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

• Published in: arXiv.org 2022-11
• Main Authors: Bublitz, Jesse, Kastner, Joel H, Hily-Blant, Pierre, veille, Thierry, Santander-García, Miguel, Alcolea, Javier, Bujarrabal, Valentin, Wilner, David J, Montez, Rodolfo, Jr, Aleman, Isabel
• Format: Article
• Language: English
• Subjects: Astrochemistry; Carbon monoxide; Emission analysis; Interferometry; Molecular gases; Molecular ions; Near infrared radiation; Photons; Planetary nebulae; Soft x rays; X ray irradiation; X-rays
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2209.13680

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.