低金属量恒星的形成：星前坍缩和球对称中的原恒星吸积
Low-Metallicity Star Formation : Prestellar Collapse and Protostellar Accretion in the Spherical Symmetry
摘要：
通过流体力学计算，结合详细的化学和辐射过程，研究了不同金属丰度的致密核的坍缩。为此，我们构建了一个简单的化学网络与15个化学物种之间的非平衡反应，它再现了丰富的重要分子冷却剂的更详细的网络非常好。这种演化一直持续到中心形成一个流体静力原恒星。在金属丰度较低的气体云中，由于冷却效率较低，坍缩过程中的温度仍然很高。利用中心的温度演化作为密度的函数，我们讨论了在尘埃冷却阶段碎裂的可能性。假设云核在这一阶段开始时有适度的伸长，则碎裂的临界金属丰度为10^{-5}Z_sun。从密度和速度分布在原恒星形成的时候，我们评估在随后的吸积阶段的质量吸积率。利用这些吸积率，我们还计算了在吸积流稳定的假设下原恒星的演化。最后，我们讨论了可能的抑制碎片加热的环境气体的原恒星辐射，这被认为是重要的当代恒星形成。我们认为对于<10^{-2}Zsun，它是可以忽略的。
英文摘要：
The collapse of dense cores with different metallicities is studied by hydrodynamical calculations coupled with detailed chemical and radiative processes. For this purpose, we construct a simple chemical network with non-equilibrium reactions among 15 chemical species, which reproduces the abundance of important molecular coolants by more detailed network very well. The evolution is followed until the formation of a hydrostatic protostar at the center. In a lower-metallicity gas cloud, the temperature during the collapse remains high owing to less efficient cooling. Using the temperature evolution at the center as a function the density, we discuss the possibility of fragmentation during the dust-cooling phase. The critical metallicity for the fragmentation is 10^{-5}Z_sun assuming moderate elongation of the cloud cores at the onset of this phase. From the density and velocity distributions at the time of protostar formation, we evaluate the mass accretion rate in the subsequent accretion phase. Using these accretion rates, we also calculate the evolution of the protostars under the assumption of stationary accretion flow. Finally, we discuss possible suppression of fragmentation by heating of the ambient gas by protostellar radiation, which is considered important in the contemporary star formation. We argue that it is negligible for <10^{-2}Zsun.
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