Astrophys. J., 690, 1497-1521 (2009/January-2)
Water, O2, and ice in molecular clouds.
HOLLENBACH D., KAUFMAN M.J., BERGIN E.A. and MELNICK G.J.
Abstract (from CDS):
We model the temperature and chemical structure of molecular clouds as a function of depth into the cloud, assuming a cloud of constant density n illuminated by an external far-ultraviolet (FUV; 6 eV <hν < 13.6 eV) flux G0(scaling factor in multiples of the local interstellar field). Extending previous photodissociation region (PDR) models, we include the freezing of species, simple grain surface chemistry, and desorption (including FUV photodesorption) of ices. We also treat the opaque cloud interior with time-dependent chemistry. Here, under certain conditions, gas-phase elemental oxygen freezes out as water ice and the elemental C/O abundance ratio can exceed unity, leading to complex carbon chemistry. Gas-phase H2 O and O2 peak in abundance at intermediate depth into the cloud, roughly AV∼ 3-8 from the surface, the depth proportional to ln(G0/n). Closer to the surface, molecules are photodissociated. Deeper into the cloud, molecules freeze to grain surfaces. At intermediate depths, PDRs are attenuated by dust extinction, but photodesorption prevents total freeze-out. For G0< 500, abundances of H2 O and O2peak at values ∼10–7, producing columns ∼1015/cm2, independent of G0 and n. The peak abundances depend primarily on the product of the photodesorption yield of water ice and the grain surface area per H nucleus. At higher values of G0, thermal desorption of O atoms from grains slightly enhances the gas-phase H2 O peak abundance and column, whereas the gas-phase O2 peak abundance rises to ∼10–5 and the column to ∼2 x 1016/cm2. We present simple analytical equations for the abundances as a function of depth, which clarify the dependence on parameters. The models are applied to observations of H2O, O2, and water ice in a number of sources, including B68, NGC 2024, and ρ Oph.
astrochemistry - ISM: abundances - ISM: lines and bands - ISM: molecules
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