Rotating massive main-sequence stars II. Simulating a population of LMC early B-type stars as a test of rotational mixing
Rotating massive main-sequence stars II. Simulating a population of LMC early B-type stars as a test of rotational mixing
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DOI：
10.1051/0004-6361/201016114
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发表时间：
2011-06-01
影响因子：
6.5
通讯作者：
Anders, P.
中科院分区：
文献类型：
--
作者：
Brott, I.;Evans, C. J.;Anders, P.
上下文大质量恒星中的旋转混合是一个应用广泛的概念，对恒星演化、核合成和恒星爆炸都有着深远的影响。最近，ESO VLT-FLAMES大质量星巡天获得了大麦哲伦星云（LMC）中大质量B型星的大样本和均匀样本的氮表面丰度。这个样本是第一个覆盖广泛的投影恒星旋转速度的样本，具有足够大的高质量数据样本，可以进行统计学上显著的分析。在这里，我们使用的样本提供了第一个严格的和定量的测试大质量恒星的旋转混合理论。我们计算了恒星演化模型的网格，使用VLT-FLAMES样本来校准一些不确定的混合过程。我们开发了一个新的人口合成代码，它使用这个网格来模拟一个大的人口与质量，年龄和旋转速度分布与VLT-火焰样本一致的恒星。然后通过观察样本中的选择效应过滤合成的群体，以使经验结果和理论预测之间的直接比较成为可能。我们的模拟再现了恒星的分数没有显着的氮富集。然而，预测的数量快速旋转增强氮是大约两倍的观测发现。此外，两组恒星（一组由缓慢旋转的富氮天体组成，另一组由快速旋转的非富氮天体组成）不能通过我们的单星星族合成来复制。除了旋转混合的物理过程似乎需要了解人口的大规模主序星的VLT火焰样本。我们讨论了双星和磁场在解释我们的结果可能发挥的作用。我们发现，人口缓慢旋转的富氮恒星是不太可能产生的质量转移和随后的潮汐自旋关闭双星系统。对大质量恒星中旋转混合作用的结论性评估需要定量分析，同时也要考虑双星和磁场的影响。
Context. Rotational mixing in massive stars is a widely applied concept, with far-reaching consequences for stellar evolution, nucleosynthesis, and stellar explosions.Aims. Nitrogen surface abundances for a large and homogeneous sample of massive B-type stars in the Large Magellanic Cloud (LMC) have recently been obtained by the ESO VLT-FLAMES Survey of Massive Stars. This sample is the first to cover a broad range of projected stellar rotational velocities, with a large enough sample of high quality data to allow for a statistically significant analysis. Here we use the sample to provide the first rigorous and quantitative test of the theory of rotational mixing in massive stars.Methods. We calculated a grid of stellar evolution models, using the VLT-FLAMES sample to calibrate some of the uncertain mixing processes. We developed a new population-synthesis code, which uses this grid to simulate a large population of stars with masses, ages, and rotational velocity distributions consistent with those from the VLT-FLAMES sample. The synthesized population is then filtered by the selection effects in the observed sample, to enable a direct comparison between the empirical results and theoretical predictions.Results. Our simulations reproduce the fraction of stars without significant nitrogen enrichment. However, the predicted number of rapid rotators with enhanced nitrogen is about twice as large as found observationally. Furthermore, two groups of stars (one consisting of slowly rotating, nitrogen-enriched objects and another consisting of rapidly rotating un-enriched objects) cannot be reproduced by our single-star population synthesis.Conclusions. Physical processes in addition to rotational mixing appear to be required to understand the population of massive main-sequence stars from the VLT-FLAMES sample. We discuss the possible role of binary stars and magnetic fields in the interpretation of our results. We find that the population of slowly rotating nitrogen-enriched stars is unlikely to be produced via mass transfer and subsequent tidal spin-down in close binary systems. A conclusive assessment of the role of rotational mixing in massive stars requires a quantitative analysis that also accounts for the effects of binarity and magnetic fields.