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The Molecular Hydrogen Deficit in Gamma-Ray Burst Afterglows | | Daniel Whalen
; Jason X. Prochaska
; Alexander Heger
; Jason Tumlinson
; | | Date: |
6 Feb 2008 | | Abstract: | Recent analysis of five gamma-ray burst (GRB) afterglow spectra reveal the
absence of molecular hydrogen absorption lines, a surprising result in light of
their large neutral hydrogen column densities and the detection of H$_2$ in
similar, more local star-forming regions like 30 Doradus in the LMC.
Observational evidence further indicates that the bulk of the neutral hydrogen
column in these sight lines lies 100 pc beyond the progenitor and that H$_2$
was 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 photons,
nine-species primordial chemistry with dust formation of H$_2$, and dust
extinction of UV photons. We find that a single GRB progenitor is sufficient to
ionize neutral hydrogen to distances of 50 - 150 pc but that a galactic
Lyman-Werner background is required to dissociate the molecular hydrogen. Only
modest intensities of 0.01 - 8 times the Galactic mean are necessary to destroy
H$_2$ 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. | | Source: | arXiv, 0802.0737 | | Services: | Forum | Review | PDF | Favorites | |
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