Abstract: Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group

The Ursa Major Moving Group: Revisiting Membership, Activity and Age

Utilizing Hipparcos parallaxes, original radial velocities and recent literature values, new Ca II H&K emission measurements, literature-based abundance estimates, and updated photometry (including recent resolved measurements of close doubles), we revisit the Ursa Major moving group membership status of some 220 stars to produce a final clean list of nearly 60 assured members based on kinematic and photometric criteria. Scatter in the velocity dispersions and H-R diagram is correlated with trial activity-based membership assignments, indicating the usefulness of photometric- and chromospheric emission-based criteria to examine membership. Closer inspection, however, shows that activity is considerably more robust at excluding membership, with failure rates of <= 15% and perhaps considerably less. Comparison of isochrones and our final UMa group members indicates an age of 500+/-100 Myr, some 200 Myr older than the canonical UMa age. Our UMa members' mean chromospheric emission levels and scatter therein are indistinguishable from values in the Hyades and smaller than evinced by members of the younger Pleiades and M34 clusters, suggesting these characteristics decline rapidly with age over 200-500 Myr. None of our UMa members demonstrate inordinately low absolute values of chromospheric emission. However, several may show residual a factor of >=2 below the Hyades-defined lower envelope. If one defines a Maunder-like Minimum in a relative sense, then the UMa results may suggest that solar-type stars spend 10% of their entire main-sequence lives in periods of precipitously low activity{ consistent with estimates from older field stars. As related asides, we identify a small number of potentially very young but isolated field stars; note that active stars (whether UMa members or not) in our sample lie very close to the solar composition zero-age main-sequence unlike Hipparcos-based positions in the H-R diagram of Pleiades dwarfs; and argue that some extant transformations of activity indices are not adequate for cool dwarfs, where Ca II infrared-triplet emission seems to be a better proxy than Halpha-values for Ca II H&K indices.