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Tracking Water, O2 and Ice in Molecular Clouds: PDRs Models with Photodesorption and Grain Chemistry
Published online by Cambridge University Press: 04 October 2008
Abstract
We have expanded our model of photodissociation regions (PDRs) to
include the freezing of O- and C-bearing species on dust grains,
simple grain surface chemistry, and desorption processes, including
photodesorption, that may be important in the surface layers of
diffuse, translucent, and dense molecular clouds. The main result of
including these processes is that a number of important gas-phase
species, including H2O and O2, peak in abundance at AV ~ few into the cloud. Most of the gas-phase column, and most of
the emission, from these species arises in the peak. Closer to the
surface, H2O and O2 are photodissociated, while deeper into
the cloud, they freeze out onto grain surfaces. The result is H2O
and O2 column densities that are nearly constant for a wide range
of gas densities, n, and for FUV fields G0$\lesssim$ 500. The roughly
constant column densities of these species provides an explanation
for the low line-of-sight average abundances of H2O observed
toward GMCs. The model results also suggest that regions of high FUV
field are the best places to search for O2.
- Type
- Research Article
- Information
- European Astronomical Society Publications Series , Volume 31: Far-Infrared Workshop 2007 , 2008 , pp. 43 - 46
- Copyright
- © EAS, EDP Sciences, 2008
References
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