Formation of Primordial Stars in a ΛCDM Universe

Abstract

Primordial stars are formed from a chemically pristine gas consisting of hydrogen and helium. They are believed to have been born at some early epoch in the history of the Universe and to have enriched the interstellar medium with synthesized heavy elements before the emergence of ordinary stellar populations. We study the formation of the first generation of stars in the standard cold dark matter model. We follow the gravitational collapse and thermal evolution of primordial gas clouds within early cosmic structures using very high-resolution, cosmological hydrodynamic simulations. Our simulation achieves a dynamic range of ∼ 10 10 in length scale. With accurate treatment of atomic and molecular physics, it allows us to study the chemo-thermal evolution of primordial gas clouds to densities up to ρ ∼ 2 × 10 −8 g cm −3 (n H ∼ 10 16 cm −3 ) without assuming any a priori equation of state; a six orders of magnitudes improvement over previous three-dimensional calculations. We implement an extensive chemistry network for hydrogen, helium and deuterium. All the relevant atomic and molecular cooling and heating processes, including cooling by collision-induced continuum emission, are implemented. For calculating optically thick H 2 cooling at high densities, we use the Sobolev method (Sobolev 1960) and evaluate the molecular line opacities for a few hundred lines. We validate the accuracy of the method by performing a spherical collapse test and comparing the results with those of accurate one-dimensional calculations that treat the line radiative transfer problem in a fully self-consistent manner.We then perform a cosmological simulation adopting the standard ΛCDM model. Dense gas clumps are formed at the centers of low mass (∼ 10 5−6 M ⊙ ) dark matter halos at redshifts z ∼ 20, and they collapse gravitationally when the cloud mass exceeds a few hundred solar masses. To examine possible gas fragmentation owing to thermal instability, we compute explicitly the growth rate of isobaric perturbations. We show that the cloud core does not fragment in either the low-density (n H ∼ 10 10 cm −3 ) or high-density (∼ 10 15 cm −3 ) regimes, where gas cooling rate is increased owing to three-body molecule formation and collision-induced emission, respectively. The cloud core becomes marginally unstable against chemo-thermal instability in the low-density regime. However, since the core is already compact at that point and correspondingly the sound-crossing time as well as the free-fall time are short, or comparable to the perturbation growth timescale, it does not fragment. Run-away cooling simply leads to fast condensation of the core to form a single proto-stellar seed. We also show that the core remains stable against gravitational deformation and fragmentation throughout the evolution. We trace in Lagrangian space the gas elements that end up at the center of the cloud, and study the evolution of the specific angular momentum. We show that, during the final dynamical collapse, small angular momentum material collapses faster...

Formation of supermassive black holes

“…Simulations of the collapse of primordial molecular clouds (Bromm et al, 1999(Bromm et al, , 2002Abel et al, 2000;Yoshida et al, 2006;Gao et al, 2007) suggest that the first generation of stars contained many 'very massive stars' (VMSs) with m > 100 M . This is because of the slow subsonic contraction -a regime set up by the main gas coolant, molecular hydrogen -further fragmentation into sub-components is not seen (although it is not clear if this is a numerical effect, rather than due to the gas physics, see Glover et al, 2008).…”

Assessing inflow rates in atomic cooling haloes: implications for direct collapse black holes

“…One potential pathway is the collapse of a primordial, i.e., metal-free, "normal" star into a stellar-mass black hole. Earlier numerical simulations performed to study the formation of the first generation of stars (Abel et al 2002;Bromm et al 2002; ⋆ Corresponding author: latif@iap.fr Yoshida et al 2006) suggested that the masses of the first stars are of the order of a few hundred solar. However, recent simulations found that the protostellar disk forming in the minihalo fragments into multiple clumps (Clark et al 2011;Greif et al 2012;Stacy et al 2012;Latif et al 2013b; and leads to the formation of multiple stars.…”

The luminosity of Population III star clusters

“…These 3 σ+ perturbations over the background dark matter density field virialized by redshifts of z ∼ 20-50 (Tegmark et al 1997;Abel et al 2002;Bromm et al 2002). With few exceptions (e.g., Turk et al 2009), these simulations suggest that the gas pooling into the halos underwent a quasi-hydrostatic contraction until they had sufficient mass to trigger runaway gravitational collapse (Abel et al 2002;Bromm et al 2002;Bromm & Loeb 2004;Yoshida et al 2006; O'Shea & Norman 2007; Yoshida et al 2008). These studies estab-⋆ e-mail: alexander.desouza@gmail.com, basu@uwo.ca lished the standard paradigm that the progenitor cloud cores of the first stars were most likely to have been massive and formed in relative isolation.…”