Knowledge Bank

The reactive molecular ions, OH$+$, H$2$O$+$, and H$3$O$+$, key probes of the oxygen chemistry of the interstellar gas, have been observed toward Orion KL with the Heterodyne Instrument for Far Infrared on board the \emph{Herschel Space Observatory}. All three $N=1-0$ fine-structure transitions of OH$+$ at 909, 971, and 1033 GHz and both fine-structure components of the doublet {\it ortho}-H$2$O$+$ $111-000$ transition at 1115 and 1139 GHz were detected, and an upper limit was obtained for H$3$O$+$. OH$+$ and H$2$O$+$ are observed purely in absorption, showing a narrow component at the source velocity of 9~km~s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, and a broad blue shifted absorption similar to that reported recently for HF and {\it para}-H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">18</mrow>}$O, and attributed to the low velocity outflow of Orion KL. We estimate column densities of OH$+$ and H$2$O$+$ for the 9~km~s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ component of $9\pm 3\times 1012$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-2</mrow>}$ and $7\pm 2\times 1012$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-2</mrow>}$, and those in the outflow of $1.9\pm 0.7\times 1013$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-2</mrow>}$ and $1.0\pm 0.3\times 1013$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-2</mrow>}$. Upper limits of $2.4\times 1012$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-2</mrow>}$ and $8.7\times 1012$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-2</mrow>}$ were derived for the column densities of {\it ortho} and {\it para}-H$3$O$+$ from transitions near 985 and 1657 GHz. The column densities of the three ions are up to an order of magnitude lower than those obtained from recent observations of W31C and W49N. A higher gas density, despite the assumption of a large ionization rate, may explain the comparatively low column densities of OH$+$ and H$2$O$+$.