Probing Dust and PAH Chemistry in Evolved Carbon-Rich Nebulae through Optical and Infrared Observations

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Abstract
This study presents optical and near-infrared photometric observations, alongside mid-infrared spectroscopic data from the ISO SWS instrument, to examine potential correlations between Aromatic Infrared Band (AIB) features and the optical properties of carbon-rich evolved stars. Identifying such correlations can provide valuable constraints on the evolutionary pathways of low- to intermediate-mass stars beyond the asymptotic giant branch (AGB) phase. Photometric measurements in the U, B, V, R, I, J, H, K, and L bands were obtained for five well-known carbon-rich objects at various post-AGB or planetary nebula (PN) stages: CRL 2688, PN M 2-43, NGC 7027, BD${+}$30${^\circ}$3639, and AFGL 2132. Our analysis reveals that all five objects exhibit prominent AIB features; however, their spectral profiles show notable variation. These differences are attributed to variations in the chemical composition and physical conditions of the surrounding circumstellar material. In particular, the 3.28$μ$m polycyclic aromatic hydrocarbon (PAH) feature is detected in all objects except AFGL 2132, indicating a potentially distinct PAH population or environmental condition in its vicinity. Although these sources share broadly similar evolutionary stages, the observed diversity in AIB characteristics underscores the complexity and heterogeneity of their circumstellar environments.
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