Observations of the Ultraviolet-bright Star III-60 in the Globular Cluster NGC 6723

Abstract

We have analyzed archival far-ultraviolet spectra of the UV-bright star III-60 in the globular cluster NGC 6723 obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Cosmic Origins Spectrograph (COS). We find that the star's photospheric parameters (effective temperature $T_{\mathrm{eff}}=44,800\pm 1200$, surface gravity $\log g=4.89\pm 0.18$, and helium abundance $\log N(\mathrm{He})/N(\mathrm{H})=-0.84\pm 0.29$) are consistent with the values derived from its optical spectrum, suggesting that optically-derived values are generally accurate for evolved stars with $T_{\mathrm{eff}}\lesssim$ 50,000 K. Relative to the cluster's RGB stars, III-60 is enhanced in nitrogen and depleted in carbon and oxygen. The star exhibits strong P Cygni profiles in both components of the N V $\lambda 1240$ doublet, but the resonance lines of other species show no evidence of a stellar wind. The star's effective temperature and luminosity place it on the evolutionary tracks of stars evolving from the blue horizontal branch, but its high mass ($\sim 1.2M_{\odot }$) indicates that it is the product of a stellar merger. Its helium, carbon, and nitrogen abundances suggest that it is following an evolutionary path similar to that of the low-carbon, intermediate helium-rich hot subdwarfs.

Accepted for publication in ApJ. arXiv admin note: text overlap with arXiv:2312.10248