Mapping NGC 7027 in New Light: CO$^+$ and HCO$^+$ Emission Reveal Its Photon- and X-ray-Dominated Regions

Abstract: The young and well-studied planetary nebula NGC 7027 harbors significant molecular gas that is irradiated by luminous, point-like UV (central star) and diffuse (shocked nebular) X-ray emission. This nebula represents an excellent subject to investigate the molecular chemistry and physical conditions within photon- and X-ray-dominated regions (PDRs and XDRs). As yet, the exact formation routes of CO$^+$ and HCO$^+$ in PN environments remain uncertain. Here, we present $\sim$2$"$ resolution maps of NGC 7027 in the irradiation tracers CO$^+$ and HCO$^+$, obtained with the IRAM NOEMA interferometer, along with SMA CO and HST 2.12~$\mu$m H$_2$ data for context. The CO$^+$ map constitutes the first interferometric map of this molecular ion in any PN. Comparison of CO$^+$ and HCO$^+$ maps reveal strikingly different emission morphologies, as well as a systematic spatial displacement between the two molecules; the regions of brightest HCO$^+$, found along the central waist of the nebula, are radially offset by $\sim$1$"$ ($\sim$900 au) outside the corresponding CO$^+$ emission peaks. The CO$^+$ emission furthermore precisely traces the inner boundaries of the nebula's PDR (as delineated by near-IR H$_2$ emission), suggesting that central star UV emission drives CO$^+$ formation. The displacement of HCO$^+$ radially outward with respect to CO$^+$ is indicative that dust-penetrating soft X-rays are responsible for enhancing the HCO$^+$ abundance in the surrounding molecular envelope, forming an XDR. These interferometric CO$^+$ and HCO$^+$ observations of NGC 7027 thus clearly establish the spatial distinction between the PDR and XDR formed (respectively) by intense UV and X-ray irradiation of molecular gas.