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

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.

Accepted to MNRAS. 19 pages, 12 figures

References in corpus (17)

• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• Dust Masses, PAH Abundances, and Starlight Intensities in the SINGS Galaxy Sample
• The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point
• Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
• Formation of fullerenes in H-containing Planetary Nebulae
• Laboratory formation of fullerenes from PAHs: Top-down interstellar chemistry
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• PAH chemistry and IR emission from circumstellar disks
• The PAH emission characteristics of the reflection nebula NGC2023
• Detection of Buckminsterfullerene emission in the diffuse interstellar medium
• The Orion HII Region and the Orion Bar in the Mid-Infrared
• The infrared spectrum of protonated buckminsterfullerene, C60H+
• Shiva: the dust destruction model
• Transformation of amorphous carbon clusters to fullerenes
• Impact of PAH photodissociation on the formation of small hydrocarbons in the Orion Bar and the Horsehead PDRs
• Detection of Cosmic Fullerenes in the Almahata Sitta Meteorite: Are They an Interstellar Heritage?
• A Principal Component Analysis of polycyclic aromatic hydrocarbon emission in NGC 7023

Cited by in corpus (2)

• Fullerenes in the circumstellar medium of Herbig Ae/Be stars: Insights from the Spitzer mid-infrared spectral catalog
• A rich molecular chemistry in the gas of the IC 348 star cluster of the Perseus Molecular Cloud