The 3.3 μm Infrared Emission Feature: Observational and Laboratory Constraints on Its Carrier

We examine the self-consistency of laboratory and observational data for potential carriers 3.3 $\mu$m infrared emission feature (IEF), a member ubiquitous family strong interstellar IEFs at 3.3, 3.4, 6.2, 7.7, 8.6, 11.2, 12.7 $\mu$m. Previous studies have shown that most Galactic sources (reflection nebulae, HII regions, planetary nebulae) show displaying similar central wavelengths, full widths half maximum, profiles. Our study is focused on band profile designated as Class A, prevalent four classes observed In contrast to observations, spectra gas phase polycyclic aromatic hydrocarbons (PAHs), widely assumed IEFs, display wavelength shifts, widths, profiles vary with temperature PAH size. present an extrapolation shifts smaller PAHs ($\le$32 carbon atoms) larger ($>$50 are thought be IEF carriers. The leads tight constraints sizes putative Reconciling observations implications pose significant challenge other carrier hypotheses.