Low-mass stars in open clusters. I. NGC 2516 and NGC 3680

We have obtained photometric and spectroscopic observations of low-mass sta rs in the young cluster NGC 2516 and the older cluster NGC 3680. Our observ ations extend the membership surveys approximately 6 mag fainter than previ ous studies in these clusters. Using fiducial main sequences representative of the cluster metallicities, we present candidate membership lists and us e these to investigate the magnetic activity and luminosity functions in th e clusters. NGC 2516 has many active M dwarfs and their properties are simi lar to those in other young clusters. The dMe stars tend to lie above/red o f the fiducial main sequence in an M-V versus V-I color-magnitude diagram. Using spectroscopic observations of the CaOH band at 6230 Angstrom, we show that CaOH is shallower in dMe stars compared with dM stars at the same col or. The absolute magnitudes of the dMe stars and dM stars can be brought in to agreement when CaOH is used as the temperature indicator, which indicate s that the magnetic activity is affecting the V-I color in these stars. The activity strength, measured by log (L-H alpha/L-bol), increases toward low er mass stars, in agreement with X-ray and chromospheric results in other y oung clusters. A few anomalous stars with low activity strength were found, in contrast to other young clusters. Comparison with a recent X-ray survey of NGC 2516 revealed very few low-mass stars with X-ray emission, indicati ng that low-mass cluster stars probably cannot explain the many X-ray sourc es without optical counterparts in the survey. The luminosity function for NGC 2516 shows that the cluster is quite extended on the sky, and mass segr egation and preferential evaporation of certain low-mass stars are probably occurring. The NGC 3680 observations are consistent with a truncation of t he cluster at similar to 1 M-. (M-V less than or similar to 5-6) stars. The absence of low-mass members indicates extensive dynamical evolution or a d eviation from a typical initial mass function for this cluster.