The cooling of shock-compressed primordial gas

Abstract

We find that at redshifts z > 10, HD line cooling allows strongly-shocked primordial gas to cool to the temperature of the cosmic microwave background (CMB). This temperature is the minimum value attainable via radiative cooling. Provided that the abundance of HD, normalized to the total number density, exceeds a critical level of ~ 10^{-8}, the CMB temperature floor is reached in a time which is short compared to the Hubble time. We estimate the characteristic masses of stars formed out of shocked primordial gas in the wake of the first supernovae, and resulting from the mergers of dark matter haloes during hierarchical structure formation to be ~ 10 M_{solar}. In addition, we show that cooling by HD enables the primordial gas in relic H II regions to cool to temperatures considerably lower than those reached via H_2 cooling alone. We confirm that HD cooling is unimportant in cases where the primordial gas does not go through an ionized phase, as in the formation process of the very first stars in z ~ 20 minihaloes of mass ~ 10^{6} M_{solar}.Comment: 10 pages, 10 figures, accepted for publication in MNRAS with minor revisions, new table adde

Enhanced direct collapse due to Lyman α feedback

“…For our study, we begin with the same one-zone model for the collapse of the primordial gas as presented in Johnson & Bromm (2006), which is very similar to other one-zone models that have been routinely applied to DCBH formation (e.g. Omukai et al 2005Omukai et al , 2008Schleicher et al 2010;Agarwal et al 2016a).…”

“…While the reader is referred to Johnson & Bromm (2006) for more details, here we describe the key ingredients in the model that we draw on for our study of the direct collapse scenario. One important update to this code has been the adoption of the H 2 self-shielding prescription presented in Wolcott-Green et al (2011;see also Hartwig et al 2015, Wolcott-Green et al 2016, which replaced the simpler prescription presented in Bromm & Loeb (2003).…”

The Very First Stars: Formation and Reionization of the Universe

“…Efficient cooling by HD can be triggered within the relic H ii regions that surround Pop III.1 stars at the end of their brief lifetimes (Alvarez et al 2006), owing to the high electron fraction that persists in the gas as it cools and recombines Yoshida et al 2007b). The efficient formation of HD can also take place when the primordial gas is collisionally ionized, such as behind the shocks driven by the first SNe or in the virialization of massive DM halos (Machida et al 2005;Johnson & Bromm 2006;Shchekinov & Vasiliev 2006).…”

“…While the initial conditions for the formation of the very first stars are known from precision measurements of cosmological parameters (Spergel et al 2007), the situation for the subsequent generations of stars is much more compex. It has become evident that Pop III star formation might actually consist of two distinct modes: one where the primordial gas collapses into a DM minihalo, and one where the metal-free gas becomes significantly ionized prior to the onset of gravitational runaway collapse (Johnson & Bromm 2006). To clearly indicate that both modes pertain to metal-free star formation, we here follow the new classification scheme suggested by Chris McKee (see McKee & Tan 2008;Johnson et al 2008).…”

“…In turn, this efficient cooling may lead to the formation of primordial stars with masses of the order of 10 M , the so-called Pop III.2 (Johnson & Bromm 2006). In general, the formation of HD, and the concomitant cooling that it provides, is found to occur efficiently in primordial gas which is strongly ionized, owing ultimately to the high abundance of electrons which serve as catalyst for molecule formation in the early 30 V. Bromm The asterisk marks the location of the most massive sink, crosses that of the second most massive one, and diamonds those of the other, smaller sinks.…”