Other
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.314r&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Other
Scientific paper
The Heterodyne Instrument for the Far Infrared (HIFI) on Herschel was used to find transitional molecules for H_3O^+: OH^+ and H_2O^+. Because both molecules react with H_2, it was expected that N(OH^+) < N(H_2O^+) < N(H_3O^+). HIFI observed OH^+ and H_2O^+ in the Orion KL region with column densities of 9 %B m 3 × 1012 cm-2 and 7 %B m 2 × 1012 cm-2. Only an upper limit was established for H_3O^+. Water was also observed at a column of ˜ 1017 cm-2. To reproduce the ionic abundances with a conventional Photodissociation Region (PDR) model at number density n = 10^4 cm-3 and at a radiation 10^4 times higher than average, a cosmic ray ionization rate of ζ_H > 10-14 s-1 is necessary. Also, such a model underproduces water (H_2O column ≈ 1013 cm-2). We explore two alternatives to the conventional PDR model: a model that includes an inflow of water, and a gas-grain PDR model which considers nearby star formation. For the water inflow model, the relationship N(OH^+) ≈ N(H_2O^+ )> N(H_3O^+) is reproduced, and the columns for these molecules agree to within a factor of 5 with observation. Water is also present with a column within an order of magnitude of observation. Other molecules, such as HCO^+, have unusually high abundances, and a very high cosmic ray ionization rate of ζ ≈ 5 × 10-15 s-1 at the edge is still required. The gas-grain PDR model has initial parameters ζ_H = 10-17 s-1 and χ = 1, which remain constant for 10^4 yr, after which χ increases to 10^4 and ζ_H to 2.5 × 10-15 s-1 at the edge. The temperature is determined by thermal balance at each point in this model. The parameters attain their final values at 2 × 10^4 yr. The model is then evolved with these new parameters to 10^6 yr, at which time N(OH^+) ≈ N(H_2O^+) ≈ N(H_3O^+) at around 1013 cm-2. The column of water lies within an order of magnitude of observation. The mechanisms involved and distinct new predictions for each of these models are discussed, as well as ways to expand upon and improve these models.
Herbst Eric
Rimmer Paul
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