2023ApJ...942...14B

The young and well-studied planetary nebula (PN) NGC 7027 harbors significant molecular gas that is irradiated by luminous, pointlike 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 ∼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 μm H₂ data for context. The CO⁺ map constitutes the first interferometric map of this molecular ion in any PN. Comparison of CO⁺ and HCO⁺ maps reveals 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 ∼1″ (∼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 H₂ 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.