Fundamental Properties of O-Type Stars
Fundamental Properties of O-Type Stars
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DOI：
10.1086/498635
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发表时间：
2004-12
期刊：
影响因子：
--
通讯作者：
S. Heap;T. Lanz;I. Hubeny
中科院分区：
文献类型：
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作者：
S. Heap;T. Lanz;I. Hubeny
本文综合分析了小麦哲伦星云中18O恒星的高分辨率、远紫外线HST光谱、引信和光谱。我们的分析是基于NLTE金属线覆盖模式大气的OSTAR2002网格，该模式大气是用我们的程序Tlusty计算的。我们系统地研究和展示了各种紫外线和光谱线对不同恒星参数的敏感性。我们得到了紫外线和光谱的一致拟合，从而得出了每颗恒星的有效温度、表面重力、表面成分和微湍流速度。恒星半径、质量和光度紧随其后。对于同一光谱亚型的恒星，我们发现用Tlusty、CMFGEN和FASTWIND模型得到的有效温度测定结果基本上是一致的，它们都低于O星的标准Tef定标。我们提出了一种新的光谱类型和有效温度之间的定标，这是基于我们对紫外金属线以及光学氢线和氦线的结果。较低的有效温度转化为电离光度，与以前的标准校准得出的光度相比，电离光度小了3倍。化学成分分析表明，约80%的程序星表面具有中等到高度的氮富集度，而显示出原始的氦、碳和氧的丰度。我们的结果支持新的恒星演化模型，该模型预测，由于旋转诱导的混合，快速自转恒星的表面在主序列相期间变得富含氮。然而，富集系数比恒星演化模型预测的要大。大多数恒星都存在“质量差异”问题，我们将其解释为快速自转降低了测量到的有效重力。因此，富氮和低光谱质量是快速自转的两种表现形式。因此，我们的研究强调了自转对我们理解大质量恒星性质的重要性，并为研究低金属丰度和高红移的大质量恒星群提供了一个框架。
We present a comprehensive analysis of high-resolution, far-ultraviolet HST STIS, FUSE, and optical spectra of 18 O stars in the Small Magellanic Cloud. Our analysis is based on the OSTAR2002 grid of NLTE metal-line-blanketed model atmospheres calculated with our code TLUSTY. We systematically explore and present the sensitivity of various UV and optical lines to different stellar parameters. We have obtained consistent fits of the UV and the optical spectrum to derive the effective temperature, surface gravity, surface composition, and microturbulent velocity of each star. Stellar radii, masses, and luminosities follow directly. For stars of the same spectral subtype, we find a general good agreement between effective temperature determinations obtained with TLUSTY, CMFGEN, and FASTWIND models, which are all lower than the standard Teff calibration of O stars. We propose a new calibration between the spectral type and effective temperature based on our results from UV metal lines, as well as optical hydrogen and helium lines. The lower effective temperatures translate into ionizing luminosities that are smaller by a factor of 3 compared to luminosities inferred from previous standard calibrations. The chemical composition analysis reveals that the surface of about 80% of the program stars is moderately to strongly enriched in nitrogen, while showing the original helium, carbon, and oxygen abundances. Our results support the new stellar evolution models that predict that the surface of fast rotating stars becomes nitrogen-rich during the main-sequence phase because of rotationally induced mixing. Enrichment factors are, however, larger than predicted by stellar evolution models. Most stars exhibit the ``mass discrepancy'' problem, which we interpret as a result of fast rotation that lowers the measured effective gravity. Nitrogen enrichment and low spectroscopic masses are therefore two manifestations of fast rotation. Our study thus emphasizes the importance of rotation in our understanding of the properties of massive stars and provides a framework for investigating populations of low-metallicity massive stars at low and high redshifts.