Radiation magnetohydrodynamics simulations of Population III star formation during the Epoch of Reionization

TL;DR

This study uses high-resolution ($7.5\,\mathrm{au}$) radiation-MHD simulations (POPSICLE) to explore Population III star formation during the Epoch of Reionization by comparing pristine primordial clouds at $z=30$ and $z=6$, with and without a strong external Lyman-Werner background. The analysis shows that the cosmic microwave background temperature floor can reduce fragmentation at $z=6$ relative to $z=30$, yielding a higher median Pop III stellar mass even in identical environments, while magnetic fields and radiation feedback modulate accretion and fragmentation. In the presence of a strong LW background, runaway collapse produces very massive stars ($M_{\star}>100\,\mathrm{M}_{\odot}$) with high accretion rates and potential mergers, dramatically increasing star formation efficiency and enabling extreme mass growth. Overall, the results indicate a non-universal Pop III IMF that varies with redshift and radiative environment, underscoring the dominant roles of CMB temperature, radiation backgrounds, and magnetic fields in shaping primordial star formation during the EoR.

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 ($z \geq 20$) 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 ($7.5\,\rm{au}$) radiation-magnetohydrodynamics simulations of identical primordial clouds exposed to the 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 which 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 ($M_{\star} \approx 70\,\rm{M_{\odot}}$) 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 ($M_{\star} > 100\,\rm{M_{\odot}}$) stars. Our results suggest that Population III 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 as compared to $z \geq 20$, with the formation of very massive and supermassive stars likely in strongly irradiated environments.

Figures (11)

• Figure 1: Evolution of the individual stellar masses of stars (top), and the mass accretion rate (bottom) for the $z = 6$ (solid) and $z=30$ (dashed) simulations. Note that the accretion rates of only the four most massive stars are shown at $z=30$ for clarity.
• Figure 2: Phase diagrams of the star-forming core in the $z=30$ and $z=6$ simulations $5000\,\rm{yr}$ since first star formation, showing the gas temperature plotted against the number density, color coded by the cell mass. The CMB temperature at $z=30$ and at $z=6$ is indicated with gray, dashed lines.
• Figure 3: Density-weighted projection plots of gas temperature, number density, and $\rm{H}_2$ and $\rm{H}^{+}$ mass fractions $5000\,\rm{yr}$ since the formation of the first star. The projections are oriented along the $\hat{L}$ axis (angular momentum vector of the star forming disk), zoomed in on the inner $0.01\,\rm{pc}$ around the center of mass (COM). The top row shows the $z=30$ simulation and the bottom row shows the run at $z=6$. White dots mark the projected positions of sink particles, which serve as proxies for Pop III stars.
• Figure 4: Same as \ref{['fig:nobg_overview_basic']} but for the radial, azimuthal (rotational), and vertical velocity. Negative radial motions reflect infall of gas towards the COM.
• Figure 5: Density-weighted projections along the angular momentum vector of the star forming disk, showing how mass inflows are impacted by magnetic fields and turbulence. From left to right, the panels show the divergence of the gas velocity field, turbulent Mach number, magnetic field strength, and the ratio of gas pressure over magnetic field pressure through plasma $\beta$. Negative values of $\nabla \cdot \vec{v}$ indicate net inflow/compression of gas. $\mathcal{M} > 1$ indicates supersonic turbulence and plasma $\beta < 1$ indicates that the magnetic field is dominant as compared to gravity.