Thermal and Fragmentation Properties of Star-forming Clouds in Low-metallicity Environments

The thermal and chemical evolution of star-forming clouds is studied for different gas metallicities, Z, using the model of Omukai (2000), 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 − 10Z⊙ and density ≈ 105cm−3. Early on, CMB radiation prevents the gas temperature to fall 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: (i) For Z = 0, all fragments are very massive, . 10M⊙, consistently with previous studies; (ii) for Z > 10 Z⊙ a few clumps go through an additional high density (& 1010cm−3) fragmentation phase driven by dustcooling, leading to low-mass fragments; (iii) The mass fraction in low-mass fragments is initially very small, but at Z ∼ 10Z⊙ it becomes dominant and continues to grow as Z is increased; (iv) as a result of the two fragmentation modes, a bimodal mass distribution emerges in 0.01 < Z/Z⊙ < 0.1. (v) For & 0.1Z⊙, the two peaks merge into a singly-peaked mass function which might be regarded as the precursor of the ordinary Salpeter-like IMF. Subject headings: cosmology: theory — galaxies: formation — stars: formation