Discovery of a Probable CH Star in the Globular Cluster M14 and Implications for the Evolution of Binaries in Clusters

We report the discovery of a probable CH star in the core of the Galacticglobular cluster M14, identified from an integrated-light spectrum of thecluster obtained with the MOS spectrograph on the CFHT. From a high- resolutionechelle spectrum of the same star obtained with the Hydra fiber positioner andbench spectrograph on the WIYN telescope, we measure a radial velocity of$-53.0\pm1.2$ km s$^{-1}$. Although this velocity is inconsistent withpublished estimates of the systemic radial velocity of M14 (eg, ${\bar {v_r}}\approx -123$ km s$^{-1}$), we use high-precision Hydra velocities for 20 starsin the central 2.6 arcminutes of M14 to calculate improved values for thecluster mean velocity and one-dimensional velocity dispersion: $-59.5\pm1.9$ kms$^{-1}$ and $8.2\pm1.4$ km s$^{-1}$, respectively. Both the star's locationnear the tip of the red giant branch in the cluster color magnitude diagram andits radial velocity therefore argue for membership in M14. Since theintermediate-resolution MOS spectrum shows not only enhanced CH absorption butalso strong Swan bands of C$_2$, M14 joins Omega Cen as the only globularclusters known to contain classical CH stars. Although evidence for itsduplicity must await additional radial velocity measurements, the CH star inM14 is probably, like all field CH stars, a spectroscopic binary with adegenerate (white dwarf) secondary. The candidate and confirmed CH stars in M14and Omega Cen, and in a number of Galactic dSph galaxies, may then owe theirexistence to the long timescales for the shrinking and coalescence of hardbinaries in low-concentration environments.