PAH Spectral Diversity in NGC 7027 and the Evolution of Aromatic Carriers

TL;DR

This study leverages JWST/MIRI‑MRS to spatially map mid-infrared PAH emission in the planetary nebula NGC 7027, addressing how circumstellar PAHs evolve into those observed in the ISM. By decomposing the major PAH bands and analyzing spatial variations, the authors identify distinct PAH subpopulations (blue: 6.205 µm/7.6 µm; red: 6.26/7.8/8.65 µm; blue 8.56 µm; 11.207 µm PAH) and a separate 11.25 µm VSG/PAH-cluster component, which correlates with H$_2$ and does not track PAH emission. Correlation analyses reveal two coupled PAH subpopulations and a third, distinct population, implying size- and structure-driven spectral differences; 11.25 µm emission is linked to VSGs/PAH clusters, not PAHs alone. The authors conclude that Class $\mathcal{B}$ PAHs are more processed and located near the star's UV field, while Class $\mathcal{A}$ PAHs are relatively pristine and found farther out, challenging the notion that PAH spectral evolution from B to A occurs mainly in the ISM. Overall, the work suggests that most PAHs ejected by PNe enter the ISM as Class $\mathcal{A}$, with UV processing driving observed spatial and spectral diversity, and it highlights the need for expanded high‑resolution studies to further unravel PAH evolution in space.

Abstract

Polycyclic Aromatic Hydrocarbons (PAHs) constitute a significant fraction of the Universe's carbon budget, playing a key role in the cosmic carbon cycle and dominating the mid-infrared spectra of astrophysical environments in which they reside. Although PAHs are known to form in the circumstellar envelopes of post-AGB stars, their formation and evolution are still not well-understood. We aim to understand how pristine complex hydrocarbons and PAHs in circumstellar environments transition to the PAHs observed in the ISM. The mid-infrared PAH spectra (5-18 micron) of the planetary nebula, NGC 7027, are investigated using spectral cubes from JWST MIRI-MRS. We report the first detection of spatially-resolved variations of the PAH spectral profiles across class A, AB, and B in all major PAH bands (6.2, 7.7, 8.6, and 11.2 micron) within a single source, NGC 7027. These variations are linked to morphological structures within NGC 7027. Clear correlations are revealed between the 6.2, 7.7, and 8.6 micron features, where the red components (6.26, 7.8, 8.65 micron) exhibit a strong correlation and the same is found for the blue components of the 6.2 and 7.7 (6.205 and 7.6 micron). The blue component of the 8.6 (8.56 micron) appears to be independent of the other components. We link this behaviour to differences in molecular structure of their PAH subpopulations. Decomposition of the 11.2 micron band confirms two previously identified components, with the broader 11.25 micron component attributed to emission from very small grains of PAH clusters rather than PAH emission. We show that PAH profile classes generally vary with proximity to the central star's UV radiation field, suggesting class B PAHs represent more processed species while class A PAHs remain relatively pristine, challenging current notions on the spectral evolution of PAHs.

Figures (20)

• Figure 1: Simplified schematic drawings of NGC 7027 as viewed from the side (top) and the front (bottom). The central star in shown in yellow. The H$_2$ emission lies primarily on the surface of the biconical shell, and is particularly bright along the rims, while the ionized gas and PAHs reside within the elliptical ring.
• Figure 2: HST/WFC3 NICMOS F212N image of NGC 7027 with JWST MIRI-MRS FOVs for channels 1 (red), 2, and 3 (white) overlaid. The black cross indicates the position of the central star. The yellow contours outline the H$_2$ gas morphology in the continuum-subtracted HST/NICMOS images from Latter2000. The small orange, pink, blue, and green boxes are the spaxels used to illustrate the AIB variability referred to hereinafter as the E ring, outer NE corner, inner region, and the SW ring, respectively. Credit: NASA/ESA, Latter2000.
• Figure 3: Illustration of the 6.2, 7-9, and 11.2 µm profile variation across the nebula. The single spaxel spectra from Fig. \ref{['fig:fov']} detailed in Table \ref{['tab:aperture_info']} are utilized here.
• Figure 4: Illustration of the profile variability with the peak position of the 6.2 µm PAH band (top) and the relative peak surface brightness of the 7.8 µm component with respect to the 7.6 µm component (bottom). Black and white contours represent the integrated 6.2 µm PAH surface brightness and the H$_2$ 0-0 S(7) integrated surface brightness, respectively (Fig. \ref{['fig:3color_images']}).
• Figure 5: Illustration of the 11.2 µm PAH band decomposition. Top: Normalized profiles of the first component, the narrowest profile, representing class $\mathcal{A}$, the broadest (transitional) profile representing class $\mathcal{B}$, and the second component used in the decomposition of the 11.2 µm feature. Bottom: Typical decomposition of the 11.2 µm band by components 1 and 2.