Polycyclic aromatic hydrocarbon spectral diversity in NGC 7027 and the evolution of aromatic carriers

¹ Department of Physics & Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
² Institute for Earth and Space Exploration, The University of Western Ontario, London, ON N6A 3K7, Canada
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Astronomy Department, University of Maryland, College Park, MD 20742, USA
⁵ NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000, USA
⁶ Carl Sagan Center, SETI Institute, 339 Bernardo Avenue, Suite 200, Mountain View, CA 94043, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 October 2025
Accepted: 15 January 2026

Abstract

Context. 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-asymptotic giant branch stars, their formation and evolution are still not well understood.

Aims. We aim to understand how pristine complex hydrocarbons and PAHs in circumstellar environments transition to the PAHs observed in the interstellar medium.

Methods. The mid-infrared PAH spectra (5–18 μm) of the planetary nebula, NGC 7027, were investigated using spectral cubes from JWST MIRI-MRS.

Results. 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 μm) 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 μm features, where the red components (6.26, 7.8, and 8.65 μm) exhibit a strong correlation and the same is found for the blue components of the 6.2 and 7.7 μm features (6.205 and 7.6 μm). The blue component of the 8.6 μm feature (8.56 μm) appears to be independent of the other components. We link this behavior to differences in the molecular structure of their PAH subpopulations. Decomposition of the 11.2 μm band confirms two previously identified components, with the broader 11.25 μm component attributed to emission from very small grains or PAH clusters rather than PAH emission.

Conclusions. 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.