METALLICITY EFFECTS ON THE CHROMOSPHERIC ACTIVITY-AGE RELATION FOR LATE-TYPE DWARFS

We show that there is a relationship between the age excess, defined a s the difference between the stellar isochrone and chromospheric ages, and the metallicity as measured by the index [Fe/H] for late-type dwa rfs. The chromospheric age tends to be lower than the isochrone age fo r metal-poor stars, and the opposite occurs for metal-rich objects. We suggest that this could be an effect of neglecting the metallicity de pendence of the calibrated chromospheric emission-age relation. We pro pose a correction to account for this dependence. We also investigate the metallicity distributions of these stars, and show that there are distinct trends according to the chromospheric activity level. Inactiv e stars have a metallicity distribution which resembles the metallicit y distribution of solar neighbourhood stars, while active stars appear to be concentrated in an activity strip on the log R-HK' x [Fe/H] dia gram. We provide some explanations for these trends, and show that the chromospheric emission-age relation probably has different slopes on the two sides of the Vaughan-Preston gap.