Examining the Class B to A Shift of the 7.7 μm PAH Band with the NASA Ames PAH IR Spectroscopic Database

Abstract

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 (λ 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 λ 7.7 ( 0.4 µm), where relatively small PAHs are consistent with class A spectra (N c ≤ 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 λ 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.

Unidentified infrared emission features in mid-infrared spectrum of comet 21P/Giacobini-Zinner

“…This experimental result implies that the ~9.2 µm feature observed in comet 21P/G-Z can be attributed to aliphatic hydrocarbons, which cannot withstand temperatures higher than ~730 K. This fact shed light on the origin of the organic materials expected due to the presence of some UIR features in comet 21P/G-Z. On the other hand, it is reported that the feature around 9.3 µm appears for PAHs with a smaller number of carbon atoms (≤ 30 carbon atoms) based on recent theoretical studies (Shannon & Boersma 2019). However, if carriers of the ~9.2 µm feature detected in comet 21P/G-Z are smaller-sized PAHs, a strong emission feature at ~12 µm should also appear (Shannon 2019, private comm.).…”

Polycyclic aromatic hydrocarbons in Seyfert and star-forming galaxies

“…In the case of the PNe the feature is narrower and shifted to longer wavelengths in comparison to SF and active galaxies. In a recent study, Shannon & Boersma (2019) found a dependence of the 7.7 𝜇m PAH band peak shift with PAH molecular size, charge, temperature and aliphatic and nitrogen content. However, the origin of the shift is yet unclear.…”

“…For this comparison, we include PNe sources since it is well known that PAH band profiles in these objects vary significantly in comparison with other sources (e.g. Peeters et al 2002;Shannon & Boersma 2019). In addition, due to the large aperture of Spitzer/IRS, local objects like PNe allow us to better isolate the effects of temperature or metallicity, because are less contaminated by the surrounding medium of the UV source than in galaxies.…”

Probing the size and charge of polycyclic aromatic hydrocarbons

“…While a truly isolated harmonic oscillator has a Lorentzian emission profile, astronomical PAH band profiles are distinct from either Lorentzian or Gaussian approximations. However, extensive comparisons have shown that the adopted emission profile does not affect observed PAH trends such as correlations between relative intensities of different features (Smith et al 2007;Galliano et al 2008), and minor to negligible differences are observed in the resulting spectra when using different emission profiles (Shannon & Boersma 2019). (15−20) 15.0 − 20.0 a The complex of interlaced PAH features generally referred to as the 7.7 µm complex.…”