The formation of massive primordial stars in the presence of moderate UV backgrounds

TL;DR

This study investigates how moderate UV backgrounds influence the formation of massive primordial stars, revealing that UV radiation suppresses fragmentation and leads to the formation of large protostars with high accretion rates in early halos.

Contribution

It provides high-resolution simulations showing the impact of UV flux on primordial star formation, highlighting a new pathway for forming massive stars in the early universe.

Findings

UV radiation suppresses fragmentation in primordial halos

Massive protostars of hundreds to thousands of solar masses form under UV irradiation

High accretion rates (0.01-0.1 M_sun/yr) observed in UV-illuminated halos

Abstract

Radiative feedback from populations II stars played a vital role in early structure formation. Particularly, photons below the Lyman limit can escape the star forming regions and produce a background ultraviolet (UV) flux which consequently may influence the pristine halos far away from the radiation sources. These photons can quench the formation of molecular hydrogen by photo-detachment of $\rm H^{-}$. In this study, we explore the impact of such UV radiation on fragmentation in massive primordial halos of a few times $\rm 10^{7}$~M${_\odot}$. To accomplish this goal, we perform high resolution cosmological simulations for two distinct halos and vary the strength of the impinging background UV field in units of $\rm J_{21}$. We further make use of sink particles to follow the evolution for 10,000 years after reaching the maximum refinement level. No vigorous fragmentation is observed in UV illuminated halos while the accretion rate changes according to the thermal properties. Our findings show that a few 100-10, 000 solar mass protostars are formed when halos are irradiated by $\rm J_{21}=10-500$ at $\rm z>10$ and suggest a strong relation between the strength of UV flux and mass of a protostar. This mode of star formation is quite different from minihalos, as higher accretion rates of about $\rm 0.01-0.1$ M$_{\odot}$/yr are observed by the end of our simulations. The resulting massive stars are the potential cradles for the formation of intermediate mass black holes at earlier cosmic times and contribute to the formation of a global X-ray background.