[论文解读] The VLT-FLAMES Tarantula Survey IV: Candidates for isolated high-mass star formation in 30 Doradus

E. Bressert, N. Bastian+16 more|arXiv (Cornell University)|Apr 16, 2012

Astrophysics and Star Formation Studies参考文献 88被引用 41

一句话总结

本文提出一种新方法，通过径向速度稳定性、空间密度约束以及与星际丝状结构的关联，识别30 Doradus区域中可能孤立形成的O型星。作者识别出15个强候选体，其中包括11颗Vz星，为大质量恒星形成中单体坍缩路径的可行性提供了关键证据。

ABSTRACT

Whether massive stars can occasionally form in relative isolation or if they require a large cluster of lower-mass stars around them is a key test in the differentiation of star formation theories as well as how the initial mass function of stars is sampled. Previous attempts to find O-type stars that formed in isolation were hindered by the possibility that such stars are merely runaways from clusters, i.e., their current isolation does not reflect their birth conditions. We introduce a new method to find O-type stars that are not affected by such a degeneracy. Using the VLT-FLAMES Tarantula Survey and additional high resolution imaging we have identified stars that satisfy the following constraints: 1) they are O-type stars that are not detected to be part of a binary system based on RV time series analysis; 2) they are designated spectral type O7 or earlier ; 3) their velocities are within 1σof the mean of OB-type stars in the 30 Doradus region, i.e. they are not runaways along our line-of-sight; 4) the projected surface density of stars does not increase within 3 pc towards the O-star (no evidence for clusters); 5) their sight lines are associated with gaseous and/or dusty filaments in the ISM, and 6) if a second candidate is found in the direction of the same filament with which the target is associated, both are required to have similar velocities. With these criteria, we have identified 15 stars in the 30 Doradus region, which are strong candidates for being high-mass stars that have formed in isolation. Additionally, we employed extensive MC stellar cluster simulations to confirm that our results rule out the presence of clusters around the candidates. Eleven of these are classified as Vz stars, possibly associated with the zero-age main sequence. We include a newly discovered W-R star as a candidate, although it does not meet all of the above criteria.

研究动机与目标

• 区分孤立大质量恒星形成与星团中逃逸的恒星，解决大质量恒星形成理论中的一个关键歧义。
• 克服识别真正孤立O型星的歧义，这些恒星可能因从星团中被弹射而出而看似孤立。
• 检验大质量恒星是否能在没有周围低质量恒星星团的情况下形成，这与单体坍缩模型的预测一致。
• 为不同恒星形成情景下的初始质量函数采样提供观测约束。
• 通过蒙特卡洛模拟验证候选恒星周围不存在隐藏星团。

提出的方法

• 利用VLT-FLAMES Tarantula巡天的径向速度时间序列，识别不在双星系统中的O型星。
• 通过空间密度分析，确认候选恒星周围3 pc范围内无显著恒星过密。
• 要求候选恒星的径向速度在30 Doradus中OB星平均速度的1σ范围内，以排除逃逸星。
• 要求候选恒星与星际介质中的气体和/或尘埃丝状结构在视线方向上相关联，表明其潜在的形成环境。
• 应用双候选一致性检验：若两个候选体位于同一条丝状结构上，则其径向速度必须相近。
• 采用蒙特卡洛恒星星团模拟，从统计上排除候选体周围存在未探测到的低质量星团的可能性。

实验结果

研究问题

• RQ1 大质量O型星是否能以孤立方式形成，而无需作为低质量恒星星团的一部分？
• RQ2 30 Doradus中O型星观测到的空间和运动学特性在多大程度上可排除其起源于星团逃逸的可能性？
• RQ3 哪些观测标准可可靠地区分真正的孤立形成与因弹射导致的表观孤立？
• RQ4 候选恒星的空间分布及其星际介质环境如何支持或反驳单体坍缩与竞争吸积模型？
• RQ5 O型星与丝状结构对齐的统计可能性有多大，是偶然导致而非物理关联？

主要发现

• 15颗O型星在30 Doradus中满足全部六项筛选标准，被确定为孤立大质量恒星形成的强候选体。
• 15个候选体中的11个被分类为Vz星，表明其位于或接近零龄主序，与近期形成一致。
• 通过二项分布概率建模（假设随机分布）确认，逃逸星与丝状结构偶然对齐的概率较低。
• 蒙特卡洛模拟证实，候选体周围不太可能存在隐藏的低质量恒星星团，支持其孤立性。
• 尽管未满足所有标准，但新发现的一颗沃尔夫-拉叶星仍被列为候选体，因其光谱特征和与星际介质的关联。
• 结果挑战了竞争吸积模型，支持单体坍缩情景作为大质量恒星形成的一种可行机制。