Chromospheric activity in low mass stars: Observational results from clusters and the field

Observations of low mass stars in the field and in open clusters are providing clues to the production and evolution of magnetic activity, many of which we still do not understand. Some molecular bands and colors respond much more strongly to the presence of activity than others. The observation that the dMe stars lie above and/or redder than the dM star main sequence in an M-V vs. V-I color magnitude diagram can be reconciled if the depth of the CaOH molecular band at 6230 Angstrom is used for the temperature indicator. Several clusters now have enough data to identify the "H alpha limit" mass (color, absolute magnitude) where chromospheric activity becomes prevalent. There is a good correlation between the Ha limit mass (color, absolute magnitude) and the age of the cluster. This age-activity relation for M dwarfs is quite different from previous age-activity-rotation relations found for earlier type stars. The activity strength in Ha and in X-rays is shown for a range of M dwarf masses in several clusters and in the field, and two unexplained results are described. A brief discussion of an ongoing investigation of binarity and activity in NGC 2420 is given.