[论文解读] Oxygen and silicon abundances in Cygnus OB2: Chemical homogeneity in a sample of OB slow rotators

S. R. Berlanas, A. Herrero+4 more|arXiv (Cornell University)|Sep 18, 2018

Stellar, planetary, and galactic studies参考文献 48被引用 9

一句话总结

本研究利用高分辨率光谱和FASTWIND模型，分析了天鹅座OB2星协中八颗早型、慢速旋转恒星的硅和氧丰度。研究发现化学成分高度均一，12 + log(O/H) = 8.65 ± 0.12 dex，12 + log(Si/H) = 7.53 ± 0.08 dex，表明尽管经历了强烈的恒星形成和大质量恒星演化，但恒星风或超新星并未显著自富集，对原始分子云的化学组成影响甚微。

ABSTRACT

Cygnus OB2 is a rich OB association in the Galaxy which has experienced intense star formation in the last 20-25 Myr. Its stellar population shows a correlation between age and Galactic longitude. Exploring the chemical composition of its stellar content we will be able to check the degree of homogeneity of the natal molecular cloud and possible effects of self-enrichment processes. Our aim is to determine silicon and oxygen abundances for a sample of eight early-type slow rotators in Cygnus OB2 in order to check possible inhomogeneities across the whole association and whether there exists a correlation of chemical composition with Galactic longitude. We have performed a spectroscopic analysis of a sample of late O and early B stars with low rotational velocity, which have been chosen so as to cover the whole association area. We have carried out an analysis based on equivalent widths of metal lines, the wings of the H Balmer lines and FASTWIND stellar atmosphere models to determine their stellar fundamental parameters as well as the silicon and oxygen surface abundances. We derive a rather homogeneous distribution of silicon and oxygen abundances across the region, with average values of 12+log(Si/H)=7.53$\pm$0.08 dex and 12+log(O/H)=8.65$\pm$0.12 dex. We find a homogeneous chemical composition in Cygnus OB2 with no clear evidence for significant chemical self-enrichment, despite indications of strong stellar winds and possible supernovae during the history of the region. Comparison with different scenarios of chemical enrichment by stellar winds and supernovae point to star forming efficiencies not significantly above 10%. The degree of homogeneity that we find is consistent with the observed Milky Way oxygen gradient based on HII regions. We also find that the oxygen scatter within Cygnus OB2 is at least of the same order than among HII regions at similar Galactocentric distance.

研究动机与目标

• 研究天鹅座OB2中化学不均一性，该星协为大质量、邻近的OB星协，已持续强烈恒星形成20–25 Myr。
• 检验来自恒星风和核心坍缩超新星的自富集是否显著改变了原始分子云的化学组成。
• 将天鹅座OB2中观测到的丰度离散度与银河系H ii区及其他星协（如猎户座OB1）进行比较。
• 基于观测到的氧和硅丰度，约束大质量星协中的恒星形成效率和富集水平。
• 评估天鹅座OB2的化学组成是否与银河系的径向氧丰度梯度一致。

提出的方法

• 利用威廉·赫歇尔望远镜（WHT）上的ISIS光谱仪，获取了天鹅座OB2中八颗晚期O型和早期B型恒星的高信噪比、高分辨率光谱。
• 对Si ii、iii、iv和氧的谱线进行等效宽度（EW）测量，以推导元素丰度。
• 使用FASTWIND恒星大气模型，通过匹配观测到的H巴尔末线轮廓以及Si II-III/IV线等效宽度比值，确定有效温度（Teff）和表面重力（log g）。
• 开展微湍流分析，以确保每颗恒星的多个谱线间丰度一致。
• 采用优化微湍流的生长曲线法计算最终丰度。
• 绘制丰度变化图，作为银河经度和银河系中心距离的函数，以评估空间上的不均一性。

实验结果

研究问题

• RQ1 天鹅座OB2星协中是否存在可测量的硅和氧丰度化学不均一性？
• RQ2 天鹅座OB2中观测到的丰度分布是否显示出大质量恒星风或核心坍缩超新星自富集的证据？
• RQ3 天鹅座OB2中的丰度离散度与同银河系中心距离的银河系H ii区相比如何？
• RQ4 考虑到观测到的化学富集缺失，天鹅座OB2的恒星形成效率在多大程度上超过10%？
• RQ5 天鹅座OB2的化学组成是否与由H ii区追踪的银河系径向氧丰度梯度一致？

主要发现

• 天鹅座OB2的平均氧丰度为12 + log(O/H) = 8.65 ± 0.12 dex，与银河系的径向氧梯度一致，且与猎户座OB1及太阳邻近区的观测值相符。
• 平均硅丰度为12 + log(Si/H) = 7.53 ± 0.08 dex，表明该星协内化学成分高度均一。
• 天鹅座OB2中观测到的丰度离散度与同银河系中心距离的银河系H ii区中的离散度处于同一数量级。
• 未发现化学组成与银河经度之间存在显著相关性，支持该区域的化学均一性。
• 缺乏可测量的富集表明，天鹅座OB2的恒星形成效率并未显著高于10%，自富集过程的影响有限。
• 结果与恒星风或超新星未造成强烈化学污染的假设一致，尽管该区域存在大质量演化恒星和高能现象的证据。