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

TL;DR

This study analyzes ultraviolet spectra of the UV-bright star III-60 in NGC 6723, confirming optical parameter accuracy, revealing chemical peculiarities, and suggesting a merger origin with evolutionary links to hot subdwarfs.

Contribution

It provides the first detailed UV spectral analysis of III-60, confirming optical parameters and identifying its chemical and evolutionary characteristics, including evidence of a stellar merger.

Findings

Photospheric parameters consistent with optical spectra.

Enhanced nitrogen and depleted carbon and oxygen.

Indications of a stellar merger origin.

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 $T_{\rm eff} = 44{,}800 \pm 1200$, surface gravity $\log g = 4.89 \pm 0.18$, and helium abundance $\log N({\rm He})/N({\rm H}) = -0.84 \pm 0.29$) are consistent with the values derived from its optical spectrum, suggesting that optically-derived values are generally accurate for evolved stars with $T_{\rm eff} \lesssim$ 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 ($\sim 1.2 \, M_{\odot}$) 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.