Simulations on a Moving Mesh: The Clustered Formation of Population Iii Protostars

Abstract

The cosmic dark ages ended a few hundred million years after the Big Bang, when the first stars began to fill the universe with new light. It has generally been argued that these stars formed in isolation and were extremely massive -perhaps 100 times as massive as the Sun. In a recent study, Clark and collaborators showed that this picture requires revision. They demonstrated that the accretion disks that build up around Population III stars are strongly susceptible to fragmentation and that the first stars should therefore form in clusters rather than in isolation. We here use a series of high-resolution hydrodynamical simulations performed with the moving mesh code AREPO to follow up on this proposal and to study the influence of environmental parameters on the level of fragmentation. We model the collapse of five independent minihalos from cosmological initial conditions, through the runaway condensation of their central gas clouds, to the formation of the first protostar, and beyond for a further 1000 years. During this latter accretion phase, we represent the optically thick regions of protostars by sink particles. Gas accumulates rapidly in the circumstellar disk around the first protostar, fragmenting vigorously to produce a small group of protostars. After an initial burst, gravitational instability recurs periodically, forming additional protostars with masses ranging from ∼ 0.1 to 10 M ⊙ . Although the shape, multiplicity, and normalization of the protostellar mass function depend on the details of the sink-particle algorithm, fragmentation into protostars with diverse masses occurs in all cases, confirming earlier reports of Population III stars forming in clusters. Depending on the efficiency of later accretion and merging, Population III stars may enter the main sequence in clusters and with much more diverse masses than are commonly assumed.

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

“…For instance, recent 3D numerical simulations show that a circumstellar disc becomes gravitationally unstable and fragments (e.g., Clark et al 2011;Greif et al 2011Greif et al , 2012Stacy & Bromm 2013). This could lead to the formation of multiple stellar systems rather than a single star in each mini-halo.…”

The luminosity of Population III star clusters

“…The disk begins to fragment within a few hundred years after its formation. This timescale is somewhat longer than that found by Clark et al (2011) and Greif et al (2011), who used sink cells with smaller radii of 1.5 AU. However, this timescale is similar to that found by Smith et al (2012a), who used sink cells with comparable radii of 20 AU (the central sink cell in our simulation being ∼10 AU in radius).…”

“…The gas to dark matter fraction in these halos is roughly 10%, and amounts to a gas mass of 10 4 to 10 5 M⊙. Numerical simulations of this collapse reveal that the gas streaming into these potential wells exhibits filamentary and knotty structure (e.g., Bromm et al 1999;Clark et al 2008;Greif et al 2011). Indeed, gravity is well known to enhance such anisotropic structure during collapse (e.g., Lin et al 1965).…”

“…Motivated by this, several recent studies have explored the fragmentary nature of primordial gas in the early universe, and have been able to resolve fragmentation in the disk-like environments surrounding the first protostars, thus challenging the standard paradigm (Stacy et al 2010;Clark et al 2011;Greif et al 2011;Vorobyov et al 2013, hereafter VDB 2013.…”

Primordial star clusters at extreme magnification

“…Due to the cooling properties of zerometallicity gas, the stellar initial mass function (IMF) of Pop III stars is predicted to be top-heavy, peaking at ∼ 10-100 M⊙ (e.g. Greif et al 2011; Hosokawa et al…”