Suppression of HD-cooling in protogalactic gas clouds by Lyman-Werner radiation

TL;DR

This paper investigates how Lyman-Werner UV radiation suppresses HD cooling in primordial gas clouds, affecting star formation by preventing the gas from reaching very low temperatures.

Contribution

It introduces a detailed model of UV suppression of HD cooling, showing that modest UV flux can prevent HD cooling by partially dissociating H_2, impacting early star formation.

Findings

A critical LW flux of J_crit ≈ 1e-22 erg/cm^2/sr/s/Hz suppresses HD cooling.

Partial H_2 dissociation prevents the gas from cooling below 150K.

HD cooling can only influence star masses in rare, early halos.

Abstract

It has been shown that HD molecules can form efficiently in metal-free gas collapsing into massive protogalactic halos at high redshift. The resulting radiative cooling by HD can lower the gas temperature to that of the cosmic microwave background, T_CMB=2.7(1+z)K, significantly below the temperature of a few 100 K achievable via H_2-cooling alone, and thus reduce the masses of the first generation of stars. Here we consider the suppression of HD-cooling by UV irradiation in the Lyman-Werner (LW) bands. We include photo-dissociation of both H_2 and HD, and explicitly compute the self-shielding and shielding of both molecules by neutral hydrogen as well as the shielding of HD by H_2. We use a simplified dynamical collapse model, and follow the chemical and thermal evolution of the gas, in the presence of a UV background. We find that a LW flux of J_crit = 1e-22 erg/cm^2/sr/s/Hz is able to suppress HD cooling and thus prevent collapsing primordial gas from reaching temperatures below 100 K. The main reason for the lack of HD cooling for J>J_crit is the partial photo-dissociation of H_2, which prevents the gas from reaching sufficiently low temperatures (T<150K) for HD to become the dominant coolant; direct HD photo-dissociation is unimportant except for a narrow range of fluxes and column densities. Since the prevention of HD-cooling requires only partial H_2 photo-dissociation, the critical flux J_crit is modest, and is below the UV background required to reionize the universe at redshift z=10-20. We conclude that HD-cooling can reduce the masses of typical stars only in rare halos forming well before the epoch of reionization.