Thermal and Fragmentation Properties of Star-forming Clouds in Low-Metallicity Environments
Thermal and Fragmentation Properties of Star-forming Clouds in Low-Metallicity Environments
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DOI：
10.1086/429955
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发表时间：
2005-03
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影响因子：
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通讯作者：
K. Omukai;T. Tsuribe;R. Schneider;A. Ferrara
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作者：
K. Omukai;T. Tsuribe;R. Schneider;A. Ferrara
使用Omukai模型研究了不同气体金属丰度Z的恒星形成云的热演化和化学演化，更新后的模型包括氘化学和宇宙微波背景辐射（CMB）的影响。当Z = 10-5 ~ 10-3 Z差，密度≥ 105 cm-3时，HD线冷却在云的热平衡中占主导地位。早期，CMB辐射阻止气体温度下降到TCMB以下，尽管这几乎没有改变低金属丰度气体的云热演化。从导出的温度演化，我们评估云/核心碎片作为金属丰度的函数，从线性微扰理论，这需要核心伸长<$B）/a ><$NL ~ 1，其中a（B）是短（长）核心轴长度。碎片的质量由热金斯质量给出，在λ = λ NL。给定这些假设和初始的（高斯）分布，我们计算碎片质量分布作为金属丰度的函数。我们发现：（1）当Z = 0时，所有的碎片都是非常大的，大约为103 M，与以前的研究一致;（2）当Z > 10-6 Z时，一些团块经历了额外的高密度（~ 1010 cm-3）由尘埃冷却驱动的碎裂阶段，导致低质量碎片;（3）低质量碎片的质量分数最初很小，但在Z ~ 10-5 Z时占主导地位，并随Z的增加而继续增加;（4）在0.01（5）当Z = 0.1时，两个峰合并为单峰质量函数，这可能是Salpeter-like初始质量函数的前身。
The thermal and chemical evolution of star-forming clouds is studied for different gas metallicities, Z, using the model of Omukai, updated to include deuterium chemistry and the effects of cosmic microwave background (CMB) radiation. HD-line cooling dominates the thermal balance of clouds when Z ~ 10-5 to 10-3 Z☉ and density ≈105 cm-3. Early on, CMB radiation prevents the gas temperature from falling below TCMB, although this hardly alters the cloud thermal evolution in low-metallicity gas. From the derived temperature evolution, we assess cloud/core fragmentation as a function of metallicity from linear perturbation theory, which requires that the core elongation ℰ ≡ (b - a)/a > ℰNL ~ 1, where a (b) is the short (long) core axis length. The fragment mass is given by the thermal Jeans mass at ℰ = ℰNL. Given these assumptions and the initial (Gaussian) distribution of ℰ, we compute the fragment mass distribution as a function of metallicity. We find that (1) for Z = 0, all fragments are very massive, ≲103 M☉, consistent with previous studies; (2) for Z > 10-6 Z☉ a few clumps go through an additional high-density (≳1010 cm-3) fragmentation phase driven by dust cooling, leading to low-mass fragments; (3) the mass fraction in low-mass fragments is initially very small, but at Z ~ 10-5 Z☉ it becomes dominant and continues to grow as Z is increased; (4) as a result of the two fragmentation modes, a bimodal mass distribution emerges in 0.01 < Z/Z☉ < 0.1; and (5) for ≳0.1 Z☉, the two peaks merge into a single-peaked mass function, which might be regarded as the precursor of the ordinary Salpeter-like initial mass function.