Radiation magnetohydrodynamics simulations of Population III star formation during the epoch of reionization

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Transdisciplinary Research Area (TRA), Argelander-Institut für Astronomie, University of Bonn, 53113 Bonn, Germany
³ Department of Physics & Astronomy, University College London, London WC1E 6BT, UK
⁴ Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA
⁵ Department of Physics and Astronomy, Rutgers University, 136 Frelinghuysen Road, Piscataway, NJ 08854, USA

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Received: 14 November 2025
Accepted: 14 January 2026

Abstract

Cosmological simulations find that pockets of star-forming gas could remain pristine up until the epoch of reionization (EoR) due to the inhomogeneous nature of metal mixing and enrichment in the early Universe. Such pristine clouds could have formed Population III stars, which could have distinct properties compared to their very high redshift (z ≥ 20) counterparts. We investigate how Population III stars form and grow during the EoR, and whether the resulting mass distribution varies with environment or across cosmic time. We perform high-resolution (7.5 au) radiation-magnetohydrodynamics simulations of identical primordial clouds exposed to the cosmic microwave background (CMB) appropriate for z = 30 and z = 6, respectively, as part of the POPSICLE project. We also run a simulation at z = 6 with a strong external Lyman-Werner (LW) background, to span across radiative environments that could host metal-free clumps during the EoR. In the limit of no external LW radiation, we find that while the evolution of the most massive star (M_* ≈ 70M_⊙) is almost identical between z = 30 and z = 6, the latter exhibits less fragmentation, leading to a smaller cluster of stars with a higher median stellar mass. In the limit of high external LW radiation, we see vigorous accretion and high star formation efficiencies, leading to the formation of very massive (M_* > 100 M_⊙) stars. Our results suggest that Population III initial mass function (IMF) could vary with redshift simply due to the CMB, independent of the environment. We find that less massive and more compact Pop III star clusters could form during the EoR compared to z ≥ 20, with the formation of very massive and supermassive stars likely in strongly irradiated environments.