Evidence from SOFIA Imaging of Polycyclic Aromatic Hydrocarbon Formation along a Recent Outflow in NGC 7027

Abstract

We report spatially resolved (FWHM$\sim 3.8-4.6"$) mid-IR imaging observations of the planetary nebula (PN) NGC 7027 taken with the 2.5-m telescope aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Images of NGC 7027 were acquired at 6.3, 6.6, 11.1, 19.7, 24.2, 33.6, and 37.1 $\mu \mathrm{m}$ using the Faint Object Infrared Camera for the SOFIA Telescope (FORCAST).The observations reveal emission from Polycyclic Aromatic Hydrocarbon (PAH) and warm dust ($T_D\sim 90$ K) from the illuminated inner edge of the molecular envelope surrounding the ionized gas and central star. The DustEM code was used to fit the spectral energy distribution of fluxes obtained by FORCAST and the archival infrared spectrum of NGC 7027 acquired by the Short Wavelength Spectrometer (SWS) on the Infrared Space Observatory (ISO). Best-fit dust models provide a total dust mass of $5.8_{-2.6}^{+2.3}\times 10^{-3}$ ${\mathrm{M}}_{\odot }$, where carbonaceous large ($a=1.5$ $\mu$m) and very small ($a\sim 12\mathring{A}$) grains, and PAHs ($3.1\mathring{A}<a<12\mathring{A}$) compose 96.5, 2.2, and 1.3 $\%$ of the dust by mass, respectively. The 37 $\mu$m optical depth map shows minima in the dust column density at regions in the envelope that are coincident with a previously identified collimated outflow from the central star. The optical depth minima are also spatially coincident with enhancements in the 6.2 $\mu$m PAH feature, which is derived from the 6.3 and 6.6 $\mu$m maps. We interpret the spatial anti-correlation of the dust optical depth and PAH 6.2 $\mu$m feature strength and their alignment with the outflow from the central star as evidence of dust processing and rapid PAH formation via grain-grain collisions in the post-shock environment of the dense ($n_H\sim 10^5{\mathrm{cm}}^{-3}$) photo-dissociation region (PDR) and molecular envelope.

arXiv comment12 pages, 6 figures, Accepted for publication in ApJ

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• Near- to mid-Infrared Observations of Galaxy Mergers: NGC2782 and NGC7727
• PAH Spectral Diversity in NGC 7027 and the Evolution of Aromatic Carriers
• Chemistry and ro-vibrational excitation of HeH${}^{+}$ in the Planetary Nebula NGC 7027
• PAH and nbL Features Detection in Planetary Nebulae NGC 7027 and BD +303639 with TIRCAM2 Instrument on 3.6m DOT
• JWST observations of the Ring Nebula (NGC 6720) -- II. PAH emission
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