Polycyclic aromatic hydrocarbon size tracers

Abstract

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, utilizing the data, models, and tools provided by the NASA Ames PAH Infrared Spectroscopic Data base. To achieve this, we used spectra from mixtures of PAHs of different ionization fractions, following a size distribution. Our work, congruent with earlier findings, shows that band ratios that include the 3.3 μm 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 μm PAH features (I$_{\Sigma _{15-20}}$) and the 6.2 or 7.7 μm 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 I6.2/I$_{\Sigma _{15-20}}$ ratio can provide estimates for the size of the small-to-intermediate PAH population within sources. Employment of the I6.2/I$_{\Sigma _{15-20}}$ and I7.7/I$_{\Sigma _{15-20}}$ ratios can be of particular interest for JWST observations limited only to ∼5–28 μm MIRI(MRS) coverage.