Polycyclic Aromatic Hydrocarbon Size Tracers

TL;DR

This study evaluates various PAH band intensity ratios as tracers for PAH size, demonstrating their effectiveness in small-to-intermediate PAHs and providing tools for interpreting JWST observations.

Contribution

It identifies the most reliable PAH band ratios for size estimation and introduces scaling methods for different PAH size distributions.

Findings

Band ratios including the 3.3 μm PAH band are effective size tracers.

Ratios involving the 15-20 μm features and 6.2 or 7.7 μm bands are also good for small-to-intermediate PAHs.

Scaling factors can adapt ratios for different PAH size distributions.

Abstract

We examine the dependence of polycyclic aromatic hydrocarbon (PAH) band intensity ratios as a function of the average number of carbon atoms and assess their effectiveness as tracers for PAH size, utilising the data, models, and tools provided by the NASA Ames PAH Infrared Spectroscopic Database. To achieve this, we used spectra from mixtures of PAHs of different ionisation fractions, following a size distribution. Our work, congruent with earlier findings, shows that band ratios that include the 3.3 ${\mu}$m PAH band provide the best PAH size tracers for small-to-intermediate sized PAHs. In addition, we find that band ratios that include the sum of the 15-20 ${\mu}$m PAH features (I$_{\Sigma_{15-20}}$) and the 6.2 or 7.7 ${\mu}$m bands also serve as good tracers for PAH size in the case of small-to-intermediate sized PAHs, for objects under a similar PAH size distribution as with the presented models. For different PAH size distributions, the application of a scaling factor to the I$_{6.2}$/I$_{\Sigma_{15-20}}$ ratio can provide estimates for the size of the small-to-intermediate PAH population within sources. Employment of the I$_{6.2}$/I$_{\Sigma_{15-20}}$ and I$_{7.7}$/I$_{\Sigma_{15-20}}$ ratios can be of particular interest for JWST observations limited only to $\sim$ 5-28 ${\mu}$m MIRI(-MRS) coverage.