Water, O2 and Ice in Molecular Clouds

Details Water, O2 and Ice in Molecular Clouds Water, O2 and Ice in Molecular Clouds

2008-09-09

arxiv.org/abs/0809.1642v2

Astrophys.J.690:1497-1521,2009

Astronomy and Astrophysics

Astrophysics

70 pages including 17 figures

Scientific paper

10.1088/0004-637X/690/2/1497

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 < E < 13.6 eV) flux G_0 (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 A_V~3-8 from the surface, the depth proportional to ln(G_0/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 G_0 < 500, abundances of H2O and O2 peak at values ~10^(-7), producing columns ~10^(15) per cm^2, independent of G_0 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 G_0, 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 ~2x10^(16) per 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 Rho Oph.

Affiliated with

Also associated with

No associations

Rating

Water, O2 and Ice in Molecular Clouds does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.

If you have personal experience with Water, O2 and Ice in Molecular Clouds, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Water, O2 and Ice in Molecular Clouds will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFWR-SCP-O-211425