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The formation of stars is a key process in the early universe with far reaching consequences for further cosmic evolution. While stars forming from truly primordial gas are thought to be on average more massive than our Sun, stars in the universe today have typical masses below one solar mass. The physical origin of this transition and the conditions under which it occurs are highly debated. Were, we focus on the importance of thermodynamics. We discuss the process of cooling-induced fragmentation and assess its influence on the initial mass function of stars. [ABSTRACT FROM AUTHOR]

We present a suite of three-dimensional, moving mesh simulations that directly resolve the collapse of the gas beyond the formation of the first protostar at the center of a dark matter minihalo. In analogy to previous studies that employed sink particles, we find that the Keplerian disk around the primary protostar fragments into a number of secondary protostars, which is facilitated by H2 collisional dissociation cooling and collision-induced emission. The further evolution of the protostellar system is characterized by strong gravitational torques that transfer angular momentum between the secondary protostars formed in the disk and the surrounding gas. This leads to the migration of about half of the secondary protostars to the center of the cloud in a free-fall time, where they merge with the primary protostar and enhance its growth to about five times the mass of the second most massive protostar. By the same token, a fraction of the protostars obtain angular momentum from other protostars via N-body interactions and migrate to higher orbits. On average, only every third protostar survives until the end of the simulation. However, the number of protostars present at any given time increases monotonically, suggesting that the protostellar system will continue to grow beyond the limited period of time simulated. [ABSTRACT FROM AUTHOR]

We study how the first galaxies were assembled under feedback from the accretion onto a central black hole (BH) that is left behind by the first generation of metal-free stars through selfconsistent, cosmological simulations. X-ray radiation fromthe accretion of gas onto BH remnants of Population III (Pop III) stars, or from high-mass X-ray binaries (HMXBs), again involving Pop III stars, influences the mode of second generation star formation. We track the evolution of the black hole accretion rate and the associated X-ray feedback startingwith the death of the Pop III progenitor star inside a minihalo and following the subsequent evolution of the black hole as the minihalo grows to become an atomically cooling galaxy. We find that X-ray photoionization heating from a stellar-mass BH is able to quench further star formation in the host halo at all times before the halo enters the atomic cooling phase. X-ray radiation from a HMXB, assuming a luminosity close to the Eddington value, exerts an even stronger, and more diverse, feedback on star formation. It photoheats the gas inside the host halo, but also promotes the formation of molecular hydrogen and cooling of gas in the intergalactic medium and in nearby minihalos, leading to a net increase in the number of stars formed at early times. Our simulations further show that the radiative feedback from the first BHs may strongly suppress early BH growth, thus constraining models for the formation of supermassive BHs. [ABSTRACT FROM AUTHOR]

It has been suggested that turbulent motions are responsible for the transport of angular momentum during the formation of Pop. III stars, however the exact details of this process have never been studied. We report the results from three dimensional SPH simulations of a rotating self-gravitating primordial molecular cloud, in which the initial velocity of solid-body rotation has been changed. We also examine the build-up of the discs that form in these idealized calculations. [ABSTRACT FROM AUTHOR]

Stars and star clusters are the fundamental visible building blocks of galaxies at present days as well as in the early universe. Today, star formation is controlled by the complex interplay between self-gravity and supersonic turbulence in the interstellar medium. Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. This determines the statistical characteristics of a stellar population such as the initial mass function or the spatial distribution of stars. The role of turbulence in primordial star formation is less well understood. In this contribution, we discuss results from recent numerical calculations that study the influence of turbulence on stellar birth in zero-metallicity and low-metallicity gas. Special emphasis lies on the distribution of stellar masses and their dependency on metallicity. [ABSTRACT FROM AUTHOR]

Using three-dimensional cosmological simulations, we study the assembly process of one of the first galaxies, with a total mass of ≃108 M⊙ collapsing at z≃10. We trace the transport of the heavy chemical elements produced and dispersed by a pair-instability supernova exploding in one of the minihalo progenitors. We find that mixing is quite efficient and is facilitated by turbulence generated by the virialization of the galaxy and photoheating from nearby stars. At the center of the newly formed galaxy, ∼105 M⊙ of cold, dense gas uniformly enriched to ∼10-3 Z⊙ are in a state of collapse, suggesting that a cluster of Population II stars will form. The first galaxies, as may be detected by the James Webb Space Telescope, would therefore already contain stellar populations familiar from lower redshifts. [ABSTRACT FROM AUTHOR]

We discuss the formation of supermassive black holes in the early universe, and how to probe their subsequent evolution with the upcoming mm/sub-mm telescope ALMA. We first focus on the chemical and radiative conditions for black hole formation, in particular considering radiation trapping and molecular dissociation effects. We then turn our attention towards the magnetic properties in the halos where the first black holes form, and show that the presence of turbulence may lead to a magnetic dynamo, which could support the black hole formation process by providing an efficient means of transporting the angular momentum. We finally focus on observable properties of high-redshift black holes with respect to ALMA, and discuss how to distinguish between chemistry driven by the starburst and chemistry driven by X-rays from the black hole. [ABSTRACT FROM AUTHOR]

We study the formation and evolution of the protostellar discs that form around the first stars in the Universe. Using sink particles, we replace the gravitationally bound gas at densities higher than 1015 cm-3 and radii greater than 3 AU from the central protostellar core, with an accreting point mass that is able to gravitationally interact with the surrounding gas. We find the disc is gravitationally (or 'Toomre') unstable, and is dominated by a strong m = 2 spiral mode. Although the angular momentum transport is dominated by a combination of gravitational torques and Reynolds stresses, which are extremely efficient mechanisms, the disc is unable to process the infalling material and grows increasingly gravitationally unstable. During the build-up of the disc, the temperature in the gas is regulated by a combination of H2 line cooling, collision-induced emission and H2 dissociation, which together help to offset heating from the gravitational collapse and feedback from the protostar. Once the disc starts to fragment, H2 dissociation keeps the gas almost isothermal as the collapse of the fragment progresses. The fragmentation occurs when the protostar/disc system is only 230 yr old and at a distance of ∼20 AU from its sibling, by which point the central protostar has a mass of ∼1 M⊙. Given the angular momentum of the new protostellar system, it is likely that the protostars will grow to become a massive binary system. [ABSTRACT FROM AUTHOR]

The theory for the formation of the first population of stars (Pop III) predicts a initial mass function (IMF) dominated by high-mass stars, in contrast to the present-day IMF, which tends to yield mostly stars with masses less than 1 M⊙. The leading theory for the transition in the characteristic stellar mass predicts that the cause is the extra cooling provided by increasing metallicity. In particular, dust can overtake H2 as the leading coolant at very high densities. To investigate this, we make use of high-resolution hydrodynamic simulations with sink particles to replace contracting protostars, and analyze the collapse and further fragmentation of star-forming clouds. We model four clouds with different metallicities (10-4, 10-5, 10-6Z⊙, and 0), and determine the properties of each cloud at the point at which it undergoes gravitational fragmentation. We find evidence for fragmentation in all four cases, and hence conclude that there is no critical metallicity below which fragmentation is impossible. Nevertheless, there is a clear change in the behavior of the clouds at Z = 105Z⊙, caused by the fact that at this metallicity, fragmentation takes longer to occur than accretion, leading to a flat mass function at lower metallicities. [ABSTRACT FROM AUTHOR]

Recent works (such as Greif et al. 2011[1] and Smith et al. 2011[2]) show that the degree of fragmentation of Population III stars depends strongly on the conditions in the halo, and halo-tohalo differences can be large. In order to provide the field with a better statistical understanding of potential halo-to-halo differences, we performed a large volume (∼ 1Mpc) pure N-body simulation of dark matter mini-haloes. We measured various physical properties such as virial ratios, spin parameter, shape, and formation times of these high-redshift (z ≥ 15) dark matter mini-haloes. This study can be used to derive initial conditions for detailed hydrodynamic simulations of star formation in individual mini-haloes. [ABSTRACT FROM AUTHOR]

We perform the first 3D simulations to include the effects of dark matter annihilation (DMA) in a calculation of the collapse of a primordial halo to near stellar densities. We couple the DMA to a time dependent chemical network, and include the effects of DMA induced ionisation. Despite an increase in the temperature of the halos, the gas temperature cannot rise above 2000 K due to cooling from H2 dissociation, and a normal Population III star forms at the centre. After the first star forms, the increased temperatures in the disk suppress further fragmentation. In our two simulated halos, fragmentation is suppressed completely in one, and only one secondary protostar forms in the second. However it is possible that in the future the baryons could become displaced from the dark matter peak and these effects could be reduced. [ABSTRACT FROM AUTHOR]

We report results from numerical simulations of star formation in the early universe that focus on gas at very high densities and very low metallicities. We argue that the gas in the central regions of protogalactic halos will fragment as long as it carries sufficient angular momentum. Rotation leads to the build-up of massive disk-like structures which fragment to form protostars. At metallicities Z≈10-5 Z⊙, dust cooling becomes effective and leads to a sudden drop of temperature at densities above n = 1012 cm-3. This induces vigorous fragmentation, leading to a very densely-packed cluster of low-mass stars. This is the first stellar cluster. The mass function of stars peaks below 1 M⊙, similar to what is found in the solar neighborhood, and comparable to the masses of the very-low metallicity subgiant stars recently discovered in the halo of our Milky Way. We find that even purely primordial gas can fragment at densities 1014 cm-3<=n<=1016 cm-3, although the resulting mass function contains only a few objects (at least a factor of ten less than the Z = 10-6 Z⊙. mass function), and is biased towards higher masses. A similar result is found for gas with Z = 10-6 Z⊙. Gas with Z <=10-6 Z⊙ behaves roughly isothermally at these densities (with polytropic exponent γ≈1.06) and the massive disk-like structures that form due to angular momentum conservation will be marginally unstable. As fragmentation is less efficient, we expect stars with Z<=10-6 Z⊙ to be massive, with masses in excess of several tens of solar masses, consistent with the results from previous studies. [ABSTRACT FROM AUTHOR]

We discuss possible implications of strong primordial magnetic fields, which may have been created during cosmic inflation or the QCD / electroweak phase transition, and explore how they influence the epoch of reionization and primordial star formation. Globally, magnetic fields may delay reionization, as the magnetic pressure suppresses gravitational collapse in small halos. In the protostellar collapse phase, we find that ambipolar diffusion heating does not change the fragmentation mass scale significantly, but may enhance the accretion rate at higher densities. [ABSTRACT FROM AUTHOR]

We present initial results from the first three-dimensional simulation of Population III (Pop III) star formation to include accretion luminosity from the central protostar, which is a significant heat source for the inner regions of the accretion disk. To test whether it can suppress fragmentation of the disk, we include a heating term in a simulation of a Pop III protostellar disk that was previously shown to be unstable in the absence of feedback. We find that while accretion luminosity alters the radial position of the fragmentation, it does not prevent it. [ABSTRACT FROM AUTHOR]