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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 2.12μm H2data 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 H2emission), 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.
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This content will become publicly available on December 1, 2025
Toward a robust physical and chemical characterization of heterogeneous lines of sight: The case of the Horsehead nebula
Context. Dense and cold molecular cores and filaments are surrounded by an envelope of translucent gas. Some of the low-Jemission lines of CO and HCO+isotopologues are more sensitive to the conditions either in the translucent environment or in the dense and cold one because their intensities result from a complex interplay of radiative transfer and chemical properties of these heterogeneous lines of sight (LoSs). Aims. We extend our previous single-zone modeling with a more realistic approach that introduces multiple layers to take account of possibly varying conditions along the LoS. We used the IRAM-30m data from the ORION-B large program toward the Horsehead nebula in order to demonstrate our method’s capability and effectiveness. Methods. We propose a cloud model composed of three homogeneous slabs of gas along each LoS, representing an outer envelope and a more shielded inner layer. We used the non-LTE radiative transfer code RADEX to model the line profiles from the kinetic temperature (Tkin), the volume density (nH2), kinematics, and chemical properties of the different layers. We then used a fast and robust maximum likelihood estimator to simultaneously fit the observed lines of the CO and HCO+isotopologues. To limit the variance on the estimates, we propose a simple chemical model by constraining the column densities. Results. A single-layer model cannot reproduce the spectral line asymmetries that result from a combination of different radial velocities and absorption effects among layers. A minimal heterogeneous model (three layers only) is sufficient for the Horsehead application, as it provides good fits of the seven fitted lines over a large part of the studied field of view. The decomposition of the intensity into three layers allowed us to discuss the distribution of the estimated physical or chemical properties along the LoS. About 80% of the12CO integrated intensity comes from the outer envelope, while ~55% of the integrated intensity of the (1 − 0) and (2 − 1) lines of C18O comes from the inner layer. For the lines of the13CO and the HCO+isotopologues, integrated intensities are more equally distributed over the cloud layers. The estimated column density ratioN(13CO)/N(C18O) in the envelope increases with decreasing visual extinction, and it reaches 25 in the pillar outskirts. While the inferredTkinof the envelope varies from 25 to 40 K, that of the inner layer drops to ~15 K in the western dense core. The estimatednH2in the inner layer is ~3 × 104cm−3toward the filament, and it increases by a factor of ten toward dense cores. Conclusions. Our proposed method correctly retrieves the physical and chemical properties of the Horsehead nebula. It also offers promising prospects for less supervised model fits of wider-field datasets.
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- Award ID(s):
- 2142300
- PAR ID:
- 10569617
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- Astronomy and Astrophysics
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 692
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A160
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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