We present a spectral analysis of the UV-bright star Y453 in M4 .
Model fits to the star ’ s optical spectrum yield T _ { eff } \sim 56 , 000 K. Fits to the star ’ s FUV spectrum , obtained with the Cosmic Origins Spectrograph ( COS ) on board the Hubble Space Telescope , reveal it to be considerably hotter , with T _ { eff } \sim 72 , 000 K. We adopt T _ { eff } = 72 , 000 \pm 2000 K and \log g = 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 , departing the AGB before third dredge-up .
It should thus exhibit the abundance pattern ( O-poor and Na-rich ) characteristic of the second-generation ( SG ) stars in M4 .
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 SG stars , while the Si and S abundances agree match the cluster values .
Abundances of the iron-peak elements ( except for iron itself ) are enhanced by 1 to 3 dex .
Rather than revealing the star ’ s origin and evolution , this pattern reflects the combined effects of diffusive and mechanical processes in the stellar atmosphere .