[论文解读] The NGC 346 massive star census: Nitrogen abundances for apparently single, narrow lined, hydrogen core burning B-type stars

P. L. Dufton, C. J. Evans+2 more|arXiv (Cornell University)|Jan 1, 2020

Stellar, planetary, and galactic studies参考文献 99被引用 12

一句话总结

本研究调查了大麦哲伦云NGC 346星团中61颗表观单星、氢核心燃烧的B型星的氮丰度，发现具有低投影旋转速度（ve sin i ≤40 km s−1）的氮富集星存在显著过剩，与包含旋转混合的单星演化模型不一致。该过剩估计约占所有单B型星的~5%，或占ve sin i <40 km s−1星体的~40%，表明存在磁场所致或双星并合等替代机制，且两种机制均与大麦哲伦云中的观测频率相容。

ABSTRACT

Analyses of large spectroscopic surveys of early-type stars in the LMC have found an excess of nitrogen enriched B-type targets with a vsini<40 km/s compared with the predictions of single star evolutionary models incorporating rotational mixing. By contrast the number of such targets with 40<vsini<80 km/s were consistent with such models. We have undertaken a similar analysis for 61 B-type similar targets, towards the young cluster, NGC 346 in the Small Magellanic Cloud (SMC). Approximately 65% could have nitrogen enhancements of less than 0.3 dex, consistent with them having experienced only small amounts of mixing due to their low rotational velocities. However as with the previous LMC surveys, an excess of stars with low projected rotational velocities, vsini<40 km/s and significant nitrogen enrichments is found. This is estimated to be approximately 5% of the total population of apparently single B-type stars or 40% of all stars with current rotational velocities of less than 40 km/s. For all three surveys, the presence of undetected binaries and other uncertainties imply that these percentages might be underestimated and indeed it is possible that all single stars with current rotational velocities of less than 40 km/s are nitrogen enriched. Possible explanations incorporate the effects of magnetic fields, via either a stellar merger followed by magnetic breaking or the evolution of a single star with a large magnetic field. Both mechanisms are compatible with the observed frequency of nitrogen-enriched stars in the Magellanic Clouds. Differences in the properties of the nitrogen-enriched stars compared with the remainder of the sample would be consistent with the former mechanism. For the latter, a qualitative comparison with evolutionary models incorporating magnetic fields is encouraging in terms of the amount of nitrogen enrichment and its presence in stars near the ZAMS.

研究动机与目标

• 确定大麦哲伦云NGC 346中表观单星、氢核心燃烧的B型星的氮丰度，其投影旋转速度较低（ve sin i ≤80 km s−1）。
• 检验具有ve sin i ≤40 km s−1的氮富集星数量是否显著超过包含旋转混合的单星演化模型的预测值。
• 评估低速B型星中氮富集的频率，并判断其是否暗示旋转混合系统性缺失，或需要磁场所致或双星并合等替代机制。
• 将结果与先前的LMC巡天（FSMS和VFTS）进行比较，以识别大麦哲伦云中的一致性模式。

提出的方法

• 对NGC 346中61颗B型星的高分辨率FLAMES/GIRAFFE数据（R ≈ 20,000）进行光谱分析，波段覆盖3854–4760 Å。
• 采用中值或σ裁剪算法对HR02光谱进行重组合，以提升信噪比和光谱质量。
• 通过轮廓拟合估算投影旋转速度（ve sin i），数据来源为Hunter et al. (2008b) 和 Dufton et al. (2019)。
• 利用3995 Å处的N ii谱线测定氮丰度，并对混合效应和大气参数进行校正。
• 将观测到的氮富集星频率与包含旋转混合的单星演化模型预测值进行比较。
• 利用计算机模拟建模在低倾角下观测到快速旋转星的预期数量，以检验与观测数据的一致性。

实验结果

研究问题

• RQ1 观测到的具有ve sin i ≤40 km s−1的氮富集B型星数量是否显著高于包含旋转混合的单星演化模型的预测值？
• RQ2 在表观单星B型星中，ve sin i <40 km s−1的氮富集星所占比例是多少？与模型预测值相比如何？
• RQ3 NGC 346中氮富集星的特性是否与双星并合后磁致制动或强磁场单星演化机制相容？
• RQ4 SMC NGC 346巡天结果与先前LMC巡天（FSMS和VFTS）相比，在低速B型星的氮富集频率方面是否存在一致性？
• RQ5 未探测到的双星或观测极限是否可能导致对低速B型星中氮富集真实频率的低估？

主要发现

• 样本中61颗SMC B型星约65%的氮增强量小于0.3 dex，与低旋转速度下旋转混合微弱的现象一致。
• 观测到具有ve sin i ≤40 km s−1的氮富集星存在过剩，估计约占样本中所有表观单B型星的~5%。
• 该过剩约占所有ve sin i <40 km s−1星体的~40%，其频率与先前两项LMC巡天中观测到的频率相似。
• 低速组中氮富集星的观测频率与包含旋转混合的单星模型预测值不一致，尤其在低速组中表现明显。
• 结果表明，由于未探测到的双星和观测限制，低速B型星中氮富集的真实频率可能被低估，甚至可能存在所有此类星体均为氮富集的可能性。
• 两种机制——双星并合后磁致制动，或强磁场单星演化——均与大麦哲伦云中观测到的氮富集星频率相容。