THE CHROMOSPHERIC ACTIVITY OF LOW-MASS STARS IN THE HYADES

High-resolution spectra or narrow-band H-alpha photometry of 106 dwarf K and M stars in the Hyades cluster have been used to determine H-alpha equivalent widths. These data reveal a sequence of Hyades members with H-alpha in absorption for photospheric temperatures hotter than about 3500 K. Within this sequence, the dispersion about the mean equivalent width-color relation is only slightly larger than our measurement errors. A second sequence of Hyades members, characterized by H-alpha emission and significant scatter at a given color, appears at effective temperatures cooler than about 4000 K. This bifurcation in H-alpha properties at T(eff) approximately 4000 K coincides with a bifurcation in the rotational characteristics of Hyades stars: the dispersion in the rotational velocities of the hotter Hyades members at a given effective temperature is small while the cooler stars exhibit significant scatter in their rotational velocities. On the basis of these data, we suggest, as has been argued by others, that low-mass stars spin down to rotational velocities dependent upon mass and age, but independent of premain-sequence angular momentum. The time scale for the rapid spin-down phase, and therefore the time scale for angular momentum evolution on the main sequence, increases toward later spectral types. Significant scatter and high rotational velocities are found among those stars cooler than T(eff) = 4000 K presumably because insufficient time has elapsed by the age of the Hyades to complete the rapid spin-down phase of their rotational evolution. We also find that the color redward of which all members of a given open cluster exhibit H-alpha emission is strongly age-dependent. The calibration of this effect could therefore yield a new tool for the determination of the relative ages of open clusters.