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arXiv 2501.01909astro-ph.SRastro-ph.GA
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球状星团 NGC 6723 中紫外亮星 III-60 的观测

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

William V. Dixon

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

本文通过FUSE和COS远紫外光谱分析球状星团NGC 6723中紫外亮星III-60，确认其光球参数与光学结果一致，并揭示其氮增丰、碳氧贫乏及恒星并合起源，演化路径类似低碳中等富氦热亚矮星。

摘要：

我们分析了球状星团 NGC 6723 中紫外亮星 III-60 的档案远紫外光谱，这些光谱由远紫外光谱探测器（FUSE）和宇宙起源光谱仪（COS）获得。我们发现该星的光球参数（有效温度 $T_{\rm eff} = 44{,}800 \pm 1200$，表面重力 $\log g = 4.89 \pm 0.18$，氦丰度 $\log N({\rm He})/N({\rm H}) = -0.84 \pm 0.29$）与其光学光谱推导出的数值一致，表明对于有效温度 $T_{\rm eff} \lesssim$ 50,000 K 的演化恒星，光学推导的数值通常是准确的。相对于该星团的红巨星支恒星，III-60 的氮元素增强，碳和氧元素贫乏。该星在 N V $λ1240$ 双线的两个分量中都表现出强烈的 P Cygni 轮廓，但其他元素的共振线没有显示恒星风的证据。该星的有效温度和光度使其位于从蓝水平支演化的恒星的演化轨迹上，但其高质量（$\sim 1.2 \\, M_{\odot}$）表明它是恒星并合的产物。其氦、碳和氮丰度表明它正沿着与低碳、中等富氦热亚矮星相似的演化路径前进。

英文摘要：

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.

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