# Turbulent processing of PAHs in protoplanetary discs -- Coagulation and freeze-out leading to depletion of gas-phase PAH

**Authors:** K. Lange, C. Dominik, A. G. G. M. Tielens  
**arXiv:** [2303.07981](https://arxiv.org/abs/2303.07981)  
**Date:** 2023-03-14  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2303.07981v1/

## Key points

- Polycyclic aromatic hydrocarbons (PAHs) are commonly found in circumstellar discs
- Continuous cycle of processing PAHs through clustering, adsorption on dust grains, and reverse processes can reduce the emission-capable PAH abundance in protoplanetary discs
- Clustering of gas-phase PAHs is very efficient, with clusters growing for years before freezing out in the disc midplane
- Depletion factor ranging between 50 and 1000 compared to standard interstellar medium (ISM) abundance of PAHs in inner discs through turbulent processing
- Smaller than circumovalene-sized PAHs favored as major gas-phase emitters of disc photospheres since larger monomers cannot photodesorb from grain surfaces

## Summary

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Polycyclic aromatic hydrocarbons (PAHs) are commonly found in circumstellar discs, and their abundance can affect the formation of planets. In this study, the authors propose a continuous cycle of processing PAHs through clustering, adsorption on dust grains, and reverse processes as key mechanisms to reduce the emission-capable PAH abundance in protoplanetary discs. This cycle is driven by vertical turbulence in the disc that mixes PAHs between the midplane and photosphere. The authors used a Monte Carlo model for photodesorption and a coagulation code to estimate the relevance and timescale of these processes in a Herbig Ae/Be disc environment. By combining these components in a 1D vertical model, they calculated the gas-phase depletion of PAHs that stick as clusters on dust grains. The results show that clustering of gas-phase PAHs is very efficient, with clusters growing for years before freezing out in the disc midplane. Once frozen on dust grain surfaces, large heat capacity prevents them from evaporating off grains in UV-rich environments such as the photosphere. Therefore, clustering followed by freeze-out can lead to depletion of gas-phase PAHs in discs over protoplanetary disc lifetimes, resulting in a depletion factor ranging between 50 and 1000 compared to standard interstellar medium (ISM) abundance of PAHs in inner discs through turbulent processing. The authors also favor smaller than circumovalene-sized PAHs as major gas-phase emitters of disc photospheres since larger monomers cannot photodesorb from grain surfaces. These gas-phase PAHs coexist with large clusters sticking on dust grains which have a close relation with both dust population and strength of vertical turbulence. This study provides insight into how turbulent processing affects PAH abundance in protoplanetary discs and highlights potential implications for planet formation.