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,\nwhen the first stars began to fill the universe with new light. It has\ngenerally been argued that these stars formed in isolation and were extremely\nmassive - perhaps 100 times as massive as the Sun. In a recent study, Clark and\ncollaborators showed that this picture requires revision. They demonstrated\nthat the accretion disks that build up around Population III stars are strongly\nsusceptible to fragmentation and that the first stars should therefore form in\nclusters rather than in isolation. We here use a series of high-resolution\nhydrodynamical simulations performed with the moving mesh code AREPO to follow\nup on this proposal and to study the influence of environmental parameters on\nthe level of fragmentation. We model the collapse of five independent minihalos\nfrom cosmological initial conditions, through the runaway condensation of their\ncentral gas clouds, to the formation of the first protostar, and beyond for a\nfurther 1000 years. During this latter accretion phase, we represent the\noptically thick regions of protostars by sink particles. Gas accumulates\nrapidly in the circumstellar disk around the first protostar, fragmenting\nvigorously to produce a small group of protostars. After an initial burst,\ngravitational instability recurs periodically, forming additional protostars\nwith masses ranging from ~ 0.1 to 10 M_sun. Although the shape, multiplicity,\nand normalization of the protostellar mass function depend on the details of\nthe sink-particle algorithm, fragmentation into protostars with diverse masses\noccurs in all cases, confirming earlier reports of Population III stars forming\nin clusters. Depending on the efficiency of later accretion and merging,\nPopulation III stars may enter the main sequence in clusters and with much more\ndiverse masses than are commonly assumed.\n