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

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.Comment: Accepted to MNRAS. 19 pages, 12 figure

Simulation of Thermal Surface Waves in a Protoplanetary Disk in a Two-Dimensional Approximation

Theoretical models predict that the obscuration of stellar radiation by irregularities on the surface of a protoplanetary disk can cause self-generating waves traveling towards the star. However, this process is traditionally simulated using the 1+1D approach, the key approximations of which - vertical hydrostatic equilibrium of the disk and vertical diffusion of IR radiation - can distort the picture. This article presents a two-dimensional radiative hydrodynamic model of the evolution of an axially symmetric gas and dust disk. Within this model, but using simplified assumptions from 1+1D models, we have reproduced the spontaneous generation and propagation of thermal surface waves. The key conclusion of our work is that taking into account two-dimensional hydrodynamics and diffusion of IR radiation suppresses the spontaneous generation and development of thermal waves observed in the 1+1D approximation. The search for the possibility of the existence of surface thermal waves should be continued by studying the problem for various parameters of protoplanetary disks.Comment: Accepted for publication in Astronomy Reports (2022

Effect of Dust Evaporation and Thermal Instability on Temperature Distribution in a Protoplanetary Disk

The thermal instability of accretion disks is widely used to explain the activity of cataclysmic variables, but its development in protoplanetary disks has been studied in less detail. We present a semi-analytical stationary model for calculating the midplane temperature of a gas and dust disk around a young star. The model takes into account gas and dust opacities, as well as the evaporation of dust at temperatures above 1000 K. Using this model, we calculate the midplane temperature distributions of the disk under various assumptions about the source of opacity and the presence of dust. We show that when all considered processes are taken into account, the heat balance equation in the region r<1 au has multiple temperature solutions. Thus, the conditions for thermal instability are met in this region. To illustrate the possible influence of instability on the accretion state in a protoplanetary disk, we consider a viscous disk model with alpha parameterization of turbulent viscosity. We show that in such a model the disk evolution is non-stationary, with alternating phases of accumulation of matter in the inner disk and its rapid accretion onto the star, leading to an episodic accretion pattern. These results indicate that this instability needs to be taken into account in evolutionary models of protoplanetary disks.Comment: Published in Astronomy Reports Vol. 67, No. 5, pp. 470-482 (2023