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

TL;DR

This work addresses why PAHs are rarely observed in T Tauri discs by modeling how X‑ray irradiation drives PAH destruction. It introduces a cascade framework where Auger‑induced vibrational energy (approximately $15-35$ eV) from X‑ray absorption leads to preferential H and H$_2$ loss and sporadic C$_2$H$_2$ ejection, with cluster evaporation and desorption from grains further depleting PAHs. The results show small PAH monomers and clusters are rapidly destroyed or desorb, while large PAH clusters are more robust and tend to freeze on grains, yielding gas‑phase PAH abundances below $1 imes 10^{-2}$ of the ISM and requiring vertical mixing to replenish observable PAHs. The findings help explain low PAH feature detections in T Tauri discs, highlight the importance of disc structure and vertical transport, and offer testable predictions for JWST observations of PAHs in protoplanetary environments.

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.

Figures (11)

• Figure 1: Excitation and ionisation process of a PAH after absorption of an X-ray. The X-ray ejects an electron from the innermost shell of a carbon atom. Then, an electron from a higher shell fills the vacant inner shell so that the freed energy ejects another electron. A fraction of the energy is carried away by the electron, the remaining fraction is transferred to vibrational excitation of the PAH. In the end, the PAH is doubly ionised and vibrationally excited by the X-ray absorption.
• Figure 2: Sketch of cascading processes for PAHs in protoplanetary discs. X-rays can trigger evaporation of PAH clusters from dust grains or the vibrational energy of the cluster is transported to the dust grains and emitted as thermal radiation. Dependent on the cluster size and PAH species, the evaporated cluster can be fully destroyed into monomers (small cluster), can lose a few of its monomers (intermediate clusters), or not lose any monomer (large clusters). Then, the subsequent absorbed X-rays break down the cluster until the entire cluster has been dissociated into molecules. Finally, the absorption of X-rays by individual PAH molecules triggers dissociation of H and H2, which cools the PAH without structural damage, followed by IR cooling. Simultaneously, acetylene (C2H2) fragments can be ejected from the PAH, damaging the carbon backbone, which is difficult to repair without complex chemistry.
• Figure 3: Comparison of dissociation rates for coronene C$_{24}$H$_{12}$ (solid lines) and ovalene C$_{36}$H$_{18}$ (dashed lines). Only for low energies where H and C dissociation starts to appear do H$_2$ and C$_2$H$_2$ loss have comparable rates to H loss; for high energies H loss always dominates. Dissociation of a dimer is always favoured because of the much weaker binding energy between two PAHs. For ovalene, the reactions appear at nearly the same temperature but at higher energies because of the larger heat capacity. Hence, the increasing size of PAHs and PAH clusters increases their stability against X-rays as the deposited energy is independent of the structure. The dissociation rate for clusters additionally depends on the size of the cluster. The typical rates for IR cooling and energy transfer to dust grains are indicated by the grey lines.
• Figure 4: Fraction of destroyed gas-phase PAHs for our four considered PAH sizes (coronene C24H12 to circumcoronene C54H18) after absorption of an X-ray followed by vibrational excitation through the release of an Auger electron.
• Figure 5: Average fraction of ejected PAH monomers from gas-phase cluster after excitation with 15 eV, 20 eV, 25 eV, 30 eV, and 35 eV (from light to dark). With larger cluster size and larger PAH size the probability of losing cluster members strongly decreases so that the cluster is stabilised against X-ray radiation.