The Stellar Initial Mass Function in Primordial Galaxies
The Stellar Initial Mass Function in Primordial Galaxies
复制标题
原初星系中恒星的初始质量函数
DOI：
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发表时间：
2002
期刊：
影响因子：
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通讯作者：
M. Umemura
中科院分区：
文献类型：
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作者：
F. Nakamura;M. Umemura
在极度缺乏金属的原星系云碎裂形成恒星的背景下，用一维数值流体力学结合H2和HD的非平衡化学研究了丝状气体云的引力崩塌。结果表明，云的演化主要受初始中心密度(Nc，0)和H2丰度(X)的影响。特别是，低密度细丝(NC，0≲10 5 cm-3)的演化在阈值H2丰度x≃3×10-3处分叉，超过这一丰度，HD冷却超过H2冷却。当中心密度(NC)达到达到≲能级布居的临界HD密度时，Nc、0≳10 5 cm-3和x LTE x的纤维收缩强烈减速，因此纤维有望在~10 7 cm-3处碎裂。碎片质量降低到≈10M☉。相反，具有NC、0≲10~(-3)cm~(-3)和x≲x的纤维的收缩受H2冷却的调节。在这种情况下，由于H2的临界密度较低，纤维倾向于在较低的密度(~104 cm-3)下碎裂，碎片质量高达≈102M☉。对于NC为0≳10 5 cm-3的高密度灯丝，由于H2和HD冷却都饱和，云层演化由H2冷却控制，因此温度保持在相对较高的值。当密度达到108-109 cm-3时，有效的三体H2生成加速了高密度纤维的收缩。在Nc，0~1012-1013 cm-3，直到H2谱线上的云变得不透明，碎片质量才会发生破碎，从而碎片质量减少到1-2M☉。结果表明，在亚太阳质量恒星中，恒星初始质量函数可能是双峰且亏损的，高质量峰值约为10或102M☉，取决于nc、0和x。如果原星系云被紫外线辐射或强激波电离，H+e≃H-+h→和H+H-νH2+e的反应可使H2丰度超过x→3×10-3，此时高质量峰值将是O(10)M☉。
In the context of star formation through fragmentation of an extremely metal deficient protogalactic cloud, the gravitational collapse of filamentary gas clouds is explored with one-dimensional numerical hydrodynamics coupled with nonequilibrium chemistry of H2 and HD. It is found that the cloud evolution is governed mainly by the initial central density (nc, 0) and H2 abundance (x). In particular, the evolution of low-density filaments (nc, 0 ≲ 105 cm-3) bifurcates at a threshold H2 abundance of x ≃ 3 × 10-3, beyond which HD cooling overwhelms H2 cooling. The contraction of a filament with nc, 0 ≲ 105 cm-3 and x ≳ x is strongly decelerated when the central density (nc) reaches a critical density of HD at which LTE level populations are achieved, and therefore the filament is expected to fragment at ~107 cm-3. The fragment mass is lowered to be ≈10 M☉. In contrast, the contraction of a filament with nc, 0 ≲ 105 cm-3 and x ≲ x is regulated by H2 cooling. In this case, the filament tends to fragment at lower density as ~104 cm-3 owing to the low critical density of H2, and the fragment mass is as high as ≈102 M☉. For a high-density filament with nc, 0 ≳ 105 cm-3, the temperature stays at a relatively high value because both H2 and HD cooling saturate, and the cloud evolution is governed by H2 cooling. The contraction of a high-density filament is accelerated by effective three-body H2 formation when the density reaches 108-109 cm-3. Fragmentation is not expected to take place until the cloud becomes opaque in H2 lines at nc, 0 ~ 1012-1013 cm-3, so that the fragment mass is reduced to 1-2 M☉. As a result, the stellar initial mass function could be bimodal and deficient in sub-solar mass stars, where the high-mass peak is around 10 or 102 M☉, dependent on nc, 0 and x. If the protogalactic clouds are ionized by UV radiation or strong shocks, the H2 abundance could exceed x ≃ 3 × 10-3 by reactions of H + e → H- + hν and H + H- → H2 + e. The high-mass peak would then be O(10) M☉.