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

Abstract

Polycyclic aromatic hydrocarbons (PAHs) have been detected in numerous circumstellar discs. We propose the continuous processing of PAHs through clustering, adsorption on dust grains, and their reverse-processes as key mechanisms to reduce the emission-capable PAH abundance in protoplanetary discs. This cycle of processing is driven by vertical turbulence in the disc mixing PAHs between the disc midplane and the photosphere. We used a theoretical Monte Carlo model for photodesorption and a coagulation code in the disc midplane 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, we calculated the gas-phase depletion of PAHs that stick as clusters on dust grains. Our results show that the clustering of gas-phase PAHs is very efficient, and that clusters with more than 100 monomers can grow for years before they are able to freeze out in the disc midplane. Once a PAH cluster is frozen on the dust grain surface, the large heat capacity of these clusters prevents them from evaporating off the grains in UV-rich environments such as the photosphere. Therefore, the clustering of PAHs followed by freeze-out can lead to a depletion of gas-phase PAHs in discs. Evaluated over the lifetime of protoplanetary discs, we find a depletion of PAHs by a factor that ranges between 50 and 1000 compared to the standard ISM abundance of PAHs in the inner disc through turbulent processing. Through these processes, we favour PAHs smaller than circumovalene as the major gas-phase emitters of the disc photosphere as larger PAH monomers cannot photodesorb from the grain surface. These gas-phase PAHs co-exist with large PAH clusters sticking on dust grains. We find a close relation between the amount of PAHs frozen out on dust grains and the dust population, as well as the strength of the vertical turbulence.

arXiv commentAccepted for Astronomy and Astrophysics. 18 pages, 14 figures

References in corpus (29)

• The interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons
• Dust coagulation in protoplanetary disks: a rapid depletion of small grains
• Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains
• ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be systems: towards an understanding of dust processing
• Closed-form expressions for particle relative velocities induced by turbulence
• The effect of dust settling on the appearance of protoplanetary disks
• Consistent dust and gas models for protoplanetary disks. I. Disk shape, dust settling, opacities, and PAHs
• The Disk Substructures at High Angular Resolution Project (DSHARP): V. Interpreting ALMA maps of protoplanetary disks in terms of a dust model
• Initial Conditions of Planet Formation: Lifetimes of Primordial Disks
• MHD simulations of the magnetorotational instability in a shearing box with zero net flux. II. The effect of transport coefficients
• ISO spectroscopy of disks around Herbig Ae/Be stars
• On-line database of the spectral properties of polycyclic aromatic hydrocarbons
• Stacked clusters of polycyclic aromatic hydrocarbon molecules
• Strong covalent bonding between two graphene layers
• Formation and destruction of polycyclic aromatic hydrocarbon clusters in the interstellar medium
• Spitzer's view on aromatic and aliphatic hydrocarbon emission in Herbig Ae stars
• Mapping PAH sizes in NGC 7023 with SOFIA
• A representative particle approach to coagulation and fragmentation of dust aggregates and fluid droplets
• Mid-IR imaging of the transitional disk of HD169142: Measuring the size of the gap
• Polycyclic aromatic hydrocarbon ionization as a tracer of gas flows through protoplanetary disk gaps
• Polycyclic Aromatic Hydrocarbon in Protoplanetary Disks around Herbig Ae/Be and T Tauri Stars
• Photochemistry of the PAH pyrene in water ice: the case for ion-mediated solid-state astrochemistry
• Effect of dust size and structure on scattered light images of protoplanetary discs
• Lack of PAH emission toward low-mass embedded young stellar objects
• Radiation thermo-chemical models of protoplanetary disks. Grain and polycyclic aromatic hydrocarbon charging
• What happened before? -- The disks around the precursors of young Herbig Ae/Be stars
• Absolute evaporation rates of non-rotating neutral PAH clusters
• PAHs in protoplanetary disks: emission and X-ray destruction
• Stability of Polycyclic Aromatic Hydrocarbon Clusters in Protoplanetary Disks

Cited by in corpus (6)

• A JWST/MIRI analysis of the ice distribution and PAH emission in the protoplanetary disk HH 48 NE
• Detectability of Polycyclic Aromatic Hydrocarbons in the Atmosphere of WASP-6 b with JWST NIRSpec PRISM
• Long-Lived Isomers of C${}_{11}$H${}_9^{+}$: New Experimental Insights from the PIRENEA Setup
• Searching for Corannulene with ALMA: Observations of the Red Rectangle Nebula
• Polycyclic aromatic hydrocarbon (PAH) abundances in the disk around T Chamaeleontis (T Cha): PAH sizes, ionization fraction, and mass during JWST observations
• Observational evidence for a possible link between PAH emission and dust trap locations in protoplanetary disks