113 research outputs found
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~cm. We found that fulleranes can be formed in the ISM,
although the fraction of carbon atoms locked in them can be maximum around
10. 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
Using HCO isotopologues as tracers of gas depletion in protoplanetary disk gaps
The widespread rings and gaps seen in the dust continuum in protoplanetary
disks are sometimes accompanied by similar substructures seen in molecular line
emission. One example is the outer gap at 100 au in AS 209, which shows that
the HCO and CO emission intensities decrease along with the
continuum in the gap, while the DCO emission increases inside the gap.
We aim to study the behavior of DCO/HCO and DCO/HCO
ratios in protoplanetary disk gaps assuming the two scenarios: the gas
depletion follows the dust depletion and only the dust is depleted.
We first modeled the physical disk structure using the thermo-chemical model
ANDES. This 1+1D steady-state disk model calculates the thermal balance of gas
and dust and includes the FUV, X-rays, cosmic rays, and other ionization
sources together with the reduced chemical network for molecular coolants.
Afterward, this physical structure was adopted for calculations of molecular
abundances with the extended gas-grain chemical network with deuterium
fractionation. Ideal synthetic spectra and 0th-moment maps were produced with
LIME.
We are able to qualitatively reproduce the increase in the DCO intensity
and the decrease in the HCO and CO intensities inside the
disk gap, which is qualitatively similar to what is observed in the outer AS
209 gap. The corresponding disk model assumes that both the gas and dust are
depleted in the gap. The model with the gas-rich gap, where only the dust is
depleted, produces emission that is too bright in all HCO isotopologues and
CO.
The DCO/HCO line ratio can be used to probe gas depletion in
dust continuum gaps outside of the CO snow line. The DCO/CO line
ratio shows a similar, albeit weaker, effect; however, these species can be
observed simultaneously with a single ALMA or NOEMA setup.Comment: 12 pages, 7 figures, Accepted for publication in Astronomy and
Astrophysic
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