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

TL;DR

This study analyzes the UV-bright star Y453 in M4, determining its temperature, mass, and chemical composition through spectral fitting, and discusses its evolutionary status and atmospheric processes.

Contribution

It provides the first detailed spectral analysis of Y453, deriving its physical parameters and chemical abundances, and links its evolution to the blue horizontal branch in M4.

Findings

Star's effective temperature ~72,000 K from FUV spectrum

Derived mass ~0.53 solar masses and luminosity log L/L_sun = 2.84

Abundance patterns suggest atmospheric diffusion and mechanical processes

Abstract

We present a spectral analysis of the UV-bright star Y453 in M4. Model fits to the star's optical spectrum yield T_eff ~ 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 ~ 72,000 K. We adopt T_eff = 72,000 +/- 2000 K and log g = 5.7 +/- 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 +/- 0.24 M_sun and luminosity log L/L_sun = 2.84 +/- 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.