Color, rotation, age, and chromospheric activity correlations in late-type main-sequence stars

A study of the rotation-activity-age relationships in a large sample of single late-type main-sequence stars is carried out, based on rotation period data from the literature, on our measurements of the Mg II k line chromospheric flux in IUE high-resolution spectra, and on Hipparcos parallaxes. The analysis of the rotation period data of open clusters and field stars indicates that the rotational period decreases linearly with increasing mass in stars of the same age and evolves as a power of age as t(0.45). This information has enabled us to disentangle the role of age in the chromospheric activity-rotation relationship, as well as to analyze the behavior of the Mg II emission flux in terms of "activity evolutionary paths" and of "activity isochrones." It is shown that the Mg II emission maintains a saturated level in very young stars until a certain age that depends on stellar mass, while at later stages it decays at a faster rate for more massive stars. We have also determined the convection turnover time from observational data and analyzed the relationship between the Mg II emission flux and the Rossby number. We reach the conclusion that there is no clear advantage in using the Rossby number instead of stellar age in describing the chromospheric Mg II losses, at least with the convective turnover time values currently in use.