15min:

PAUL B. RIMMER, Department of Physics, The Ohio State University, Columbus, OH 43210; ERIC HERBST, Departments of Astronomy, Chemistry and Physics, The Ohio State University, Columbus, OH 43210.

The reactive ions OH⁺ and H₂O⁺ 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 H₃O⁺ was observed, establishing an upper limit of ~ 10¹² cm^-2. This is unexpected, because both OH⁺ and H₂O⁺ react with molecular hydrogen to form H₃O⁺. The primary destruction of H₃O⁺ 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 = 1 and _H2 = 5 × 10^-17 s^-1 at the edge. Water ice collects on the grains at this time, and then as stars form, increases to 10⁴ and _H2 becomes 5 × 10^-15 s^-1 at the edge. At all times, temperature is calculated via thermal balance using the Meudon PDR code. At A _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 OH⁺ and H₂O⁺ formation. On the other hand, the increased electron fraction depletes the H₃O⁺. The results of this model agree to within a factor of 5 with observation, and place the H₃O⁺ column at ~ 5 × 10¹¹ cm^-2. We will discuss the model and its results for the OH⁺ and H₂O⁺ ions as well as predicted abundances for other species.