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The activity, variability, and rotation of lower main-sequence Hyades stars
Radick, Richard R.; Thompson, D. T.; Lockwood, G. W.; Duncan, D. K.; Baggett, W. E.
AA(Sacramento Peak Observatory, Sunspot, NM), AB(Lowell Observatory, Flagstaff, AZ), AC(Lowell Observatory, Flagstaff, AZ), AD(Mount Wilson and Las Campanas Observatoires, Pasadena, CA), AE(New Mexico State University, Las Cruces)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 321, Oct. 1, 1987, p. 459-472. (ApJ Homepage)
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NASA/STI Keywords:
Late Stars, Main Sequence Stars, Stellar Activity, Stellar Rotation, Variable Stars, Calcium, Emission Spectra, Potassium, Stellar Spectra, Stellar Spectrophotometry
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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.

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