Radiative Feedback in Population III Protostellar Growth: The Impact of HI Shielding

TL;DR

This study uses radiation-magnetohydrodynamics simulations to explore how HI shielding affects radiative feedback and protostellar growth in Population III stars, revealing that shielding can enhance accretion and increase stellar masses.

Contribution

It provides the first detailed analysis of HI shielding's role in Pop III star formation, highlighting its impact on feedback mechanisms and stellar mass accumulation.

Findings

HI shielding weakens LW feedback, allowing higher accretion rates.

HII regions remain compact due to high gas densities and replenishment.

Shielding leads to larger final stellar masses compared to models without shielding.

Abstract

We present a suite of radiation-magnetohydrodynamics simulations from the POPSICLE project that follow the long-term growth (~50 kyr) of primordial protostars while self-consistently coupling radiation, turbulence, and magnetic fields. The simulation suite is designed to quantify the relative impacts of the pathways of radiative feedback in Pop III stars - the extreme-ultraviolet (EUV) ionization and Lyman-Werner (LW) dissociation - by considering simulations with and without their inclusion. We find that without HI shielding, LW feedback alone can suppress and ultimately terminate accretion. With HI shielding, the large column densities near the protostar significantly weaken LW feedback. In the polar direction, atomic hydrogen fully shields LW radiation where $H_2$ self-shielding alone is insufficient. This leads to lower gas temperatures near the protostar and higher accretion rates, yielding larger final stellar masses than in models without shielding. The HII regions remain compact and confined to less than about 100 AU measured outward from the sink accretion radius (75 AU) due to high gas densities and continuous gas replenishment that inhibit the thermal pressure-driven breakout of the ionization front even for high ionizing luminosities. These results demonstrate that the interplay of gas dynamics, shielding, and radiative feedback can significantly alter the growth of Pop III stars. We discuss the implications for the initial mass function of primordial stars and the influence of feedback from early stellar populations.