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

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.