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.

arXiv commentAccepted to MNRAS. 19 pages, 12 figures

References in corpus (44)

• Dust Grain Size Distributions and Extinction in the Milky Way, LMC, and SMC
• 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
• Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains
• The rich 6 to 9 um spectrum of interstellar PAHs
• Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating
• The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point
• The global dust SED: Tracing the nature and evolution of dust with DustEM
• The CH out-of-plane bending modes of PAH molecules in astrophysical environments
• Formation of buckminsterfullerene (C60) in interstellar space
• C${}_{60}$ in Reflection Nebulae
• Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
• Polycyclic aromatic hydrocarbon processing in interstellar shocks
• Formation of fullerenes in H-containing Planetary Nebulae
• Polycyclic aromatic hydrocarbon processing in a hot gas
• Formation of PAHs and Carbonaceous Solids in Gas-Phase Condensation Experiments
• Laboratory formation of fullerenes from PAHs: Top-down interstellar chemistry
• Evolution of PAHs in photodissociation regions: Hydrogenation and charge states
• Top-down formation of fullerenes in the interstellar medium
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• The formation of fullerenes: clues from new C60, C70, and (possible) planar C24 detections in Magellanic Cloud Planetary Nebulae
• Velocity-resolved [CII] emission and [CII]/FIR Mapping along Orion with Herschel
• The inner wind of IRC+10216 revisited: New exotic chemistry and diagnostic for dust condensation in carbon stars
• Mapping PAH sizes in NGC 7023 with SOFIA
• Interstellar Fullerene Compounds and Diffuse Interstellar Bands
• PAH chemistry and IR emission from circumstellar disks
• The PAH emission characteristics of the reflection nebula NGC2023
• The Carriers of the Interstellar Unidentified Infrared Emission Features: Aromatic or Aliphatic?
• The formation of cosmic fullerenes from arophatic clusters
• Physical Properties of Fullerene-containing Galactic Planetary Nebulae
• Spitzer IRS Detection of Molecular Hydrogen Rotational Emission Towards Translucent Clouds
• 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+
• C${}_{60}$ in Photodissociation Regions
• Shiva: the dust destruction model
• Transformation of amorphous carbon clusters to fullerenes
• Stochastic grain heating and mid-infrared emission in protostellar cores
• Fullerenes and proto-fullerenes in interstellar carbon dust
• 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?
• Orion Bar as a window to the evolution of PAHs
• Fullerenes in the IC 348 star cluster of the Perseus Molecular Cloud
• A Principal Component Analysis of polycyclic aromatic hydrocarbon emission in NGC 7023

Cited by in corpus (4)

• 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
• Hydrogenated amorphous carbon grains as an alternative carrier of the 9$-$13$\mu$m plateau feature in the fullerene planetary nebula Tc 1
• JWST observations of photodissociation regions. IV. Carbonaceous emission band sub-components in NGC 7023 have distinct spatial distributions