On the Initial Mass Function of Population III Stars
On the Initial Mass Function of Population III Stars
复制标题
论III族恒星的初始质量函数
DOI：
--
复制
发表时间：
2000
期刊：
影响因子：
--
通讯作者：
M. Umemura
中科院分区：
文献类型：
--
作者：
F. Nakamura;M. Umemura
利用一维和二维流体动力学模拟以及氢分子形成的非平衡过程来探索丝状原始气体云的塌缩和破碎。云演化是根据初始中心密度 nc = 10-106 cm-3 计算的。模拟表明，根据初始密度，原始细丝有两种断裂情况。如果细丝具有相对较低的初始密度，例如 nc ≲ 105 cm-3，则由于氢气冷却效率较低，径向收缩很慢，并且在高于临界密度的密度下明显减速，其中针对氢气分子的旋转水平实现了 LTE 群体，并且冷却时间尺度相应地变得比自由落体时间尺度长。在中心密度达到 108-109 cm-3 之前，这种细丝往往会碎裂成致密的团块，此时三体反应的 H2 冷却是有效的，并且碎片质量大于几十 M☉。相反，如果细丝最初的密度为 nc ≳ 105 cm-3，则在三体反应的帮助下更有效的氢气冷却可以使细丝收缩至 n ~ 1012 cm-3。当密度达到 n ~ 1012 cm-3 后，灯丝在光学上变得厚至 H2 线，随后径向收缩几乎停止。在最后的静水阶段，由于长丝的高密度，碎片质量降低至 ≈1 M☉。碎片质量对初始密度的依赖性可以转化为对随机高斯密度场的局部振幅或母云塌陷历元的依赖性。因此，预测星族 III 恒星的初始质量函数可能是双峰的，峰值为 ≈102 和 ≈1 M☉，其中相对高度可能是坍缩历元的函数。简要讨论了高红移和重子暗物质对 III 族恒星金属富集的影响。
The collapse and fragmentation of filamentary primordial gas clouds are explored using one- and two-dimensional hydrodynamical simulations coupled with the nonequilibrium processes of hydrogen molecule formation. The cloud evolution is computed from the initial central density nc = 10-106 cm-3. The simulations show that depending on the initial density, there are two occasions for the fragmentation of primordial filaments. If a filament has relatively low initial density such as nc ≲ 105 cm-3, the radial contraction is slow as a result of less effective H2 cooling and appreciably decelerates at densities higher than a critical density, where LTE populations are achieved for the rotational levels of H2 molecules and the cooling timescale becomes accordingly longer than the free-fall timescale. This filament tends to fragment into dense clumps before the central density reaches 108-109 cm-3, where H2 cooling by three-body reactions is effective and the fragment mass is more massive than some tens of M☉. In contrast, if a filament is initially as dense as nc ≳ 105 cm-3, the more effective H2 cooling with the help of three-body reactions allows the filament to contract up to n ~ 1012 cm-3. After the density reaches n ~ 1012 cm-3, the filament becomes optically thick to H2 lines and the radial contraction subsequently almost stops. At this final hydrostatic stage, the fragment mass is lowered down to ≈1 M☉ because of the high density of the filament. The dependence of the fragment mass upon the initial density could be translated into the dependence on the local amplitude of random Gaussian density fields or the epoch of the collapse of a parent cloud. Hence, it is predicted that the initial mass function of Population III stars is likely to be bimodal with peaks of ≈102 and ≈1 M☉, where the relative heights could be a function of the collapse epoch. Implications for the metal enrichment by Population III stars at high redshifts and baryonic dark matter are briefly discussed.