Role of the H$_2^+$ channel in the primordial star formation under strong radiation field and the critical intensity for the supermassive star formation

TL;DR

This study examines how the H_2^+ channel influences molecular hydrogen formation in primordial gas clouds under strong radiation, revealing its dominance in soft spectra and impact on the critical intensity for supermassive star formation.

Contribution

It provides a detailed analysis of the vibrationally-resolved H_2^+ kinetics and clarifies the conditions under which the H_2^+ channel dominates in primordial environments.

Findings

H_2^+ channel dominates in soft radiation fields with T_rad < 7000 K.

The critical radiation intensity for supermassive star formation depends on H_2^+ level populations.

LTE assumption underestimates critical intensity for soft spectra.

Abstract

We investigate the role of the H_2^+ channel on H_2 molecule formation during the collapse of primordial gas clouds immersed in strong radiation fields which are assumed to have the shape of a diluted black-body spectra with temperature T_rad. Since the photodissociation rate of H_2^+ depends on its level population, we take full account of the vibrationally-resolved H_2^+ kinetics. We find that in clouds under soft but intense radiation fields with spectral temperature T_rad < 7000 K, the H_2^+ channel is the dominant H_2 formation process. On the other hand, for harder spectra with T_rad > 7000 K, the H^- channel takes over H_2^+ in the production of molecular hydrogen. We calculate the critical radiation intensity needed for supermassive star formation by direct collapse and examine its dependence on the H_2^+ level population. Under the assumption of local thermodynamic equilibrium (LTE) level population, the critical intensity is underestimated by a factor of a few for soft spectra with T_rad < 7000 K. For harder spectra, the value of the critical intensity is not affected by the level population of H_2^+. This result justifies previous estimates of the critical intensity assuming LTE populations since radiation sources like young and/or metal-poor galaxies are predicted to have rather hard spectra.