First star formation in extremely early epochs

TL;DR

This study explores the formation of the first stars at extremely early cosmic epochs ($z ge 100$), revealing that the cosmic microwave background significantly influences their mass and formation process, potentially leading to supermassive stars.

Contribution

It demonstrates that first stars could form much earlier than previously thought, with their masses heavily affected by CMB effects at high redshifts, especially above $z ge 100$.

Findings

First stars with masses over 1000 solar masses can form at $z ge 100$.

At $z ge 500$, atomic cooling dominates, enabling formation of supermassive stars around $10^5$ solar masses.

CMB effects limit H$_2$ formation, altering the typical mass and formation epoch of the first stars.

Abstract

First stars play crucial roles in development of the universe, influencing events like cosmic reionization and the chemical enrichment. While first stars are conventionally thought to form at around $z \sim 20-30$ in the standard $\Lambda$ Cold Dark Matter ($\Lambda$CDM) cosmology, observational constraints on small-scale density fluctuations remain limited, possibly differing significantly from the scale-invariant fluctuations assumed in the $\Lambda$CDM model. Should this be the case, the formation of first stars could occur much earlier than typically predicted. In this study, we investigate the formation process of first stars in the extremely early epochs of $z \gtrsim 100$ in the post-recombination universe. At such early times, the effects of the warm cosmic microwave background (CMB) become significant. We calculate the collapse of primordial star-forming clouds using a one-zone thermo-chemical model that accounts for CMB influences on radiative heating, Compton cooling, and photodissociation reactions. We found that the impact of the CMB on the evolution is limited at $z \lesssim 100$, with the temperature evolution closely resembling the conventional model. However, within the range $100 \lesssim z \lesssim 400$, the formation of H$_2$ via the H$^-$ channel is impeded by H$^-$ photodetachment induced by the CMB, leading to higher temperatures compared to standard one. Consequently, first stars with masses exceeding $1000 ~\mathrm{M}_\odot$ can emerge at $z \gtrsim 100$. Furthermore, at $z \gtrsim 500$, the temperature evolution becomes nearly isothermal solely due to atomic cooling, as H$_2$ formation is entirely suppressed. In such cases, supermassive stars with masses around $\sim 10^5 ~\mathrm{M}_\odot$ are expected to form solely via atomic cooling. These findings emphasize the significant variation in the typical mass of the first stars depending on the epoch of formation.