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

We present a spectral analysis of the UV-bright star Y453 in M4. Model fits to the star's optical spectrum yield \teff\ $\sim 56,000$ K. Fits to the star's FUV spectrum, obtained with the Cosmic Origins Spectrograph (COS) on board the {\em Hubble Space Telescope}, reveal it to be considerably hotter, with \teff\ $\sim$ 72,000 K. We adopt \teff\ = $72,000 \pm 2000$ K and \logg\ = $5.7 \pm 0.2$ as our best-fit parameters. Scaling the model spectrum to match the star's optical and near-infrared magnitudes, we derive a mass $M_* = 0.53 \pm 0.24\; M_{\sun}$ and luminosity $\log L/L_{\sun} = 2.84 \pm 0.05$, consistent with the values expected of an evolved star in a globular cluster. Comparing the star with post-horizontal branch evolutionary tracks, we conclude that it most likely evolved from the blue horizontal branch. We derive the star's photospheric abundances of He, C, N, O, Si, S, Ti, Cr, Fe, and Ni. CNO abundances are roughly 0.25 dex greater than those of the cluster's second-generation RGB stars, while the Si and S abundances agree with those of the cluster. Abundances of the iron-peak elements (except for iron itself) are enhanced by 1 to 3 dex. This pattern represents an equilibrium among gravitational settling and other diffusive and mechanical processes in the stellar atmosphere.