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 − 10 −3 Z ⊙ and density ≈ 10 5 cm −3 . Early on, CMB radiation prevents the gas temperature to fall below T CM B , 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 > E NL ∼ 1, where a (b) is the short (long) core axis length. The fragment mass is given by the thermal Jeans mass at E = E NL . 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, 10 3 M ⊙ , consistently with previous studies; (ii) for Z > 10 −6 Z ⊙ a few clumps go through an additional high density ( 10 10 cm −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 ∼ 10 −5 Z ⊙ 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.
We study the emission from an old supernova remnant (SNR) with an age of around 105 yr and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 105 yr, proton acceleration is efficient enough to emit TeV γ‐rays both at the shock of the SNR and that in the GMC. The maximum energy of primarily accelerated electrons is so small that TeV γ‐rays and X‐rays are dominated by hadronic processes, π0‐decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 105 yr, there are few high‐energy particles emitting TeV γ‐rays because of the energy‐loss effect and/or the wave‐damping effect occurring at low‐velocity isothermal shocks. For old SNRs or SNR–GMC interacting systems capable of generating TeV γ‐ray emitting particles, we calculated the ratio of TeV γ‐ray (1–10 TeV) to X‐ray (2–10 keV) energy flux and found that it can be more than ∼102. Such a source showing large flux ratio may be a possible origin of recently discovered unidentified TeV sources.
Dynamical collapse and fragmentation of low-metallicity cloud cores is studied using three-dimensional hydrodynamical calculations, with particular attention devoted whether the cores fragment in the dust-cooling phase or not. The cores become elongated in this phase, being unstable to non-spherical perturbation due to the sudden temperature decrease. In the metallicity range of 10 −6 − 10 −5 Z ⊙ , cores with an initial axis ratio 2 reach a critical value of the axis ratio ( 30) and fragment into multiple small clumps. This provides a possible mechanism to produce low-mass stars in ultra-metal-poor environments.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.