Dark-matter halo mergers as a fertile environment for low-mass Population III star formation

Abstract

While Population III stars are typically thought to be massive, pathways towards lower-mass Pop III stars may exist when the cooling of the gas is particularly enhanced. A possible route is enhanced HD cooling during the merging of dark-matter halos. The mergers can lead to a high ionization degree catalysing the formation of HD molecules and may cool the gas down to the cosmic microwave background (CMB) temperature. In this paper, we investigate the merging of mini-halos with masses of a few 10 5 M ⊙ and explore the feasibility of this scenario. We have performed threedimensional cosmological hydrodynamics calculations with the ENZO code, solving the thermal and chemical evolution of the gas by employing the astrochemistry package KROME. Our results show that the HD abundance is increased by two orders of magnitude compared to the no-merging case and the halo cools down to ∼60 K triggering fragmentation. Based on Jeans estimates the expected stellar masses are about 10 M ⊙ . Our findings show that the merging scenario is a potential pathway for the formation of low-mass stars.

Primordial star formation under the influence of far ultraviolet radiation: 1540 cosmological haloes and the stellar mass distribution

“…fHD/fH 2 increases sharply in the density range 10 6 cm −3 < nH,cen < 10 7 cm −3 and eventually exceeds 10 −3 . We classify our 1540 samples of primordial clouds as one of three possible cases, depending on fHD/fH 2 during the collapse: H2-cooling cases (1186 cases), HD-cooling cases (151), and the intermediate cases (203), where the numbers 6 Another possible route for HD-cooling primordial starformation is suggested by some studies (Uehara & Inutsuka 2000;Shchekinov & Vasiliev 2006;Prieto et al 2012;Bovino et al 2014;Prieto et al 2014). In this scenario, the merging of multiple dark matter haloes induces the formation of shockwaves in which HD formation is enhanced.…”

The numerical frontier of the high-redshift Universe

“…If the electron abundance is significantly enhanced with respect to the postrecombination value, HD cooling may become important in other circumstances as well. An elevated electron abundance is produced by the virialization shocks of atomic cooling halos or during mergers (Greif & Bromm 2006;Greif et al 2008;Shchekinov & Vasiliev 2006;Vasiliev & Shchekinov 2008;Prieto et al 2012;Bovino et al 2014b;Prieto et al 2014), as well as in SN remnants (Mac Low & Shull 1986;Uehara & Inutsuka 2000;Mackey et al 2003;Machida et al 2005;Vasiliev & Shchekinov 2005a;Vasiliev et al 2008). The H 2 abundance in the post-shock gas increases to values well above those found in minihalos, which allows the gas to cool to temperatures low enough that chemical fractionation occurs and HD cooling takes over.…”

The chemical evolution of self-gravitating primordial disks

“…The latter requires the formation of a self-gravitating disk, and its subsequent fragmentation via gravitational instabilities. Indeed, simulations starting from cosmological initial conditions have now confirmed the formation of self-gravitating disks in minihalos as well as their fragmentation Greif et al 2011Greif et al , 2012Latif et al 2013b;Bovino et al 2014b;Susa et al 2014). While these simulations followed the evolution for up to ∼1000 yr depending on resolution, the Kelvin-Helmholtz timescale for a protostar to reach the main sequence is ∼10 6 yr, implying that no final conclusions can be drawn about the properties of the stellar system, and that the simulations so far have only explored the evolution of the disks at very early stages.…”

“…The latter can be understood as the viscous heating rate that strongly depends on the angular velocity of the gas, considerably increases toward smaller scales, and ultimately exceeds the cooling rate of a molecular gas. While previous investigations were predominantly exploring a free-fall collapse using one-zone models (e.g., Omukai 2001;Omukai et al 2005;Glover & Abel 2008;Glover 2015) or the modeling of the early stages of disk formation (e.g., Regan & Haehnelt 2009;Clark et al 2011;Greif et al 2011Greif et al , 2012Latif et al 2013a,b;Bovino et al 2014b;Prieto et al 2013;Regan et al 2014;Susa et al 2014;Becerra et al 2015), we explore the chemical and thermal evolution of these disks at their later stages, in particular, during the presence of a central massive object. For this purpose, we consider different disk models and initial chemical conditions, representing both an initially atomic and molecular gas.…”