Mapping NGC 7027 in New Light: CO<sup>+</sup> and HCO<sup>+</sup> Emission Reveal Its Photon- and X-Ray-dominated Regions
Jesse Bublitz, Joel H. Kastner, P. Hily-Blant, T. Forveille, M. Santander-García, J. Alcolea, V. Bujarrabal, David J. Wilner, Rodolfo Montez, Isabel Aleman
Abstract
Abstract 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 μ 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 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 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.