OB stars at the lowest Local Group metallicity: GTC-OSIRIS observations of Sextans A
OB stars at the lowest Local Group metallicity: GTC-OSIRIS observations of Sextans A
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DOI：
10.1051/0004-6361/201425533
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发表时间：
2015-10
期刊：
影响因子：
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通讯作者：
I. Camacho;I. Camacho;Miriam García;A. Herrero;A. Herrero;S. Simón-Díaz;S. Simón-Díaz
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文献类型：
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作者：
I. Camacho;I. Camacho;Miriam García;A. Herrero;A. Herrero;S. Simón-Díaz;S. Simón-Díaz
我们的目标是在六分星A中找到OB恒星并进行分类，然后确定这些低金属丰度区域中蓝色大质量恒星的精确恒星参数$(Z \sim 0.1 \rm Z_{\odot})$。利用UBV光度法、无变红指数Q和GALEX成像技术，建立了六分星a中蓝色大质量候选恒星的列表，获得了部分候选恒星的低分辨率（R $\sim$ 1000） GTC-OSIRIS光谱，并进行了光谱分类。对于已确认的o型星，我们导出了初步的恒星参数。目标选择标准和观测是成功的，并产生了六分星a中ob型恒星的第一个光谱图谱。从整个观测到的18颗恒星样本中，有12颗被归类为早期ob型，其中包括5颗o型恒星。所有目标恒星的径向速度都与它们的六分星A成员一致，尽管其中三个显示出明显的偏差。我们使用恒星大气代码FASTWIND确定了o型恒星的恒星参数，并重新考察了亚smc温度尺度。其中两颗o型恒星与相对强劲的风和更高的氦丰度相一致，尽管结果还不是决定性的。我们讨论了OB星在HRD中的位置。初始恒星的质量从略低于20倍到40倍太阳质量。目标选择方法在Sextans A中效果良好，证实了Garcia ＆ Herrero（2013）中开发的程序。恒星的温度与其他星系的发现一致。其中一些目标值得后续光谱分析，因为有迹象表明它们的性质失控、氦丰度增加或风力相对较强。我们观察到HI和OB之间的关联类似于不规则星系IC1613，证实了之前的结果，即六分星a最近的恒星形成目前正在H\， {\scI}腔的边缘附近进行。
Our aim is to find and classify OB stars in Sextans A, to later determine accurate stellar parameters of these blue massive stars in this low metallicity region $(Z \sim 0.1 \rm Z_{\odot})$. Using UBV photometry, the reddening-free index Q and GALEX imaging, we built a list of blue massive star candidates in Sextans A. We obtained low resolution (R $\sim$ 1000) GTC-OSIRIS spectra for a fraction of them and carried out spectral classification. For the confirmed O-stars we derive preliminary stellar parameters. The target selection criteria and observations were successful and have produced the first spectroscopic atlas of OB-type stars in Sextans A. From the whole sample of 18 observed stars, 12 were classified as early OB-types, including 5 O-stars. The radial velocities of all target stars are in agreement with their Sextans A membership, although three of them show significant deviations. We determined the stellar parameters of the O-type stars using the stellar atmosphere code FASTWIND, and revisited the sub-SMC temperature scale. Two of the O-stars are consistent with relatively strong winds and enhanced helium abundances, although results are not conclusive. We discuss the position of the OB stars in the HRD. Initial stellar masses run from slightly below 20 up to 40 solar masses. The target selection method worked well for Sextans A, confirming the procedure developed in Garcia \& Herrero (2013). The stellar temperatures are consistent with findings in other galaxies. Some of the targets deserve follow-up spectroscopy because of indications of a runaway nature, an enhanced helium abundance or a relatively strong wind. We observe a correlation between HI and OB associations similar to the irregular galaxy IC1613, confirming the previous result that the most recent star formation of Sextans A is currently on-going near the rim of the H\,{\sc I} cavity.