Examining the class B-to-A shift of the 7.7 ${\mu}$m PAH band with the NASA Ames PAH IR Spectroscopic Database

Abstract

We present insights into the behavior of the astronomical 7.7 ${\mu}$m polycyclic aromatic hydrocarbon (PAH) emission complex as gleaned from analyzing synthesized spectra, utilizing the data and tools from the NASA Ames PAH IR Spectroscopic Database. We specifically study the influence of PAH size, charge, aliphatic content and nitrogen substitution on the profile and peak position of the 7.7 ${\mu}$m feature (${\lambda}$7.7). The 7.7 ${\mu}$m band is known to vary significantly from object-to-object in astronomical observations, but the origin of these variations remains highly speculative. Our results indicate that PAH size can accommodate the largest shift in ${\lambda}$7.7 (~0.4 ${\mu}$m), where relatively small PAHs are consistent with class A spectra (fewer than 60 carbon atoms) while large PAHs are consistent with red/very red class B spectra. Aliphatic PAHs, of which our sample only contains a few, can produce redshifts typically around 0.15 ${\mu}$m; changes in ionization fraction, depending on the species, produce shifts up to 0.1 ${\mu}$m; and nitrogen substitution has no effect on ${\lambda}$7.7. Within the limits of our study, the class B-to-A transition is best explained with a changing PAH size distribution, with a relatively minor role assigned to aliphatic content and varying charge states. The resulting astronomical picture is that the photochemical evolution of PAHs moving from shielded class C/B environments into exposed ISM-like class A environments may be intrinsically different from the reverse class A-to-B transition of interstellar PAHs being incorporated into newly-forming star systems.