Low-Mass Stars in Open Clusters. I. NGC 2516 and NGC 3680

Abstract

We have obtained photometric and spectroscopic observations of low-mass stars in the young cluster NGC 2516 and the older cluster NGC 3680. Our observations extend the membership surveys approximately 6 mag fainter than previous studies in these clusters. Using fiducial main sequences representative of the cluster metallicities, we present candidate membership lists and use these to investigate the magnetic activity and luminosity functions in the clusters. NGC 2516 has many active M dwarfs and their properties are similar to those in other young clusters. The dMe stars tend to lie above/red of the fiducial main sequence in an MV versus V-I color-magnitude diagram. Using spectroscopic observations of the CaOH band at 6230 ?, we show that CaOH is shallower in dMe stars compared with dM stars at the same color. The absolute magnitudes of the dMe stars and dM stars can be brought into agreement when CaOH is used as the temperature indicator, which indicates that the magnetic activity is affecting the V-I color in these stars. The activity strength, measured by log (LH?/Lbol), increases toward lower mass stars, in agreement with X-ray and chromospheric results in other young 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, indicating that low-mass cluster stars probably cannot explain the many X-ray sources without optical counterparts in the survey. The luminosity function for NGC 2516 shows that the cluster is quite extended on the sky, and mass segregation and preferential evaporation of certain low-mass stars are probably occurring. The NGC 3680 observations are consistent with a truncation of the cluster at ~1 M? (MV 5?6) stars. The absence of low-mass members indicates extensive dynamical evolution or a deviation from a typical initial mass function for this cluster.