. The low-mass stars in these two clusters exhibit a similar correlation between rotation and coronal activity as is found in several other young open clusters. That is, there is a large spread in coronal activity for stars with v sin i < 25 km s(-1), where we assume there is an intrinsic link between increasing rotation and increasing activity superimposed upon which are a variety of observational and physical mechanisms that act to smear out this relation; above v sin i similar to 25 km s(-1), all of the low-mass stars have log (L-X/L-bol) similar to -3.0, the canonical ''saturation'' limit. Our measurements of the H alpha equivalent widths are consistent with a similar relationship holding for chromospheric activity. One and possibly two of our spectra for M dwarf members of the IC clusters show broad wings for the H alpha profile, which we attribute to a flare event or to microflares. Since spectra of a small sample of late-type M dwarfs in the Pleiades also showed similarly broad H alpha wings, this suggests that flare frequencies for very young M dwarfs may be quite high.