Dissociation and destruction of PAHs and PAH clusters induced by absorption of X-rays in protoplanetary discs around T Tauri stars

TL;DR

This study models how X-ray irradiation in protoplanetary discs around T Tauri stars causes PAHs and their clusters to dissociate and desorb, explaining the observed scarcity of PAH emission features.

Contribution

The paper presents a new model of X-ray absorption and dissociation processes affecting PAHs and clusters in protoplanetary discs, highlighting their rapid destruction and implications for PAH abundance.

Findings

Small PAH clusters quickly desorb and dissociate.

Large PAH clusters remain intact and frozen on dust grains.

Gas-phase PAH abundance is significantly reduced over time.

Abstract

Only 8% of the protoplanetary discs orbiting a T Tauri star show emission features of polycyclic aromatic hydrocarbons (PAHs). As PAHs are strong absorbers of UV radiation, they contribute to the heating of the discs photosphere, shielding of UV radiation that drives photo-chemistry in the disc, and their abundance is a key parameter to determine the strength of photo-evaporative disc winds. We want to understand the photochemical evolution of PAHs in protoplanetary discs around T Tauri stars and thus explain the absence of PAH features. We want to determine whether PAHs are destroyed because of the X-ray emission from their host stars or whether PAHs can withstand these conditions. We developed a model for the absorption of X-rays by PAHs. X-rays with more energy than the K edge of carbon will double ionise PAHs and will vibrationally excite them by ~ 15-35 eV. With a Monte Carlo model, we modelled the dissociation of H, H2, and C2H2 from PAH monomers. Furthermore, we modelled the dissociation of PAH clusters and the desorption of PAH clusters from dust grains caused by X-ray excitation. We find that small PAH clusters will quickly desorb and dissociate into individual molecules. PAH molecules experience rapid loss of H and acetylene C2H2 by the high excitation and will lose C2H2 on average after three X-ray excitations. However, large PAH clusters can stay intact and frozen out on dust grains. Based on our results, we expect a gas-phase PAH abundance that is lower than 0.01 times the ISM abundance and will rapidly decrease over time due to the dissociation of small clusters that are subsequently destroyed. To maintain a higher abundance, replenishment processes must exist such as vertical mixing. Large PAH clusters remain in the disc, frozen out on dust grains, but barely emit PAH features because of their strong thermal coupling to dust grains.