Astrophysics > Astrophysics of Galaxies
[Submitted on 11 Jan 2016 (v1), last revised 16 Mar 2016 (this version, v2)]
Title:Analytic HI-to-H2 Photodissociation Transition Profiles
View PDFAbstract: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.
Submission history
From: Shmuel Bialy [view email][v1] Mon, 11 Jan 2016 21:00:00 UTC (1,089 KB)
[v2] Wed, 16 Mar 2016 14:45:10 UTC (1,927 KB)
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