Observations of the Ultraviolet-bright Star III-60 in the Globular Cluster NGC 6723
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| Formato: | Preprint |
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2025
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| _version_ | 1866909447753826304 |
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| author | Dixon, William V. |
| author_facet | Dixon, William V. |
| contents | 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 $λ1240$ 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2501_01909 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Observations of the Ultraviolet-bright Star III-60 in the Globular Cluster NGC 6723 Dixon, William V. Solar and Stellar Astrophysics Astrophysics of Galaxies 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 $λ1240$ 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. |
| title | Observations of the Ultraviolet-bright Star III-60 in the Globular Cluster NGC 6723 |
| topic | Solar and Stellar Astrophysics Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2501.01909 |