WIYN OPEN CLUSTER STUDY. XXXII. STELLAR RADIAL VELOCITIES IN THE OLD OPEN CLUSTER NGC 188

Abstract

(Abridged) We present the results of our ongoing radial-velocity (RV) survey\nof the old (7 Gyr) open cluster NGC 188. Our WIYN 3.5m data set spans a time\nbaseline of 11 years, a magnitude range of 12<=V<=16.5 (1.18-0.94 MSun), and a\n1 deg. diameter region on the sky. With the addition of a Dominion\nAstrophysical Observatory (DAO) data set we extend our bright limit to V = 10.8\nand, for some stars, extend our time baseline to 35 years. Our magnitude limits\ninclude solar-mass main-sequence stars, subgiants, giants, and blue stragglers\n(BSs), and our spatial coverage extends radially to 17 pc (~13 core radii). For\nthe WIYN data we find a measurement precision of 0.4 km/s for narrow-lined\nstars. We have measured RVs for 1046 stars in the direction of NGC 188, finding\n473 to be likely cluster members. We detect 124 velocity-variable cluster\nmembers, all of which are likely to be dynamically hard-binary stars. Using our\nsingle member stars, we find an average cluster RV of -42.36 +/- 0.04 km/s. We\nuse our precise RV and proper-motion membership data to greatly reduce\nfield-star contamination in our cleaned color-magnitude diagram, from which we\nidentify six stars of note that lie far from a standard single-star isochrone.\nWe find the binaries to be centrally concentrated, providing evidence for the\npresence of mass segregation in NGC 188. We observe the BSs to populate a\nbimodal spatial distribution that is not centrally concentrated, suggesting\nthat we may be observing two populations of BSs in NGC 188, including a\ncentrally concentrated distribution as well as a halo population. Finally, we\nfind NGC 188 to have a global RV dispersion of 0.64 +/- 0.04 km/s. When\ncorrected for unresolved binaries, the NGC 188 RV dispersion has a nearly\nisothermal radial distribution. We use this mean-corrected velocity dispersion\nto derive a virial mass of 2300 +/- 460 MSun.\n