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We present a spectral analysis of the UV-bright star Y453 in M4. Model fitsto the star's optical spectrum yield T_eff ~ 56,000 K. Fits to the star's FUVspectrum, obtained with the Cosmic Origins Spectrograph (COS) on board theHubble Space Telescope, reveal it to be considerably hotter, with T_eff ~72,000 K. We adopt T_eff = 72,000 +/- 2000 K and log g = 5.7 +/- 0.2 as ourbest-fit parameters. Scaling the model spectrum to match the star's optical andnear-infrared magnitudes, we derive a mass M_* = 0.53 +/- 0.24 M_sun andluminosity log L/L_sun = 2.84 +/- 0.05, consistent with the values expected ofan evolved star in a globular cluster. Comparing the star with post-horizontalbranch evolutionary tracks, we conclude that it most likely evolved from theblue horizontal branch, departing the AGB before third dredge-up. It shouldthus exhibit the abundance pattern (O-poor and Na-rich) characteristic of thesecond-generation (SG) stars in M4. We derive the star's photosphericabundances of He, C, N, O, Si, S, Ti, Cr, Fe, and Ni. CNO abundances areroughly 0.25 dex greater than those of the cluster's SG stars, while the Si andS abundances agree match the cluster values. Abundances of the iron-peakelements (except for iron itself) are enhanced by 1 to 3 dex. Rather thanrevealing the star's origin and evolution, this pattern reflects the combinedeffects of diffusive and mechanical processes in the stellar atmosphere.

Observations of the Ultraviolet-Bright Star Y453 in the Globular Cluster M4 (NGC 6121)