The activity, variability, and rotation of lower main-sequence Hyades stars

Abstract

High-precision differential b, y photometric observations of 24 Hyades stars, spectral types F4 V to K8 V, were made at Lowell Observatory during late 1983 and early 1984. It was found that all 18 stars of spectral type F8 V or later in this sample were variable, confirming that low-level photometric variability is a ubiquitous and persistent characteristic of Hyades stars later than spectral type F8 V. On the other hand, the six stars earlier than spectral type F7 V were not detectably variable. Contemporaneous Ca II H + K emission flux measurements for five of the 24 stars, spectral types F8 V to G2 V, were obtained at Mount Wilson Observatory and showed that H + K emission flux varies inversely with respect to photometric brightness on both long-term and rotational time scales. Rotation periods were measured for a total of 23 Hyades stars ranging in spectral type from F8 V to K8 V. It was found that Hyades stars conform reasonably well to the 'Rossby relation', which relates chromospheric activity to the ratio of rotation period and convective turnover time scale.