Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency
Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency
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DOI：
10.1093/mnras/stw637
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发表时间：
2015-11
影响因子：
4.8
通讯作者：
Y. Sakurai;E. Vorobyov;T. Hosokawa;N. Yoshida;K. Omukai;H. Yorke
中科院分区：
文献类型：
--
作者：
Y. Sakurai;E. Vorobyov;T. Hosokawa;N. Yoshida;K. Omukai;H. Yorke
SMSs的形成是在早期宇宙中播种SMBHs的潜在途径。形成SMSs的一个关键问题是恒星紫外线反馈，这可能会限制恒星通过吸积的质量增长。本文研究了自引力环盘在实际变吸积条件下的吸积SMS及其紫外发射率的演化。首先，我们进行了二维流体动力学模拟，以跟踪长期的原恒星吸积，直到恒星质量超过$10^4~M_\odot$。由于引力不稳定，圆盘碎片形成了许多小团块，它们迅速向内迁移，落在恒星上。由此产生的吸积历史是高度依赖时间的：短暂的偶发性吸积爆发之后是较长的相对静止的阶段。我们表明，所谓的直接坍缩模型的星周盘更不稳定，并且在较短的时间尺度上产生比正常的Pop III病例更大的变异性。我们利用获得的吸积历史进行后处理恒星演化计算。我们的结果表明，尽管吸积速率具有很强的可变性，但随着恒星质量的增加，恒星半径单调增加，有效温度在$T_{\rm eff} \simeq 5000$ K处几乎恒定。由于恒星紫外光子的低通量，由此产生的紫外反馈太弱而无法阻碍质量吸积，从而验证了我们在流体动力学模拟过程中没有恒星反馈的隐含假设。恒星演化对可变吸积的不敏感归因于这样一个事实：典型的变率时间尺度（$\lesssim 10^3$年）太短，无法影响恒星结构。我们认为，在恒星质量达到$\gtrsim 10^5~M_\odot$后，这种演化将继续下去，直到SMS最终在广义相对论不稳定性下坍缩产生一个巨大的黑洞。
The formation of SMSs is a potential pathway to seed SMBHs in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper we study the evolution of an accreting SMS and its UV emissivity under conditions of realistic variable accretion from a self-gravitating circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the long-term protostellar accretion until the stellar mass exceeds $10^4~M_\odot$. The disc fragments due to gravitational instability, creating a number of small clumps that rapidly migrate inward to fall onto the star. The resulting accretion history is thus highly time-dependent: short episodic accretion bursts are followed by longer, relative quiescent phases. We show that the circumstellar disc for the so-called direct collapse model is more unstable and generates greater variability over shorter timescales than normal Pop III cases. We conduct a post-process stellar evolution calculation using the obtained accretion history. Our results show that, regardless of the strong variability of the accretion rates, the stellar radius monotonically increases with almost constant effective temperature at $T_{\rm eff} \simeq 5000$ K as the stellar mass increases. The resulting UV feedback is too weak to hinder mass accretion due to the low flux of stellar UV photons, thus verifying our implicit assumption of no stellar feedback during the hydrodynamic simulations. The insensitivity of stellar evolution to variable accretion is attributed to the fact that typical timescales of variability, $\lesssim 10^3$ years, are too short to affect the stellar structure. We argue that this evolution will continue until the SMS eventually collapses to produce a massive black hole by the general relativistic instability after the stellar mass reaches $\gtrsim 10^5~M_\odot$.