第一颗恒星：原恒星反馈下的质量增长
The First Stars: Mass Growth Under Protostellar Feedback
摘要：
我们进行三维宇宙学模拟，研究无金属，人口III（流行III）的辐射反馈下的恒星的增长。我们从z=100开始模拟，追踪气体和暗物质的演化，直到第一个微晕的形成。然后，我们跟踪微晕内气体的坍缩，直到密度达到n = 10^12 cm^-3，此时我们用一个下沉粒子来代替高密度粒子，以代表成长中的原恒星。我们模拟的原恒星发出的莱曼-沃纳（LW）辐射的影响，并采用射线跟踪计划，以遵循周围的H II区域在未来5000年的增长。我们发现，一个磁盘组装周围的第一个原恒星，辐射反馈将不会阻止磁盘进一步分裂，形成多个流行III星。中性氢的电离和H_2的光解离导致稠密气体被加热到几千开尔文，并且这个温暖的区域以气体声速向外膨胀。一旦这个温暖区域的范围变得与盘的大小相等，盘的质量就会下降，而对原恒星的吸积率就会降低一个数量级。当最大的汇增长到~ 20 M_sol而第二个汇增长到7 M_sol时，我们估计主汇在到达主层序时将接近~ 30 M_sol的渐近值。因此，我们的模拟表明，最有可能的结果是一个巨大的Pop III双星。然而，我们只模拟一个minihalo，和minihalo之间的统计变化可能是巨大的。如果Pop III恒星通常无法成长到超过几十个太阳质量，这将对早期宇宙中的对不稳定超新星的发生以及今天可观测到的最古老恒星中的Pop III化学特征产生重要影响。
英文摘要：
We perform three-dimensional cosmological simulations to examine the growth of metal-free, Population III (Pop III) stars under radiative feedback. We begin our simulation at z=100 and trace the evolution of gas and dark matter until the formation of the first minihalo. We then follow the collapse of the gas within the minihalo up to densities of n = 10^12 cm^-3, at which point we replace the high-density particles with a sink particle to represent the growing protostar. We model the effect of Lyman-Werner (LW) radiation emitted by the protostar, and employ a ray-tracing scheme to follow the growth of the surrounding H II region over the next 5000 yr. We find that a disk assembles around the first protostar, and that radiative feedback will not prevent further fragmentation of the disk to form multiple Pop III stars. Ionization of neutral hydrogen and photodissociation of H_2 by LW radiation leads to heating of the dense gas to several thousand Kelvin, and this warm region expands outward at the gas sound speed. Once the extent of this warm region becomes equivalent to the size of the disk, the disk mass declines while the accretion rate onto the protostars is reduced by an order of magnitude. This occurs when the largest sink has grown to ~ 20 M_sol while the second sink has grown to 7 M_sol, and we estimate the main sink will approach an asymptotic value of ~ 30 M_sol by the time it reaches the main sequence. Our simulation thus indicates that the most likely outcome is a massive Pop III binary. However, we simulate only one minihalo, and the statistical variation between minihaloes may be substantial. If Pop III stars were typically unable to grow to more than a few tens of solar masses, this would have important consequences for the occurence of pair-instability supernovae in the early Universe as well as the Pop III chemical signature in the oldest stars observable today.
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