Knowledge Bank

The reactive ions $OH+$ and $H2O+$ have been observed in an outflow in front of the Orion KL region at significant column densities of $\sim 1013$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-2</mrow>}$ with the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory. No $H3O+$ was observed, establishing an upper limit of $\approx 1012$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-2</mrow>}$. 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 $\chi =1$ and $\zeta H2=5\times 10-17$ s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ at the edge. Water ice collects on the grains at this time, and then as stars form, $\chi$ increases to $104$ and $\zeta H2$ becomes $5\times 10-15$ s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ at the edge. At all times, temperature is calculated via thermal balance using the Meudon PDR code. At $A\_\{\backslash rm\; V\}\; \&lt;\; 4$ 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 $\approx 5\times 1011$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-2</mrow>}$. We will discuss the model and its results for the $OH+$ and $H2O+$ ions as well as predicted abundances for other species.