The characteristic stellar mass as a function of redshift

We present a model for the star formation process during the initial collapse of dark matter haloes at redshifts z=0-30. We derive a simple expression for the characteristic stellar mass scale during this initial burst of star formation. In our picture, this characteristic scale reflects both the minimum temperature to which the gas can cool (determined by the metallicity and the temperature of the cosmic microwave background) and the pressure of overlying baryons in the collapsing halo. This prescription reproduces both the large mass scales found in simulations of Population III star formation and the near solar values observed for star formation at low redshift.Comment: 8 pages, 4 figures, accepted for publication in MNRA

Do all stars in the solar neighbourhood form in clusters? A cautionary note on the use of the distribution of surface densities

Bressert et al. recently showed that the surface density distribution of low-mass, young stellar objects (YSOs) in the solar neighbourhood is approximately log-normal. The authors conclude that the star formation process is hierarchical and that only a small fraction of stars form in dense star clusters. Here, we show that the peak and the width of the density distribution are also what follow if all stars form in bound clusters which are not significantly affected by the presence of gas and expand by two-body relaxation. The peak of the surface density distribution is simply obtained from the typical ages (few Myr) and cluster membership number (few hundred) typifying nearby star-forming regions. This result depends weakly on initial cluster sizes, provided that they are sufficiently dense (initial half mass radius of <0.3 pc) for dynamical evolution to be important at an age of a few Myr. We conclude that the degeneracy of the YSO surface density distribution complicates its use as a diagnostic of the stellar formation environment.Comment: 5 pages, 3 figures, MNRAS Letter; Updated to match final journal styl

Accretion of sub-stellar companions as the origin of chemical abundance inhomogeneities in globular clusters

Globular clusters exhibit abundance variations, defining `multiple populations', which have prompted a protracted search for their origin. Properties requiring explanation include: the high fraction of polluted stars ($\sim 40{-}90$~percent, correlated with cluster mass), the absence of pollution in young clusters and the lower pollution rate with binarity and distance from the cluster centre. We present a novel mechanism for late delivery of pollutants into stars via accretion of sub-stellar companions. In this scenario, stars move through a medium polluted with AGB and massive star ejecta, accreting material to produce companions with typical mass ratio $q\sim 0.1$. These companions undergo eccentricity excitation due to dynamical perturbations by passing stars, culminating in a merger with their host star. The accretion of the companion alters surface abundances via injected pollutant. Alongside other self-enrichment models, the companion accretion model can explain the dilution of pollutant and correlation with intra-cluster location. The model also explains the ubiquity and discreteness of the populations and correlations of enrichment rates with cluster mass, cluster age and stellar binarity. Abundance variations in some clusters can be broadly reproduced using AGB and massive binary ejecta abundances from the literature. In other clusters, some high companion mass ratios ($q\gtrsim 1$) are required. In these cases, the available mass budget necessitates a variable degree of mixing of the polluted material with the primary star, deviations from model ejecta abundances or mixing of internal burning products. We highlight the avenues of further investigation which are required to explore some of the key processes invoked in this model.Comment: 29 pages, 20 figures, accepted for publication in MNRA

The first multidimensional view of mass loss from externally FUV irradiated protoplanetary discs

Computing the flow from externally FUV irradiated protoplanetary discs requires solving complicated and expensive photodissociation physics iteratively in conjunction with hydrodynamics. Previous studies have therefore been limited to 1D models of this process. In this paper we compare 2D-axisymmetric models of externally photoevaporating discs with their 1D analogues, finding that mass loss rates are consistent to within a factor four. The mass loss rates in 2D are higher, in part because half of the mass loss comes from the disc surface (which 1D models neglect). 1D mass loss rates used as the basis for disc viscous evolutionary calculations are hence expected to be conservative. We study the anatomy of externally driven winds including the streamline morphology, kinematic, thermal and chemical structure. A key difference between the 1D and 2D models is in the chemical abundances. For instance in the 2D models CO can be dissociated at smaller radial distances from the disc outer edge than in 1D calculations because gas is photodissociated by radiation along trajectories that are assumed infinitely optically thick in 1D models. Multidimensional models will hence be critical for predicting observable signatures of environmentally photoevaporating protoplanetary discs.Comment: 15 pages, 12 figures. Accepted for publication in the MNRAS main journa