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, 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 similar to 10(-5) to 10(-3) Z(circle dot) and density similar to 10(5) 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 epsilon &3bond; (b - a)/a > epsilon(NL) similar to 1, where a(b) is the short (long) core axis length. The fragment mass is given by the thermal Jeans mass at epsilon = epsilon(NL). Given these assumptions and the initial (Gaussian) distribution of epsilon, we compute the fragment mass distribution as a function of metallicity. We find that (1) for Z = 0, all fragments are very massive, less than or similar to 10(3) M-circle dot, consistent with previous studies; (2) for Z > 10(-6) Z(circle dot) a few clumps go through an additional high-density (greater than or similar to 10(10) 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 similar to 10(-5) Z(circle dot) 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(circle dot) < 0.1; and (5) for greater than or similar to 0.1 Z(circle dot), 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.