IS WATER ICE THE PRECURSOR TO OH$^+$ AND H$_2$O$^+$ IN ORION KL?
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Date
2011
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Ohio State University
Abstract
The reactive ions $\rm{OH^+}$ and $\rm{H_2O^+}$ have been observed in an outflow in front of the Orion KL region at significant column densities of $\sim 10^{13}$ cm$^{-2}$ with the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory. No $\rm{H_3O^+}$ was observed, establishing an upper limit of $\approx 10^{12}$ cm$^{-2}$. This is unexpected, because both $\rm{OH^+}$ and $\rm{H_2O^+}$ react with molecular hydrogen to form $\rm{H_3O^+}$. The primary destruction of $\rm{H_3O^+}$ 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 $\chi = 1$ and $\zeta_{\rm H2} = 5 \times 10^{-17}$ s$^{-1}$ at the edge. Water ice collects on the grains at this time, and then as stars form, $\chi$ increases to $10^4$ and $\zeta_{\rm H2}$ becomes $5 \times 10^{-15}$ s$^{-1}$ at the edge. At all times, temperature is calculated via thermal balance using the Meudon PDR code. At $A_{\rm V} < 4$ 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 $\rm{OH^+}$ and $\rm{H_2O^+}$ formation. On the other hand, the increased electron fraction depletes the $\rm{H_3O^+}$. The results of this model agree to within a factor of $5$ with observation, and place the $\rm{H_3O^+}$ column at $\approx 5 \times 10^{11}$ cm$^{-2}$. We will discuss the model and its results for the $\rm{OH^+}$ and $\rm{H_2O^+}$ ions as well as predicted abundances for other species.
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Author Institution: Department of Physics, The Ohio State University, Columbus, OH 43210; Departments of Astronomy, Chemistry and Physics, The Ohio State University, Columbus, OH 43210