低金属丰度星系 NGC 6822 中沃尔夫-拉叶星的不同星族：完整样本的光学和紫外光谱定量分析
Distinct Population of Wolf-Rayet Stars in the Low-Metallicity Galaxy NGC 6822. Quantitative analysis of optical and UV spectra of the complete sample
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中文总结 AI 辅助
通过定量光谱分析 NGC 6822 中全部四颗 WR 星，发现其星族与 SMC 不同，并首次在低金属丰度下发现无氢 WN 星及一颗低光度 qWR 星。
中文摘要 AI 辅助
背景：沃尔夫-拉叶（WR）星在低金属丰度星系中尤为罕见，且对其认识仍十分有限。对邻近小麦哲伦云（SMC）星系中据信完整的 WR 星族进行的详细研究得出了令人困惑的结果。目的：邻近星系 NGC 6822 的金属丰度与 SMC 相似，因此为比较研究提供了绝佳的实验室。我们旨在通过定量光谱学方法，推导出 NGC 6822 中每颗已知 WR 星的恒星参数和星风参数，并将结果与 SMC 的 WR 星族进行比较。方法：我们利用甚大望远镜（VLT）和哈勃空间望远镜（HST）获得了新的光学和紫外光谱观测数据。我们采用波茨坦沃尔夫-拉叶（PoWR）模型大气代码进行分析，将合成光谱与观测到的光谱能量分布、光学光谱和紫外光谱进行一致性拟合。结果：我们通过光谱学证实，样本中所有四颗星均属于氮序列，且为早型子型（WN3-WN6）。所有恒星均显示出双星性的迹象。样本中仅有一颗恒星的大气中氢含量较高，这与 SMC 中 WN 星族的典型特征相符，而另外两颗恒星则不含氢。最后，第四颗恒星的光谱和参数与 HD 45166 双星系统中 qWR 型强磁合并产物恒星极为相似。结论：我们样本的恒星性质多样。我们首次在金属丰度约为 1/7 太阳金属丰度（Z☉）的条件下发现了无氢 WN 型恒星。根据演化计算，NGC 6822 中的 WR 星起源于初始质量低于 40 倍太阳质量（M☉）的前身星。我们报告了在 NGC 6822 中发现一颗低光度剥离型 qWR 恒星，并得出结论：尽管宿主星系的金属丰度相似，但 NGC 6822 与 SMC 中的 WR 星族是截然不同的。
英文摘要
Context: Wolf-Rayet (WR) stars are particularly rare in low-metallicity galaxies and remain poorly understood. Detailed studies of the presumably complete population of WR stars in the nearby Small Magellanic Cloud (SMC) galaxy led to perplexing results. Aims: The nearby galaxy NGC 6822 has a similar metallicity to the SMC, and hence provides an excellent laboratory for a comparative study. We aim to derive the stellar and wind parameters of each known WR star in NGC 6822 by means of quantitative spectroscopy. The results are compared with the WR population of the SMC. Methods: New optical and UV spectroscopic observations have been obtained with the Very Large Telescope (VLT) and Hubble Space Telescope (HST). We employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code for our analysis, consistently fitting synthetic spectra to the observed spectral energy distribution, and optical and UV spectra. Results: We spectroscopically confirm all four stars in our sample belong to the nitrogen sequence with early subtypes (WN3-WN6). All stars show indications of binarity. Only one star in our sample has a high fraction of hydrogen in its atmosphere, as typical for the WN population in the SMC, and two other stars in our sample are hydrogen-free. Finally, the spectrum and parameters of the fourth object closely resemble those of the qWR-type strongly magnetic merger-product star in the HD 45166 binary. Conclusions: The stellar properties of our sample are diverse. We have identified the first H-free WN-type stars at metallicities $\sim$ 1/7Z$_\odot$. According to evolutionary calculations, the WR stars in NGC 6822 stem from progenitors with initial masses below 40 M$_\odot$. We report a discovery of a low luminosity stripped qWR star in NGC 6822, and conclude that the WR star population in NGC 6822 and the SMC are distinct despite similar metallicities of their host galaxies.
发表机构
- Universität Potsdam（波茨坦大学）
- KU Leuven（荷语鲁汶大学）
- Tel Aviv University（特拉维夫大学）
- Universität Heidelberg（海德堡大学）
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