Spectral variations among different scenarios of PAH processing or formation

TL;DR

This study investigates how spectral features of PAHs vary under different formation or destruction scenarios, revealing key ratios and diagnostics for understanding PAH processing in astrophysical environments.

Contribution

It introduces new PAH charge-size diagnostic diagrams and identifies effective spectral ratios to distinguish PAH processing scenarios.

Findings

Smaller PAHs significantly influence PAH band strengths and plateau emissions.

The 3.3/(11.2+11.0) μm ratio effectively traces PAH size and processing scenario.

New diagnostic diagrams provide insights into PAH size distribution in sources.

Abstract

We examine the variations in the spectral characteristics and intensities of PAHs in two different scenarios of PAH processing (or formation): (1) small PAHs are being destroyed (or equivalently large PAHs are being formed, referred to as SPR i.e. small PAHs removed), and (2) large PAHs are being destroyed (or equivalently small PAHs are being formed referred to as LPR i.e. large PAHs removed). PAH emission was measured considering both the presence or absence of plateau components. The variation in the PAH band intensities as a function of the average number of carbon atoms <N$_{C}$> has the highest dynamic range in the SPR case suggesting that smaller PAHs have higher impact on the PAH band strengths. The plateaus show overall declining emission with <N$_{C}$>, and their higher dynamic range in the SPR case also suggests that smaller PAHs are mainly contributing to the plateau emission. The 7.7/(11.0+11.2) $\mu$m PAH band ratio presents the least amount of variance with the lowest dynamic range, rendering this ratio as the better choice for tracing PAH charge. The 3.3/(11.2+11.0) $\mu$m PAH band ratio is the only ratio that has both a monotonic variance and fully separated values among the SPR and LPR scenarios, highlighting its efficiency as PAH size tracer but also allowing the characterization of the dominant scenario of processing or formation in a given region or source. We present new PAH charge $-$ size diagnostic diagrams, which can provide insights on the average, maximum, or minimum N$_{C}$ within astrophysical sources.