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 ~ 10 -5 to 10 -3 Z ☉ and density ≈10 5 cm -3 . Early on, CMB radiation prevents the gas temperature from falling below T CMB , 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 ≡ ( b – a )/ a > NL ~ 1, where a ( b ) is the short (long) core axis length. The fragment mass is given by the thermal Jeans mass at = NL . Given these assumptions and the initial (Gaussian) distribution of , we compute the fragment mass distribution as a function of metallicity. We find that (1) for Z = 0, all fragments are very massive, ≲10 3 M ☉ , consistent with previous studies; (2) for Z > 10 -6 Z ☉ a few clumps go through an additional high-density (≳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 ~ 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.