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 Galactic globular cluster M14 (=NGC 6402 = C1735-032), identified from an integrated-light spectrum of the cluster obtained with the MOS spectrograph on the Canada-France-Hawaii telescope. From a high-resolution echelle spectrum of the same star obtained with the Hydra fiber positioner and bench spectrograph on the WIYN telescope, we measure a radial velocity of -53.0 ± 1.2 km s^-1. Although this velocity is inconsistent with published estimates of the systemic radial velocity of M14 (e.g., v_r⁻ ≈ -123 km s^-1), we use high-precision Hydra velocities for 20 stars in the central 26 of M14 to calculate improved values for the cluster mean velocity and one-dimensional velocity dispersion: -59.5 ± 1.9 km s^-1 and 8.2 ± 1.4 km s^-1, respectively. Both the star's location near the tip of the red giant branch in the cluster color-magnitude diagram and its radial velocity therefore argue for membership in M14. Since the intermediate-resolution MOS spectrum shows not only enhanced CH absorption but also strong Swan bands of C₂, M14 joins ω Cen as the only globular clusters known to contain "classical" CH stars. Although evidence for its duplicity must await additional radial velocity measurements, the CH star in M14 is probably, like all field CH stars, a spectroscopic binary with a degenerate (white dwarf) secondary. The candidate and confirmed CH stars in M14 and ω Cen, and in a number of Galactic dSph galaxies, may then owe their existence to the long timescales for the shrinking and coalescence of hard binaries in low-concentration environments.