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

Context. Only 8% of the protoplanetary discs orbiting a T Tauri star show emission features of polycyclic aromatic hydrocarbons (PAHs). Their abundance is therefore little known. As PAHs are strong absorbers of UV radiation, they contribute to the heating of the disc photosphere, shielding of UV radiation that drives photo-chemistry in the disc, and their abundance is a key parameter for determining the strength of photo-evaporative disc winds. Soon, high-quality data obtained with the James Webb Space Telescope (JWST) will become available with new data to interpret.
Aims. 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.
Methods. We developed a model for the absorption of X-rays by PAHs. X-rays with more energy than the K edge of carbon doubly ionise PAHs and 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.
Results. We find that small PAH clusters quickly desorb and dissociate into individual molecules. PAH molecules experience rapid loss of H and acetylene C₂H₂ by the high excitation and lose C₂H₂ on average after three X-ray excitations. However, large PAH clusters (coronene C₂₄H₁₂: 50 cluster members, circumcoronene C₅₄H₁₈: 3 cluster members) can stay intact and frozen out on dust grains.
Conclusions. Based on our results, we expect a gas-phase PAH abundance that is lower than 0.01 times the ISM abundance and that rapidly decreases over time due to the dissociation of small clusters that are subsequently destroyed. To maintain a higher abundance, replenishment processes such as vertical mixing must exist. 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.