[论文解读] Numerical Evidence for Dark Star Formation: A Comment on "Weakly Interacting Massive Particle Dark Matter and First Stars: Suppression of Fragmentation in Primordial Star Formation" by Smith et al. 2012, ApJ 761, 154

Paolo Gondolo, Katherine Freese+2 more|arXiv (Cornell University)|Apr 28, 2013

Stellar, planetary, and galactic studies被引用 3

一句话总结

本文认为，Smith 等人（2012）的模拟为暗星形成提供了强有力的数值支持，表明暗物质湮灭可抑制气体碎裂，并在高密度（~10^17 cm⁻³）条件下促成平衡态暗星的形成。模拟结果表明，坍缩过程中形成的吸积核即为暗星，其稳定性由湮灭加热维持，同时在它们周围形成吸积盘；而次级吸积核则在远处形成，从而保留了暗星的暗物质晕及其能量来源。

ABSTRACT

(abridged) This comment is intended to show that simulations by Smith et al. (S12) support the Dark Star (DS) scenario and even remove some potential obstacles. Our previous work illustrated that the initial hydrogen densities of the first equilibrium DSs are high, ~10^{17}/cm^3 for the case of 100 GeV WIMPs, with a stellar radius of ~2-3 AU. Subsequent authors have somehow missed the fact that equilibrium DSs have the high densities they do. S12 have numerically simulated the effect of dark matter annihilation on the contraction of a protostellar gas cloud en route to forming the first stars. They show results at a density ~5 10^{14}/cm^3, slightly higher than the value at which annihilation heating prevails over cooling. However, they are apparently unable to reach the ~10^{17}/cm^3 density of our hydrostatic DS solutions. We are in complete agreement with their physical result that the gas keeps collapsing to densities > 5 10^{14}/cm^3, as it must before equilibrium DSs can form. However we are in disagreement with some of the words in their paper which imply that DSs never come to exist. It seems to us that S12 supports the DS scenario. They use the sink particle approach to treat the gas that collapses to scales smaller than their resolution limit. We argue that their sink is effectively a DS, or contains one. An accretion disk forms as more mass falls onto the sink, and the DS grows. S12 not only confirm our predictions about DS in the range where the simulations apply, but also solve a potential obstruction to DS formation by showing that dark matter annihilation prevents the fragmentation of the collapsing gas. Whereas fragmentation might perturb the dark matter away from the DS and remove its power source, instead S12 show that further sinks, if any, form only far enough away as to leave the DS undisturbed in the comfort of its dark matter surroundings.

研究动机与目标

• 解决文献中关于暗星是否能在气体碎裂担忧下形成的问题上的分歧。
• 证明 Smith 等人的模拟实际上支持暗星情景，与某些解释相反。
• 主张模拟中的吸积核粒子因其高密度和暗物质湮灭的能量沉积，可有效代表暗星。
• 表明暗物质湮灭可稳定吸积盘并防止碎裂，从而保留暗星的暗物质环境。
• 建议未来工作应开展更高分辨率的模拟，并改进对高能暗物质湮灭产物的建模。

提出的方法

• 使用三维流体动力学模拟结合吸积核粒子，以模拟原初星系晕中气体的坍缩。
• 鉴于其高密度（~10^17 cm⁻³）和暗物质湮灭的能量沉积，将吸积核粒子视为有效暗星。
• 采用吸积核粒子方法，设置约 5 AU 的吸积半径，以处理未解析的小尺度动力学。
• 建模 100 GeV WIMP 湮灭产物的能量沉积，包括电磁级联过程及与气体的热化。
• 通过 Toomre 参数 Q > 1 评估盘的稳定性，以判断碎裂是否被抑制。
• 将模拟结果与解析暗星解进行比较，以验证平衡态暗星的存在。

实验结果

研究问题

• RQ1 Smith 等人的模拟是否支持在原初气体云中形成平衡态暗星？
• RQ2 暗物质湮灭是否能抑制原初气体坍缩过程中的碎裂，从而促成暗星形成？
• RQ3 模拟中的吸积核粒子是否因其物理特性与能量沉积而可有效视为暗星？
• RQ4 模拟中形成的吸积盘是否因暗物质加热而保持对碎裂的稳定性？
• RQ5 100 GeV WIMPs 的高能湮灭产物如何影响坍缩气体中的能量沉积与热化？

主要发现

• Smith 等人的模拟证实，暗物质湮灭可抑制碎裂，从而允许单个中心原恒星的形成。
• 模拟达到的气体密度约为 ~5×10^14 cm⁻³，接近但尚未完全达到形成平衡态暗星所需的 ~10^17 cm⁻³。
• 模拟中的吸积核粒子质量约为 ~10 M⊙，密度足够高，可容纳暗星，其内部条件与暗星解一致。
• 在中心吸积核周围约 1000 AU 处形成吸积盘，其稳定性由湮灭加热维持，Toomre 参数 Q > 1。
• 次级吸积核仅在距离 >1000 AU 处形成，远离中心暗星，从而保留了其暗物质晕与能量来源。
• 模拟通过表明碎裂不会破坏中心暗物质分布，解决了暗星形成的一个关键障碍。