The Molecular Hydrogen Deficit in Gamma‐Ray Burst Afterglows

Abstract

Recent analysis of five gamma-ray burst (GRB) afterglow spectra reveals the absence of molecular hydrogen absorption lines, a surprising result in light of their large neutral hydrogen column densities and the detection of H2 in similar, more local star-forming regions such as 30 Doradus in the Large Magellanic Cloud (LMC). Observational evidence further indicates that the bulk of the neutral hydrogen column in these sight lines lies more than 100 pc from the GRB progenitor and that H2 was likely absent prior to the burst, suggesting that direct flux from the star, FUV background fields, or both suppressed its formation. We present one-dimensional radiation hydrodynamical models of GRB host galaxy environments, including self-consistent radiative transfer of both ionizing and Lyman-Werner (L-W) photons, nine-species primordial chemistry with dust formation of H2, and dust extinction of UV photons. We find that a single GRB progenitor is sufficient to ionize neutral hydrogen to distances of 50-100 pc, but that a galactic L-W background is required to dissociate molecular hydrogen in the ambient ISM. Intensities of 0.1-100 times the Galactic mean are necessary to destroy H2 in the cloud, depending on its density and metallicity. The minimum radii at which neutral hydrogen will be found in afterglow spectra is insensitive to the mass of the progenitor or the initial mass function (IMF) of its cluster, if present.