磁效应促进富金属原子冷却晕中的超大质量恒星形成
Magnetic Effects Promote Supermassive Star Formation in Metal-enriched Atomic-cooling Halos
AI总结：
该研究通过磁流体动力学模拟发现，富金属原子冷却晕中磁场会因原恒星作用放大，促进低质量原恒星并合与主原恒星质量增长，且极贫金属环境下增长速率最高，影响直接坍缩黑洞形成判据。
摘要：
中等质量黑洞（质量≥10⁵M⊙）是早期宇宙（红移z~6）中超大质量黑洞（质量~10⁹M⊙）起源的有潜力的候选天体。Chon & Omukai（2020）首次指出富金属原子冷却晕（ACHs）中存在直接坍缩黑洞（DCBH）形成的可能，这放宽了DCBH的形成判据。另一方面，Hirano等人（2021）的研究表明，磁效应会促进无金属ACHs中的DCBH形成。我们开展了一组磁流体动力学模拟，以研究金属丰度Z/Z⊙=0、10⁻⁵和10⁻⁴的磁化ACHs中的恒星形成过程。我们的模拟显示，富金属ACHs中原恒星的质量吸积率低于无金属ACHs中的吸积率。然而，大量原恒星会从引力和热不稳定的富金属气体云中形成。在这种情况下，由于原恒星的自旋缠绕磁力线，磁场会迅速增强。被放大的磁场区域会因原恒星的轨道运动和吸积气体的旋转而向外扩展。放大后的磁场从吸积气体中提取角动量，促进低质量原恒星的并合，并提升主原恒星的质量增长速率。我们得出结论：无论初始磁场强度如何，富金属ACHs中总会发生磁场放大，这会影响DCBH的形成判据。此外，我们还发现了与此前无磁模拟定性不同的趋势：在Z/Z⊙=10⁻⁵的极贫金属ACHs中，质量增长速率达到最大值。
英文摘要：
Intermediate-mass black holes (with $\geq\!10^5\,M_\odot$) are promising candidates for the origin of supermassive black holes (with $\sim\!10^9\,M_\odot$) in the early universe (redshift $z\sim6$). Chon & Omukai (2020) firstly pointed out the direct collapse black hole (DCBH) formation in metal-enriched atomic-cooling halos (ACHs), which relaxes the DCBH formation criterion. On the other hand, Hirano et al. (2021) showed that the magnetic effects promote the DCBH formation in metal-free ACHs. We perform a set of magnetohydrodynamical simulations to investigate star formation in the magnetized ACHs with metallicities $Z/Z_\odot = 0$, $10^{-5}$, and $10^{-4}$. Our simulations show that the mass accretion rate onto the protostars becomes lower in metal-enriched ACHs than that of metal-free ACHs. However, many protostars form from gravitationally and thermally unstable metal-enriched gas clouds. Under such circumstances, the magnetic field rapidly increases as the magnetic field lines wind up due to the spin of protostars. The region with the amplified magnetic field expands outwards due to the orbital motion of protostars and the rotation of the accreting gas. The amplified magnetic field extracts the angular momentum from the accreting gas, promotes the coalescence of the low-mass protostars, and increases the mass growth rate of the primary protostar. We conclude that the magnetic field amplification is always realized in the metal-enriched ACHs regardless of the initial magnetic field strength, which affects the DCBH formation criterion. In addition, we find a qualitatively different trend from the previous unmagnetized simulations in that the mass growth rate is maximal for the extremely metal-poor ACHs with $Z/Z_\odot = 10^{-5}$.
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