Title:
Analytic H i-to-H2 Photodissociation Transition Profiles
Authors:
Bialy, Shmuel; Sternberg, Amiel
Affiliation:
AA(Raymond and Beverly Sackler School of Physics & Astronomy, Tel Aviv University, Ramat Aviv 69978, Israel 0000-0002-0404-003X), AB(Raymond and Beverly Sackler School of Physics & Astronomy, Tel Aviv University, Ramat Aviv 69978, Israel)
Publication:
The Astrophysical Journal, Volume 822, Issue 2, article id. 83, pp. (2016). (ApJ Homepage)
Publication Date:
05/2016
Origin:
IOP
Astronomy Keywords:
galaxies: star formation, ISM: clouds, methods: analytical, photon-dominated region: PDR, radiative transfer
DOI:
10.3847/0004-637X/822/2/83
Bibliographic Code:
2016ApJ...822...83B
Abstract
We present a simple analytic procedure for generating atomic (H i) to molecular
(`H`2) density profiles for optically thick hydrogen gas clouds illuminated by
far-ultraviolet radiation fields. Our procedure is based on the analytic theory for
the structure of one-dimensional H i/`H`2 photon-dominated regions,
presented by Sternberg et al. Depth-dependent atomic and molecular density
fractions may be computed for arbitrary gas density, far-ultraviolet field intensity,
and the metallicity-dependent H2 formation rate coefficient, and dust absorption
cross section in the Lyman–Werner photodissociation band. We use our
procedure to generate a set of {{H}} {{I}}{-}{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 {{H}} {{I}} and `H`2 density profiles depend primarily on the
Sternberg “? G parameter” (dimensionless) that determines the dust optical
depth associated with the total photodissociated {{H}} {{I}} column. We derive a
universal analytic formula for the {{H}} {{I}}{-}{to}{-}`H`2 transition points as a
function of just ? G. Our formula will be useful for interpreting emission-line
observations of H i/`H`2 interfaces, for estimating star formation thresholds,
and for sub-grid components in hydrodynamics simulations.