Talk

The Rotation and Chromospheric Activity of the Solar-Type Stars in M67

Axel Brandenburg, Nordita

Giampapa et al. (2019, ApJ, submitted, http://www.nordita.org/preprints, no.~2017-121) reported the results of a program to measure rotation periods and chromospheric activity in solar-type members of the approximately solar-age and solar-metallicity open cluster, M67. They utilized Kepler/K2 data from Campaign 5 along with contemporaneous spectroscopic observations of the Ca II H and K resonance lines as obtained with the 3.5-m WIYN telescope on Kitt Peak in conjunction with its Hydra multi-object spectrograph. They found that the solar-type M67 stars exhibit a correlation of activity with Rossby number that is opposite to that seen in the inhomogeneous (in age and metallicity) sample of field stars from the Mt. Wilson HK Survey. This unconventional behavior was theoretically predicted for stars characterized by Rossby number (the ratio of rotation period to convective turnover time) exceeding that of the Sun. Such stars are expected to have antisolar differential rotation, but with an increased angular velocity contrast between rapidly rotating poles and a slow equator. Brandenburg & Giampapa (2018, ApJL 855, L22, hereafter BG18) discussed the applicability of these results in the context of the activity and rotation of the M67 solar-type stars based on preliminary determinations of their rotation periods. However, many of the rotation periods utilized previously in BG18 have now been revised. In view of these revised rotation period measurements, we revisit the analysis presented in BG18 that was based, in part, on preliminary periods for 19 solar-type M67 stars. We are now left with only 8 single stars with reliable periods in the present sample. Of these, five are from the original sample, while for another three stars, no periods were previously determined (S724, S777, and S802). However, in the context of trends in dynamo action in solar-type stars, we find that our results remain consistent with the evidence for antisolar differential rotation as BG18 previously discussed.