WATER, O2, AND ICE IN MOLECULAR CLOUDS Export

@article{citeulike:4169161, abstract = {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 FUV (6 eV < h < 13.6 eV) flux G0 (scaling factor in multiples of the local interstellar field). Extending previous photodissociation region 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 H2O 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 photodissociation rates are attenuated by dust extinction, but photodesorption prevents total freezeout. For G0 < 500, abundances of H2O and O2 peak at values ∼ 10−7, producing columns ∼ 1015 cm−2, 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 enhances the gas phase H2O peak abundance and column slightly, whereas the gas phase O2 peak abundance rises to ∼ 10−5 and the column to ∼ 2 × 1016 cm−2. We present simple analytic 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  Oph.}, author = {Hollenbach, D. and Kaufman, M. J. and Bergin, E. A. and Melnick, G. J.}, citeulike-article-id = {4169161}, citeulike-linkout-0 = {http://arxiv.org/PS_cache/arxiv/pdf/0809/0809.1642v2.pdf}, keywords = {abundances, and, bands, ism, ismlines, ismmolecules}, posted-at = {2009-03-12 19:04:30}, priority = {2}, title = {WATER, O2, AND ICE IN MOLECULAR CLOUDS}, url = {http://arxiv.org/PS_cache/arxiv/pdf/0809/0809.1642v2.pdf}, year = {2009} }

TY - JOUR ID - citeulike:4169161 L3 - citeulike-article-id:4169161 N2 - 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 FUV (6 eV < h < 13.6 eV) flux G0 (scaling factor in multiples of the local interstellar field). Extending previous photodissociation region 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 H2O 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 photodissociation rates are attenuated by dust extinction, but photodesorption prevents total freezeout. For G0 < 500, abundances of H2O and O2 peak at values ∼ 10−7, producing columns ∼ 1015 cm−2, 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 enhances the gas phase H2O peak abundance and column slightly, whereas the gas phase O2 peak abundance rises to ∼ 10−5 and the column to ∼ 2 × 1016 cm−2. We present simple analytic 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  Oph. TI - WATER, O2, AND ICE IN MOLECULAR CLOUDS KW - abundances KW - and KW - bands KW - ism KW - ismlines KW - ismmolecules AU - Hollenbach, D AU - Kaufman, MJ AU - Bergin, EA AU - Melnick, GJ PY - 2009/// UR - http://arxiv.org/PS_cache/arxiv/pdf/0809/0809.1642v2.pdf ER -