We present insights into the behavior of the astronomical 7.7 µm 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 µm feature ( \lambda _ { 7.7 } ) .
The 7.7 µm 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 } ( \simeq 0.4 µm ) , where relatively small PAHs are consistent with class A spectra ( N _ { c } \leq 60 ) 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 µm ; changes in ionization fraction , depending on the species , produce shifts up to 0.1 µm ; and nitrogen substitution has no effect on \lambda _ { 7.7 } .
Within the limits of our study , the class B→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→B transition of interstellar PAHs being incorporated into newly-forming star systems .