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

TL;DR

This study uses SOFIA mid-infrared imaging to investigate PAH formation and dust processing in the planetary nebula NGC 7027, revealing spatial correlations between PAH features and dust structures related to stellar outflows.

Contribution

It provides spatially resolved evidence of PAH formation linked to stellar outflows and grain collisions in NGC 7027, a novel insight into dust evolution in planetary nebulae.

Findings

PAH emission correlates with outflow regions.

Dust optical depth minima coincide with PAH enhancements.

Rapid PAH formation occurs via grain collisions in post-shock environments.

Abstract

We report spatially resolved (FWHM$\sim3.8-4.6"$) 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 ($T_D\sim90$ 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 $5.8^{+2.3}_{-2.6}\times10^{-3}$ $\mathrm{M}_\odot$, where carbonaceous large ($a=1.5$ $\mu$m) and very small ($a \sim12\AA$) grains, and PAHs ($3.1\AA<a<12\AA$) 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 ($n_H\sim10^5\,\mathrm{cm}^{-3}$) photo-dissociation region (PDR) and molecular envelope.