Star formation

About: Star formation is a research topic. Over the lifetime, 37405 publications have been published within this topic receiving 1808161 citations. The topic is also known as: astrogenesis.

The universal mass accretion history of cold dark matter haloes

Abstract: We use the extended Press-Schechter formalism to investigate the rate at which cold dark matter haloes accrete mass. We discuss the shortcomings of previous methods that have been used to compute the mass accretion histories of dark matter haloes, and present an improved method based on the N-branch merger tree algorithm of Somerville & Kolatt. We show that this method no longer suffers from inconsistencies in halo formation times, and compare its predictions with high-resolution N-body simulations. Although the overall agreement is reasonable, there are slight inconsistencies which are most easily interpreted as a reflection of ellipsoidal collapse (as opposed to spherical collapse assumed in the Press-Schechter formalism). We show that the average mass accretion histories follow a simple, universal profile, and we present a simple recipe for computing the two scale-parameters which is applicable to a wide range of halo masses and cosmologies. Together with the universal profiles for the density and angular momentum distributions of cold dark matter haloes, these universal mass accretion histories provide a simple but accurate framework for modelling the structure and formation of dark matter haloes. In particular, they can be used as a backbone for modelling various aspects of galaxy formation where one is not interested in the detailed effects of merging. As an example we use the universal mass accretion history to compute the rate at which dark matter haloes accrete mass, which we compare with the cosmic star formation history of the Universe.

Early evolution of stellar groups and clusters: environmental effects on forming planetary systems

Abstract: This paper studies the dynamical evolution of young groups/clusters, with N = 100-1000 members, from their embedded stage out to ages of ~10 Myr. We use N-body simulations to explore how their evolution depends on the system size N and the initial conditions. Motivated by recent observations suggesting that stellar groups begin their evolution with subvirial speeds, this study compares subvirial starting states with virial starting states. Multiple realizations of equivalent cases (100 simulations per initial condition) are used to build up a robust statistical description of these systems, e.g., the probability distribution of closest approaches, the mass profiles, and the probability distribution for the radial location of cluster members. These results provide a framework from which to assess the effects of groups/clusters on the processes of star and planet formation and to study cluster evolution. The distributions of radial positions are used in conjunction with the probability distributions of the expected far-ultraviolet (FUV) luminosities (calculated here as a function of cluster size N) to determine the radiation exposure of circumstellar disks. The distributions of closest approaches are used in conjunction with scattering cross sections (calculated here as a function of stellar mass using ~105 Monte Carlo scattering experiments) to determine the probability of disruption for newly formed solar systems. We use the nearby cluster NGC 1333 as a test case in this investigation. The main conclusion of this study is that clusters in this size range have only a modest effect on forming planetary systems. The interaction rates are low, so that the typical solar system experiences a single encounter with closest approach distance b ~ 1000 AU. The radiation exposure is also low, with median FUV flux G0 ~ 900 (1.4 ergs s-1 cm-2), so that photoevaporation of circumstellar disks is only important beyond 30 AU. Given the low interaction rates and modest radiation levels, we suggest that solar system disruption is a rare event in these clusters.

Constraints on a Universal Stellar Initial Mass Function from Ultraviolet to Near-Infrared Galaxy Luminosity Densities

Abstract: We obtain constraints on the slope of a universal stellar initial mass function (IMF) over a range of model cosmic star formation histories (SFHs) using z ≈ 0.1 luminosity densities in the range from 0.2 to 2.2 μm. The age-IMF degeneracy of the integrated spectra of stellar populations can be broken for the universe as a whole by using direct measurements of (relative) cosmic SFH from high-redshift observations. These have only marginal dependence on uncertainties in the IMF, whereas fitting to local luminosity densities depends strongly on both cosmic SFH and the IMF. We fit to these measurements using population synthesis and find the best-fit IMF power-law slope to be Γ = 1.15 ± 0.2 (assuming dN/d log m ∝ m-Γ for 0.5-120 M☉ and m-0.5 for 0.1-0.5 M☉). This M > 0.5 M☉ slope is in good agreement with the Salpeter IMF slope (Γ = 1.35). A strong upper limit of Γ < 1.7 is obtained, which effectively rules out the Scalo IMF because its fraction of high-mass stars is too low. This upper limit is at the 99.7% confidence level if we assume a closed-box chemical evolution scenario and 95% if we assume constant solar metallicity. Fitting to the Hα line luminosity density, we obtain a best-fit IMF slope in good agreement with that derived from broadband measurements. Marginalizing over cosmic SFH and IMF slope, we obtain (95% confidence ranges) Ωstars = × 10-3 h-1 for the stellar mass density, ρSFR = × 10-2 h M☉ yr-1 Mpc-3 for the star formation rate density, and ρL = × 1035 h W Mpc-3 for the bolometric, attenuated, stellar luminosity density (0.09-5 μm). Comparing this total stellar emission with an estimate of the total dust emission implies a relatively modest average attenuation in the UV (1 mag at 0.2 μm).

Feedback from central black holes in elliptical galaxies: two-dimensional models compared to one-dimensional models

Abstract: We extend the black hole (BH) feedback models of Ciotti, Ostriker, and Proga to two dimensions. In this paper, we focus on identifying the differences between the one-dimensional and two-dimensional hydrodynamical simulations. We examine a normal, isolated L * galaxy subject to the cooling flow instability of gas in the inner regions. Allowance is made for subsequent star formation, Type Ia and Type II supernovae, radiation pressure, and inflow to the central BH from mildly rotating galactic gas which is being replenished as a normal consequence of stellar evolution. The central BH accretes some of the infalling gas and expels a conical wind with mass, momentum, and energy flux derived from both observational and theoretical studies. The galaxy is assumed to have low specific angular momentum in analogy with the existing one-dimensional case in order to isolate the effect of dimensionality. The code then tracks the interaction of the outflowing radiation and winds with the galactic gas and their effects on regulating the accretion. After matching physical modeling to the extent possible between the one-dimensional and two-dimensional treatments, we find essentially similar results in terms of BH growth and duty cycle (fraction of the time above a given fraction of the Eddington luminosity). In the two-dimensional calculations, the cool shells forming at 0.1-1?kpc from the center are Rayleigh-Taylor unstable to fragmentation, leading to a somewhat higher accretion rate, less effective feedback, and a more irregular pattern of bursting compared with the one-dimensional case.