996 research outputs found
Improving the thin-disk models of circumstellar disk evolution. The 2+1-dimensional model
Circumstellar disks of gas and dust are naturally formed from contracting
pre-stellar molecular cores during the star formation process. To study various
dynamical and chemical processes that take place in circumstellar disks prior
to their dissipation and transition to debris disks, the appropriate numerical
models capable of studying the long-term disk chemodynamical evolution are
required. We present a new 2+1-dimensional numerical hydrodynamics model of
circumstellar disk evolution, in which the thin-disk model is complemented with
the procedure for calculating the vertical distributions of gas volume density
and temperature in the disk. The reconstruction of the disk vertical structure
is performed at every time step via the solution of the time-dependent
radiative transfer equations coupled to the equation of the vertical
hydrostatic equilibrium. We perform a detailed comparison between circumstellar
disks produced with our previous 2D model and with the improved 2+1D approach.
The structure and evolution of resulting disks, including the differences in
temperatures, densities, disk masses and protostellar accretion rates, are
discussed in detail. The new 2+1D model yields systematically colder disks,
while the in-falling parental clouds are warmer. Both effects act to increase
the strength of disk gravitational instability and, as a result, the number of
gravitationally bound fragments that form in the disk via gravitational
fragmentation as compared to the purely 2D thin-disk simulations with a
simplified thermal balance calculation.Comment: Accepted for publication in Astronomy & Astrophysic
An alternative model for the origin of gaps in circumstellar disks
Motivated by recent observational and numerical studies suggesting that
collapsing protostellar cores may be replenished from the local environment, we
explore the evolution of protostellar cores submerged in the external
counter-rotating environment. These models predict the formation of
counter-rotating disks with a deep gap in the gas surface density separating
the inner disk (corotating with the star) and the outer counter-rotating disk.
The properties of these gaps are compared to those of planet-bearing gaps that
form in disks hosting giant planets. We employ numerical hydrodynamics
simulations of collapsing cores that are replenished from the local
counter-rotating environment, as well as numerical hydrodynamic simulations of
isolated disks hosting giant planets, to derive the properties of the gaps that
form in both cases. Our numerical simulations demonstrate that counter-rotating
disks can form for a wide range of mass and angular momentum available in the
local environment. The gap that separates both disks has a depletion factor
smaller than 1%, can be located at a distance from ten to over a hundred AU
from the star, and can propagate inward with velocity ranging from 1 AU/Myr to
>100 AU/Myr. Unlike our previous conclusion, the gap can therefore be a
long-lived phenomenon, comparable in some cases to the lifetime of the disk
itself. For a proper choice of the planetary mass, the viscous \alpha-parameter
and the disk mass, the planet-bearing gaps and the gaps in counter-rotating
disks may show a remarkable similarity in the gas density profile and depletion
factor, which may complicate their observational differentiation.Comment: 13 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
The Origin of Episodic Accretion Bursts in the Early Stages of Star Formation
We study numerically the evolution of rotating cloud cores, from the collapse
of a magnetically supercritical core to the formation of a protostar and the
development of a protostellar disk during the main accretion phase. We find
that the disk quickly becomes unstable to the development of a spiral structure
similar to that observed recently in AB Aurigae. A continuous infall of matter
from the protostellar envelope makes the protostellar disk unstable, leading to
spiral arms and the formation of dense protostellar/protoplanetary clumps
within them. The growing strength of spiral arms and ensuing redistribution of
mass and angular momentum creates a strong centrifugal disbalance in the disk
and triggers bursts of mass accretion during which the dense
protostellar/protoplanetary clumps fall onto the central protostar. These
episodes of clump infall may manifest themselves as episodes of vigorous
accretion rate (\ge 10^{-4} M_sun/yr) as is observed in FU Orionis variables.
Between these accretion bursts, the protostar is characterized by a low
accretion rate (< 10^{-6} M_sun/yr). During the phase of episodic accretion,
the mass of the protostellar disk remains less than or comparable to the mass
of the protostar.Comment: 5 pages, 2 figures, accepted for publication in ApJ
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
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