Properties of M Dwarfs in Clusters and the Field

Summary

We report on magnetic activity and luminosity function results from our field and cluster surveys of low mass stars. Magnetic activity in M dwarfs has several notable effects on the colors, magnitudes and molecular bandstrengths. The presence of activity only up to a limiting mass (color, magnitude) in a coeval population can be used as an age indicator. We have calibrated several age-activity relations using new observations of dMe stars in M67 to anchor the relations at large age. The changes in activity strength along the M dwarf sequence are discussed. The luminosity functions for several clusters show evidence for mass segregation and two clusters appear to have lost their low mass population. Unusually rapid dynamical evolution or a skewed initial mass function could account for these results. Either explanation would have implications for the number of brown dwarfs and very low mass stars expected in the field at the present epoch.

Introduction

We have been carrying out large surveys of M dwarfs in the field (Reid et al. 1995, hereafter PMSU1, Hawley, Gizis & Reid 1996, hereafter PMSU2) and in nearby open clusters (Hawley, Tourtellot & Reid 1998, hereafter HTR98). Although these stars, on the whole, might not qualify as “very low mass” (VLM) stars for this conference, they are interesting to study in order to understand the properties that might affect stars even further down the main sequence.