Stellar Kinematic Groups. II. Reexamination of the Membership, Activity, and Age of the Ursa Major GroupThe Astronomical Journal
PublisherThe American Astronomical Society
AbstractUtilizing Hipparcos parallaxes, original radial velocities and recent literature values, new Ca ii H and 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 photomet-ric criteria. Scatter in the velocity dispersions and H-R diagram is correlated with trial activity-based mem-bership assignments, indicating the usefulness of criteria based on photometric and chromospheric emission to examine membership. Closer inspection, however, shows that activity is considerably more robust at excluding membership, failing to do so only for 15% of objects, perhaps considerably less. Our UMa mem-bers demonstrate nonzero vertex deviation in the Bottlinger diagram, behavior seen in older and recent stud-ies of nearby young disk stars and perhaps related to Galactic spiral structure. Comparison of isochrones and our ﬁnal UMa group members indicates an age of 500 Æ100 Myr, some 200 Myr older than the canonically quoted UMa age. Our UMa kinematic=photometric members’ mean chromospheric emission levels, rota-tional velocities, and scatter therein are indistinguishable from values in the Hyades and smaller than those 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, but several may show residual ﬂuxes a factor of !2 below a 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, which is consistent with estimates from older ﬁeld stars. As related asides, we note six evolved stars (among our UMa nonmembers) with distinctive kinematics that lie along a 2 Gyr isochrone and appear to be late-type counterparts to disk F stars deﬁning intermediate-age star streams in previous studies, identify a small num-ber 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, for which Ca ii infrared triplet emission seems to be a better proxy than H-based values for Ca ii H and K indices.
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