Observations and analysis of CH${}^{+}$ vibrational emissions from the young, carbon-rich planetary nebula NGC 7027: a textbook example of chemical pumping

Abstract

We discuss the detection of 14 rovibrational lines of CH${}^{+}$, obtained with the iSHELL spectrograph on NASA's Infrared Telescope Facility (IRTF) on Maunakea. Our observations in the 3.49 - 4.13 $\mu$m spectral region, obtained with a 0.375" slit width that provided a spectral resolving power $\lambda /\Delta \lambda \sim 80,000$, have resulted in the unequivocal detection of the $R(0)-R(3)$ and $P(1)-P(10)$ transitions within the $v=1-0$ band of CH${}^{+}$. The $R$-branch transitions are anomalously weak relative to the $P$-branch transitions, a behavior that is explained accurately by rovibronic calculations of the transition dipole moment reported in a companion paper (Changala et al. 2021). Nine infrared transitions of H${}_2$ were also detected in these observations, comprising the $S(8)$, $S(9)$, $S(13)$ and $S(15)$ pure rotational lines; the $v=1-0$ $O(4)-O(7)$ lines, and the $v=2-1$ $O(5)$ line. We present a photodissociation region model, constrained by the CH${}^{+}$ and H${}_2$ line fluxes that we measured, that includes a detailed treatment of the excitation of CH${}^{+}$ by inelastic collisions, optical pumping, and chemical ("formation") pumping. The latter process is found to dominate the excitation of the observed rovibrational lines of CH${}^{+}$, and the model is remarkably successful in explaining both the absolute and relative strengths of the CH${}^{+}$ and H${}_2$ lines.

arXiv comment38 pages, 15 figures, accepted for publication in the Astrophysical Journal

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Cited by in corpus (7)

• PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar
• Self-generated ultraviolet radiation in molecular shock waves II. CH+ and the interpretation of emission from shock ensembles
• PDRs4All XI. Detection of infrared CH${}^{+}$ and CH${}_3^{+}$ rovibrational emission in the Orion Bar and disk d203-506: evidence of chemical pumping
• Observations and analysis of CH${}^{+}$ vibrational emissions from the young, carbon-rich planetary nebula NGC 7027: a textbook example of chemical pumping
• Anomalous intensities in the infrared emission of CH${}^{+}$ explained by quantum nuclear motion and electric dipole calculations
• Chemistry and ro-vibrational excitation of HeH${}^{+}$ in the Planetary Nebula NGC 7027
• Infrared line emissions from atoms and atomic ions in NGC 7027: improved wavelength determinations for infrared metal lines and a probable detection of Zn${}^{5+}$