The Production of O_2 in Molecular Shocks: HOP Observations of Orion H_2Peak 1

Astronomy and Astrophysics – Astronomy

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The Herschel Oxygen Project (HOP) is an Open Time Key Project to search for O_2 in dense clouds, shocked regions, PDRs, and regions associated with massive star formation. HOP observations of the shocked gas near Orion KL, at the position of H2 Peak 1, show relatively narrow submillimeter lines (Δ v˜ 4 km/s) in the 487, 774 and 1121 GHz tansitions of O_2. We interpret this emission using the C-shock models of Kaufman & Neufeld. A strong, confirmed prediction of C-shock models is that neutral-neutral reactions at temperatures greater than ˜400 K drive all of the oxygen nuclei not locked in CO into H_2O, with an abundance ˜ 10-4, and produce almost no O_2. However, very mild C-shocks (v˜ 10-15 km/s) heat gas sufficiently to begin the neutral-neutral reaction sequence but result in nearly equal abundances of H_2O and O_2, each with fractional abundance ˜ 10-6. Thus the mild shocks produce a far larger column of O_2 than shocks at speeds greater than 20 km/s. We compare the models with the emission observed in H2 Peak 1, constraining the shock velocity, gas density and filling factor of the mildly shocked gas.

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