Aliphatics and Aromatics in the Universe: The Pre-JWST Era

Abstract The so-called “unidentified infrared emission” (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 μ m ubiquitously seen in a wide variety of astrophysical regions are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. Astronomical PAHs often have an aliphatic component (e.g., aliphatic sidegroups like methyl –CH 3 may be attached as functional groups to PAHs) as revealed by the detection in many UIE sources of the aliphatic C–H stretching feature at 3.4 μ m. With its unprecedented sensitivity and spatial resolution, and high spectral resolution, the James Webb Space Telescope (JWST) holds great promise for revolutionizing the studies of aliphatics and aromatics in the Universe. To facilitate analyzing JWST observations, we present a theoretical framework for determining the aliphatic fractions ( η ali ) of PAHs (the fractions of C atoms in aliphatic units) from the emission intensity ratios of the 3.4 μ m aliphatic C–H feature to the 3.3 μ m aromatic C–H feature. To demonstrate the effectiveness of this framework, we compile the 3.3 and 3.4 μ m UIE data obtained in the pre-JWST era for an as-complete-as-possible sample, and then apply the framework to these pre-JWST data. We derive a median aliphatic fraction of 〈 η ali 〉 ≈ 5.4%, and find that the aliphatic fractions are the highest in protoplanetary nebulae illuminated by cool stars lacking ultraviolet radiation. Nevertheless, the “hardness” of stellar photons is not the only factor affecting the PAH aliphaticity, and other factors such as the starlight intensity may also play an important role.