标题： 略金属 enrichment 云中通过超级竞争吸积形成的超大质量恒星 显示英文标题

标题： Supermassive Star Formation via Super Competitive Accretion in Slightly Metal-enriched Clouds

摘要： 直接坍缩黑洞（DCBH）质量为$\gtrsim 10^{5}~M_{\odot}$的形成是高红移超大质量黑洞起源的一个有前景的场景。 人们通常认为，只有原始气体中才能形成DCBH，因为金属富集会增强冷却能力，并导致碎片化成更小的部分。 然而，在这种环境中实际发生的情况尚未被详细探索。 在这里，我们研究了金属富集对云的影响，通过流体力学模拟来跟踪不同程度金属富集（$Z/Z_{\odot}=10^{-6}-10^{-3}$）情况下云的演化。 在低于$Z/Z_{\odot}=10^{-6}$时，金属丰度没有影响，超大质量恒星形成的同时也伴随少量低质量恒星的产生。 当金属丰度更高（$Z/Z_{\odot} \gtrsim 5 \times 10^{-6}$）时，尽管尘埃冷却确实促进了云核的碎裂并产生了约几千个低质量恒星，但吸积流优先将气体供给中央的大质量恒星，使其成长为超大质量恒星，就像在原始情况中一样。 我们将这种形成模式称为{\it 超竞争性吸积}，在这种模式下，只有中央少数几颗恒星成长为超大质量恒星，而大量其他恒星则争夺着气体储库。 一旦金属丰度超过$10^{-3}~Z_{\odot}$并且金属线冷却变得有效，由于降低的吸积率，中央恒星无法成长为超大质量恒星。 通过超级竞争性吸积形成的超大质量恒星为种子黑洞开启了一个新的窗口，这放宽了对金属丰度的要求并提高了种子黑洞的数量。 显示英文摘要

摘要： Direct collapse black hole (DCBH) formation with mass $\gtrsim 10^{5}~M_{\odot}$ is a promising scenario for the origin of high-redshift supermassive black holes. It has usually been supposed that the DCBH can only form in the primordial gas since the metal enrichment enhances the cooling ability and causes the fragmentation into smaller pieces. What actually happens in such an environment, however, has not been explored in detail. Here, we study the impact of the metal enrichment on the clouds, conducting hydrodynamical simulations to follow the cloud evolution in cases with different degree of the metal enrichment $Z/Z_{\odot}=10^{-6}-10^{-3}$. Below $Z/Z_{\odot}=10^{-6}$, metallicity has no effect and supermassive stars form along with a small number of low-mass stars. With more metallicity $Z/Z_{\odot} \gtrsim 5 \times 10^{-6}$, although the dust cooling indeed promotes fragmentation of the cloud core and produces about a few thousand low-mass stars, the accreting flow preferentially feeds the gas to the central massive stars, which grows supermassive as in the primordial case. We term this formation mode as the {\it super competitive accretion}, where only the central few stars grow supermassive while a large number of other stars are competing for the gas reservoir. Once the metallicity exceeds $10^{-3}~Z_{\odot}$ and metal-line cooling becomes operative, the central star cannot grow supermassive due to lowered accretion rate. Supermassive star formation by the super competitive accretion opens up a new window for seed BHs, which relaxes the condition on metallicity and enhances the seed BH abundance.