A Probable Correlation Between Chromospheric Activity and Age in Main-Sequence Stars.

Observation shows that the average intensity of H and K emission is much higher for main-sequence stars of types G0-K2 in the Hyades, Praesepe, Coma, and Pleiades clusters than for similar local field stars and that it is appreciably higher for the Pleiades than for the other clusters. It is also found, from observations of local visual binaries, that the H-K intensities in the members of such pairs tend to be very similar when allowance is made for spectral-type differences. It is concluded that the most probable explanation of these facts is that the H-K intensity in a main-sequence star and hence the general degree of its chromospheric activity bear an inverse relationship to its age. This concept of chromospheric evolution is consistent with all the observations presented here, as well as with the known kinematical differences between dMe and ordinary dM stars. If, however, stars can be formed in the general field, as well as in clusters, then the observed differences in chromospheric activity would be related to place of origin rather than to age. The probable bearing of average surface magnetic-field strength on the question of chromospheric activity is discussed briefly.