STRUCTURE OF THE DENSE MOLECULAR GAS IN THE HELIX NEBULA: LARGE SCALE MAPPING OF HCO$^{+}$

Author Institution: Department of Chemistry, University of Arizona, PO Box 210041, 1306 East University Blvd, Tucson, AZ, 85721, USA; Department of Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, SwitzerlandThe Helix Nebula, NGC 7293, has been mapped in HCO$^{+}$ J = 1 $\rightarrow$ 0 emission with a 70$^{\prime}$$^{\prime}$ spatial resolution (1.68 km s$^{-1}$ velocity resolution) using the Arizona Radio Observatory (ARO) 12 m telescope on Kitt Peak. Over 200 individual positions covering an area of roughly 1000 $\times$ 800 $^{\prime}$$^{\prime}$ were observed down to a 3$\sigma$ noise level of \textasciitilde 20 mK. HCO$^{+}$ was detected at over three-quarters of the positions, with the majority of spectra showing multiple velocity components indicative of a complex kinematic structure. The column density of HCO$^{+}$ across the Helix ranges from N$_{tot}$ $\~{}$ 1.4 $\times$ 10$^{11}$ to $\~{}$2.4 $\times$ 10$^{12}$ cm$^{-2}$, with an average N$_{tot}$ $\~{}$ 4 $\times$ 10$^{11}$ cm$^{-2}$. The HCO$^{+}$ distribution, which traces gas with densities \textasciitilde 10$^{5}$ cm$^{-3}$, is similar to that of CO and H$_{2}$ , with certain point symmetries and red-and blue-shifted regions common in all three molecules; these data are also consistent with observations of atomic lines. The HCO$^{+}$ emission appears to trace two ring-like structures emanating from the central star: one is blue-shifted and the other red-shifted with respect to the star, and possibly trace the edges of a bipolar outflow