[论文解读] Photoevaporation and close encounters: how the environment around Cygnus OB2 affects the evolution of protoplanetary disks

M. G. Guarcello, J. J. Drake+11 more|arXiv (Cornell University)|May 5, 2016

Astrophysics and Star Formation Studies被引用 41

一句话总结

本研究探讨了天鹅座OB2大质量星协的严酷环境如何通过光致蒸发和近距离相遇加速原行星盘的消散。通过将空间分辨的盘分数测量结果与局部紫外辐射及恒星密度相关联，作者发现，在O型星周围0.5 pc范围内，远紫外（FUV）辐射主导了盘的侵蚀；而极端紫外（EUV）辐射则在星协范围内驱动了广泛的盘损失，星际物质的遮蔽作用解释了盘分数的平滑下降。近距离相遇导致的盘破坏可忽略不计，仅影响≤1%的盘，证实光致蒸发是主导的反馈机制。

ABSTRACT

In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cygnus OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun, and hosts hundreds of massive stars and thousands of low mass members. In this paper, we analyze the spatial variation of the disk fraction in Cygnus OB2 and we study its correlation with the local values of Far and Extreme ultraviolet radiation fields and the local stellar surface density. We present definitive evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV field and large stellar density. In particular, the FUV radiation dominates disks dissipation timescales in the proximity (i.e. within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars due to absorption by the intervening intracluster material. We find that disk dissipation due to close stellar encounters is negligible in Cygnus OB2, and likely to have affected 1% or fewer of the stellar population. Disk dissipation is instead dominated by photoevaporation. We also compare our results to what has been found in other young clusters with different massive populations, concluding that massive associations like Cygnus OB2 are potentially hostile to protoplanetary disks, but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.

研究动机与目标

• 理解像天鹅座OB2这样的大质量星协中的环境反馈如何影响原行星盘的演化及消散 timescale。
• 确定光致蒸发（FUV和EUV）与近距离恒星相遇对密集大质量星团中盘侵蚀的相对贡献。
• 评估天鹅座OB2中盘分数的空间变化是否由环境因素引起，而非观测偏差或内部恒星形成触发。
• 将天鹅座OB2中的盘存活率与其他年轻星团进行比较，并推断银河系中盘安全环境的普遍性。

提出的方法

• 基于钱德拉天鹅座OB2遗产巡天数据，通过识别具有盘和不具盘的星团成员，获得了空间分辨的盘分数测量结果。
• 基于靠近O型星的距离，并利用恒星群体合成模型，估算局部远紫外（FUV）和极端紫外（EUV）辐射场。
• 计算每个空间像素单元的恒星面密度，以评估动力学相遇在盘破坏中的作用。
• 通过统计检验盘分数与局部环境参数（紫外通量、恒星密度）之间的相关性，排除投影效应或灵敏度不均一的影响。
• 应用光致蒸发模型，估算在FUV和EUV照射下盘侵蚀的时间尺度，同时考虑中间物质的遮蔽作用。
• 通过估算相遇率和盘破坏截面，量化近距离恒星相遇的贡献，结果表明在观测区域内影响可忽略。

实验结果

研究问题

• RQ1 在天鹅座OB2中，导致盘分数空间下降的主导环境机制是什么？
• RQ2 大质量恒星产生的FUV和EUV辐射场如何影响星协不同区域中原行星盘消散的时间尺度？
• RQ3 在天鹅座OB2中，近距离恒星相遇对盘侵蚀的贡献有多大？与光致蒸发相比是否具有显著影响？
• RQ4 遮蔽物质（如致密云团成分）的存在如何影响天鹅座OB2中盘的观测分布？
• RQ5 像天鹅座OB2这样的大质量星协通常对盘演化具有敌意，还是存在足够长的盘可存活以支持行星形成的区域？

主要发现

• 天鹅座OB2中的盘分数从低紫外区域的~40%平滑下降至高紫外、高密度环境的~18%，表明其强烈依赖于环境因素。
• 在O型星周围约0.5 pc范围内，远紫外（FUV）辐射主导了盘的侵蚀，且靠近大质量恒星时，光致蒸发时间尺度显著缩短。
• 极端紫外（EUV）辐射强度足以在星协整个范围内驱动盘质量损失，但其效率随距离增加而下降，这是由于中间物质的吸收作用。
• 观测到的盘分数平滑下降与残余云物质的遮蔽作用一致，后者吸收EUV光子，降低了远距离处的电离程度。
• 近距离恒星相遇对盘破坏的贡献可忽略不计，仅影响高密度区域中≤1%的盘群体，排除了动力学破坏作为主要机制的可能性。
• 尽管环境严酷，外区及稀疏星团中仍有大量盘幸存，表明银河系中存在普遍的盘安全环境，有利于行星形成。