Efficiency of the top-down PAH-to-fullerene conversion in UV irradiated environments

TL;DR

This study models the conversion of PAHs to fullerenes in UV-irradiated environments, highlighting conditions that favor fullerene formation and comparing predictions with observations in specific interstellar regions.

Contribution

It introduces a comprehensive model of PAH-to-fullerene conversion considering various chemical processes and environmental factors, extending understanding of fullerene formation in space.

Findings

Fullerene formation occurs in both H-poor and modest H-density environments.

Fulleranes can form in the ISM, but with very low carbon fraction (~10^{-9}).

Model predictions align well with observations near ionizing stars in Orion and NGC 7023.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) and fullerenes play a major role in the physics and chemistry of the interstellar medium. Based on a number of recent experimental and theoretical investigations we developed a model in which PAHs are subject to photo-dissociation (carbon and hydrogen loss) and hydrogenation. We take into account that dehydrogenated PAHs may fold into closed structures -- fullerenes. Fullerenes, in their turn, can be also hydrogenated, becoming fulleranes, and photo-dissociated, losing carbon and hydrogen atoms. The carbon loss leads to shrinking of fullerene cages to smaller ones. We calculate the abundance of PAHs and fullerenes of different sizes and hydrogenation level depending on external conditions: the gas temperature, intensity of radiation field, number density of hydrogen atoms, carbon atoms, and electrons. We highlight the conditions, which are favourable for fullerene formation from PAHs, and we conclude that this mechanism works not only in H-poor environment but also at modest values of hydrogen density up to 10$^{4}$~cm$^{-3}$. We found that fulleranes can be formed in the ISM, although the fraction of carbon atoms locked in them can be maximum around 10$^{-9}$. We applied our model to two photo-dissociation regions, Orion Bar and NGC 7023. We compare our estimates of the fullerene abundance and synthetic band intensities in these objects with the observations and conclude that our model gives good results for the closest surroundings of ionising stars. We also demonstrate that additional fullerene formation channels should operate along with UV-induced formation to explain abundance of fullerenes far from UV sources.