Is Water Ice the Precursor to OH^+ and H_2O^+ in Orion Kl?

Details Is Water Ice the Precursor to OH^+ and H_2O^+ in Orion Kl? Is Water Ice the Precursor to OH^+ and H_2O^+ in Orion Kl?

Jun 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011mss..confefa04r&link_type=abstract

"International Symposium On Molecular Spectroscopy, 66th Meeting, Held 20-24 June, 2011 at Ohio State University. http://molspec

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Mini-Symposium: The Thz Cosmos

Scientific paper

The reactive ions OH^+ and H_2O^+ have been observed in an outflow in front of the Orion KL region at significant column densities of ˜ 1013 Cm-2 with the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory. No H_3O^+ was observed, establishing an upper limit of ≈ 1012 Cm-2. This is unexpected, because both OH^+ and H_2O^+ react with molecular hydrogen to form H_3O^+. The primary destruction of H_3O^+ is by recombination with electrons. We explore the low velocity Orion KL outflow with a gas-grain PDR model where UV radiation, cosmic rays, X-rays, and temperature depend on both depth into the cloud and time. The model starts with cold core conditions and a radiation field of χ = 1 and ζH2 = 5 × 10-17 S-1 at the edge. Water ice collects on the grains at this time, and then as stars form, χ increases to 10^4 and ζH2 becomes 5 × 10-15 S-1 at the edge. At all times, temperature is calculated via thermal balance using the Meudon PDR code. At AV < 4 into the cloud the water desorbs off grains and becomes ionized by cosmic rays and X-rays, and dissociated by UV photons, increasing the rates of OH^+ and H_2O^+ formation. On the other hand, the increased electron fraction depletes the H_3O^+. The results of this model agree to within a factor of 5 with observation, and place the H_3O^+ column at ≈ 5 × 1011 Cm-2. We will discuss the model and its results for the OH^+ and H_2O^+ ions as well as predicted abundances for other species.

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