arXiv 1205.44442012-05-20astro-ph.SR

O星中的氮线光谱学-- III。最早的O星

Nitrogen line spectroscopy in O-stars -- III. The earliest O-stars

J. G. Rivero Gonzalez, J. Puls, P. Massey, F. Najarro

备注：: Main paper: 23 pages, 14 figures. Appendix: 20 pages, 20 figures. Accepted by Astronomy & Astrophysics
DOI：: 10.1051/0004-6361/201218955

摘要：

这是旨在分析O型恒星中氮丰度的系列论文中的第三篇，以进一步限制大质量恒星的早期演化。我们提供了第一个理论预测的NIV 4058/NIII 4640发射线的比例依赖于各种参数，并面对他们从早期LMC/SMC O-星样本的分析结果。恒星和风的参数确定的H/He/N线的线轮廓拟合，利用氦和氮电离平衡。使用NLTE大气代码FASTWIND计算相应的合成光谱。虽然有一个单调的关系之间的发射线比和Teff，所有其他参数是相等的，理论预测表明额外的依赖关系，最值得注意的是，对氮丰度。这些基本的预测证实了大气代码CMFGEN的结果。从氮电离平衡推断，最早的O型星的有效温度比以前的研究表明的要高得多。与早期的结果一致，有效温度从超巨星到矮星在LMC的所有光谱类型增加。对于给定的光度类，观测到的NIV 4058/NIII 4640发射线比与Teff之间的关系对于我们的样本恒星来说是相当单调的，并且与我们的模型预测相当一致。光谱子型内的散射主要由丰度效应产生。我们的研究结果表明，Walborn等人（2002）的分类方案能够提供光谱类型和Teff之间的有意义的关系，只要有可能区分光度类别。这在低Z环境（如SMC）中可能难以实现，因为风强度相当低。根据我们的预测，分类方案的主要偏差是由于氮含量。

英文摘要：

This is the third paper in a series aiming at the analysis of nitrogen abundances in O-type stars, to enable further constraints on the early evolution of massive stars. We provide first theoretical predictions for the NIV4058/NIII4640 emission line ratio in dependence of various parameters, and confront them with results from the analysis of a sample of early LMC/SMC O-stars. Stellar and wind parameters are determined by line profile fitting of H/He/N lines, exploiting the helium and nitrogen ionization balance. Corresponding synthetic spectra are calculated using the NLTE atmospheric code FASTWIND. Though there is a monotonic relationship between the emission line ratio and Teff, all other parameters being equal, theoretical predictions indicate additional dependencies, most notably, on the nitrogen abundance. These basic predictions are confirmed by results from atmospheric code CMFGEN. The effective temperatures for the earliest O-stars, inferred from the nitrogen ionization balance, are partly considerably hotter than indicated by previous studies. Consistent with earlier results, effective temperatures increase from supergiants to dwarfs for all spectral types in the LMC. The relation between observed NIV4058/NIII4640 emission line ratio and Teff, for a given luminosity class, turned out to be quite monotonic for our sample stars, and fairly consistent with our model predictions. The scatter within a spectral sub-type is mainly produced by abundance effects. Our findings suggest that the Walborn et al. (2002) classification scheme is able to provide a meaningful relation between spectral type and Teff, provided that it is possible to discriminate for the luminosity class. This might be difficult to achieve in low-Z environments such as the SMC, owing to rather low wind-strengths. According to our predictions, the major bias of the classification scheme is due to nitrogen content.

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