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We investigate the role of the H_2^+ channel on H_2 molecule formation duringthe collapse of primordial gas clouds immersed in strong radiation fields whichare assumed to have the shape of a diluted black-body spectra with temperatureT_rad. Since the photodissociation rate of H_2^+ depends on its levelpopulation, we take full account of the vibrationally-resolved H_2^+ kinetics.We find that in clouds under soft but intense radiation fields with spectraltemperature T_rad < 7000 K, the H_2^+ channel is the dominant H_2 formationprocess. 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 calculatethe critical radiation intensity needed for supermassive star formation bydirect collapse and examine its dependence on the H_2^+ level population. Underthe assumption of local thermodynamic equilibrium (LTE) level population, thecritical intensity is underestimated by a factor of a few for soft spectra withT_rad < 7000 K. For harder spectra, the value of the critical intensity is notaffected by the level population of H_2^+. This result justifies previousestimates of the critical intensity assuming LTE populations since radiationsources like young and/or metal-poor galaxies are predicted to have rather hardspectra.

monthly notices of the royal astronomical society

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