Analytic HI-to-H2 Photodissociation Transition Profiles

Abstract

We present a simple analytic procedure for generating atomic-to-molecular (HI-to-H${}_2$) density profiles for optically thick clouds illuminated by far-ultraviolet radiation. Our procedure is based on the analytic theory for the structure of one-dimensional HI/H${}_2$ photon-dominated regions, presented by Sternberg et al. (2014). Depth-dependent HI and H${}_2$ density fractions may be computed for arbitrary gas density, far-ultraviolet field intensity, and the metallicity dependent H${}_2$ formation rate coefficient, and dust absorption cross section. We use our procedure to generate a set of HI-to-H${}_2$ transition profiles for a wide range of conditions, from the weak- to strong-field limits, and from super-solar down to low metallicities. We show that if presented as functions of dust optical depth the HI and H${}_2$ density profiles depend primarily on the Sternberg "$\alpha G$ parameter" (dimensionless) that determines the dust optical depth associated with the total photodissociated HI column. We derive a universal analytic formula for the HI-to-H${}_2$ transition points as a function of just $\alpha G$. Our formula will be useful for interpreting emission-line observations of HI/H${}_2$ interfaces, for estimating star-formation thresholds, and for sub-grid components in hydrodynamics simulations.

Accepted for publication in ApJ. 13 pages, 9 Figures

References in corpus (11)

• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• Molecular Hydrogen and Global Star Formation Relations in Galaxies
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• The distribution of atomic hydrogen in EAGLE galaxies: morphologies, profiles, and HI holes
• The penetration of FUV radiation into molecular clouds
• H2 distribution during formation of multiphase molecular clouds
• Rotationally Warm Molecular Hydrogen in the Orion Bar
• Time-dependent H2 formation and protonation
• HD/H2 as a probe of the roles of gas, dust, light, metallicity and cosmic rays in promoting the growth of molecular hydrogen in the diffuse interstellar medium
• Water Formation During the Epoch of First Metal Enrichment

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