Thermal and Fragmentation Properties of Star‐forming Clouds in Low‐Metallicity Environments

Abstract

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.

TOPoS

“…Clearly, in order to form the low-mass objects observed, one would need to resort to a high-density cooling process, such as dust cooling. This is the only process that brings the gas into the right regime of masses consistent with the observations (Omukai et al 2005;Clark et al 2008;Klessen et al 2012;Schneider et al 2012a,b;Dopcke et al 2013;Chiaki et al 2014Chiaki et al , 2015. It seems thus more realistic to assume that all the stars in Fig.…”

“…As soon as the metallicity rises above a certain threshold, which is thought to lie between 10 −6 and 10 −5 Z (e.g. Omukai et al 2005;Schneider et al 2012a;Glover 2013), two main cooling channels emerge which could lead to the efficient formation of low-mass stars and result in a stellar mass spectrum similar to the present-day IMF (Kroupa 2002;Chabrier 2003). These cooling processes are -fine structure transitions of C ii and O i (Bromm & Loeb 2003) and; -dust cooling (Schneider et al 2006(Schneider et al , 2012aOmukai et al 2008;Dopcke et al 2011Dopcke et al , 2013.…”

The First Billion Years project: the escape fraction of ionizing photons in the epoch of reionization

“…With the increase of the metallicity of the star-forming gas, the IMF switches rapidly from the top-heavy Pop III IMF to a much more bottom-heavy IMF, characteristic of present-day star formation (Bromm et al 2001;Schneider et al 2003;Omukai et al 2005;Dopcke et al 2013). For the metal-enriched Pop II stars we assume a Chabrier IMF (Chabrier 2003), which has a Salpeter slope at the high-mass end and extends down to sub-solar masses.…”

The dust content of high-zsubmillimeter galaxies revealed byHerschel

“…Dust allows the formation of low-mass stars in low-metalicity environments while it inhibits the formation of massive stars, hence affects the IMF (e.g. Omukai et al 2005). Dust greatly enhances the formation of many molecules whose transitions provide the main cooling mechanism for molecular clouds to form stars.…”