Polycyclic Aromatic Hydrocarbons as an Extraterrestrial Atmospheric Technosignature

Polycyclic aromatic hydrocarbons (PAHs) are prevalent in the Universe and interstellar medium but are primarily attributed to anthropogenic sources on Earth, such as fossil fuel combustion and firewood burning. Drawing upon the idea of PAHs being suitable candidates for technosignatures, we investigate the detectability of those PAHs that have available absorption cross sections in the atmospheres of Earth-like exoplanets (orbiting G-type stars at a distance of 10 pc) with an 8 m mirror of the Habitable Worlds Observatory (HWO). Specifically, we focus on Naphthalene, Anthracene, Phenanthrene, and Pyrene. Our simulations indicate that under current-Earth-like conditions, detecting PAH signatures between 0.2 and 0.515 mu m is infeasible. To account for the historical decline in PAH production post the industrial revolution, we explore varying PAH concentrations to assess instrumental capabilities of detecting civilizations resembling modern Earth. We also evaluate telescope architectures (6 m, 8 m, and 10 m mirror diameters) to put our results into the context of the future HWO mission. With these four molecules, PAH detection remains infeasible, even at concentrations 10 times higher than current levels. While larger mirrors provide some advantages, they fail to resolve the spectral signatures of these molecules with significant signal-to-noise ratios. The UV absorption features of PAHs-caused by pi-orbital -> pi*-orbital electronic transitions-serve as valuable markers, due to their distinct and detectable nature, preserved by the aromatic stability of PAHs. Additional lab measurements are necessary to gather absorption cross-sectional data beyond UV for more abundant PAHs. This may help further in improving the detectability of these molecules.