In this paper we introduce a new radiative transfer code SPRAI (Simplex Photon Radiation in the Arepo Implementation) based on the SimpleX radiation transfer method. This method, originally used only for post-processing, is now directly integrated into the Arepo code and takes advantage of its adaptive unstructured mesh. Radiated photons are transferred from the sources through the series of Voronoi gas cells within a specific solid angle. From the photon attenuation we derive corresponding photon fluxes and ionization rates and feed them to a primordial chemistry module. This gives us a self-consistent method for studying dynamical and chemical processes caused by ionizing sources in primordial gas. Since the computational cost of the SimpleX method does not scale directly with the number of sources, it is convenient for studying systems such as primordial star-forming halos that may form multiple ionizing sources.

We present version 2 of the radiation transfer module sprai (Simplex Photon Radiation in the arepo Implementation). sprai is a novel method for solving the equations of transfer on an unstructured mesh using a variant of the short characteristics approach. It has several advantages compared to other approaches: its computational cost is independent of the number of radiation sources (unlike typical ray-tracing methods) and it is less diffusive than moment-based methods. Version 1 of sprai (presented in Jaura et al. 2018) has already been shown to do an excellent job of modelling the growth of R-type ionization fronts in low density gas. However, it does not perform so well with D-type fronts in denser gas unless run with a small time-step. Version 2 of the code addresses this weakness in the algorithm, allowing us to dramatically improve its performance in dense gas. Version 2 of sprai also includes two important updates to the microphysics treated in the code: a revised multi-frequency framework that allows us to model helium photoionization, and a treatment of the effects of radiation pressure. In this paper, we describe these enhancements to sprai and also present several tests of the code.

Radiative feedback from massive Population III (Pop. III) stars in the form of ionising and photodissociating photons is widely believed to play a central role in shutting off accretion on to these stars. Understanding whether and how this occurs is vital for predicting the final masses reached by these stars and the form of the Pop. III stellar initial mass function. To help us better understand the impact of UV radiation from massive Pop. III stars on the gas surrounding them, we carry out high resolution simulations of the formation and early evolution of these stars, using the AREPO moving-mesh code coupled with the innovative radiative transfer module SPRAI. Contrary to most previous results, we find that the ionising radiation from these stars is trapped in the dense accretion disk surrounding them. Consequently, the inclusion of radiative feedback has no significant impact on either the number or the total mass of protostars formed during the 20 kyr period that we simulate. We show that the reason that we obtain qualitatively different results from previous studies of Pop. III stellar feedback lies in how the radiation is injected into the simulation. HII region trapping only occurs if the photons are injected on scales smaller than the local scale height of the accretion disk, a criterion not fulfilled in previous 3D simulations of this process. Finally, we speculate as to whether outflows driven by the magnetic field or by Lyman-α radiation pressure may be able to clear enough gas away from the star to allow the HII region to escape from the disk.

The lack of observations of abundance patterns originating in pair-instability supernovae has been a long-standing problem in relation to the first stars. This class of supernovae is expected to have an abundance pattern with a strong odd–even effect, making it substantially different from present-day supernovae. In this study, we use a cosmological radiation hydrodynamics simulation to model such supernovae and the subsequent formation of the second generation of stars. We incorporate streaming velocities for the first time. There are 14 star-forming minihalos in our 1 cMpc h −1 box, leading to 14 supernovae occurring before redshift z = 19.5, where we start reducing the complexity of the simulation. Following the explosions, extremely metal-poor stars form in 10 halos via internal and external enrichment, which makes it the most common outcome. Only one halo does not recollapse during the simulations. This result is at variance with the current (lack of) observations of metal-poor stars with pair-instability supernova abundance patterns, suggesting that these very massive stars might be rare even in the early universe. The results from this simulation also give us insights into what drives different modes of recollapse and what determines the mixing behavior of metals after very energetic supernovae.