47 research outputs found
Gravitoviscous protoplanetary disks with a dust component. II. Spatial distribution and growth of dust in a clumpy disk
Spatial distribution and growth of dust in a clumpy protoplanetary disk
subject to vigorous gravitational instability and fragmentation is studied
numerically with sub-au resolution using the FEOSAD code. Hydrodynamics
equations describing the evolution of self-gravitating and viscous
protoplanetary disks in the thin-disk limit were modified to include a dust
component consisting of two parts: sub-micron-sized dust and grown dust with a
variable maximum radius. The conversion of small to grown dust, dust growth,
friction of dust with gas, and dust self-gravity were also considered. We found
that the disk appearance is notably time-variable with spiral arms, dusty
rings, and clumps, constantly forming, evolving, and decaying. As a
consequence, the total dust-to-gas mass ratio is highly non-homogeneous
throughout the disk extent, showing order-of-magnitude local deviations from
the canonical 1:100 value. Gravitationally bound clumps formed through
gravitational fragmentation have a velocity pattern that deviates notably from
the Keplerian rotation. Small dust is efficiently converted into grown dust in
the clump interiors, reaching a maximum radius of several decimeters.
Concurrently, grown dust drifts towards the clump center forming a massive
compact central condensation (70-100 ). We argue that protoplanets
may form in the interiors of inward migrating clumps before they disperse
through the action of tidal torques. We foresee the formation of protoplanets
at orbital distances of several tens of au with initial masses of gas and dust
in the protoplanetary seed in the (0.25-1.6) and (1.0-5.5)
limits, respectively. The final masses of gas and dust in the
protoplanets may however be much higher due to accretion from surrounding
massive metal-rich disks/envelopes.Comment: 13 pages, 10 figures; The abstract is abridged to meet ArXiv size
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Gravitoviscous protoplanetary disks with a dust component. I. The importance of the inner sub-au region
The central region of a circumstellar disk is difficult to resolve in global
numerical simulations of collapsing cloud cores, but its effect on the
evolution of the entire disk can be significant. We use numerical hydrodynamics
simulations to model the long-term evolution of self-gravitating and viscous
circumstellar disks in the thin-disk limit. Simulations start from the
gravitational collapse of prestellar cores of 0.5--1.0~ and both
gaseous and dusty subsystems were considered, including a model for dust
growth. The inner unresolved 1.0 au of the disk is replaced with a central
"smart" cell (CSC) -- a simplified model that simulates physical processes that
may occur in this region. We found that the mass transport rate through the CSC
has an appreciable effect on the evolution of the entire disk. Models with slow
mass transport form more massive and warmer disks and they are more susceptible
to gravitational instability and fragmentation, including a newly identified
episodic mode of disk fragmentation in the T Tauri phase of disk evolution.
Models with slow mass transport through the CSC feature episodic accretion and
luminosity bursts in the early evolution, while models with fast transport are
characterized by a steadily declining accretion rate with low-amplitude
flickering. Dust grows to a larger, decimeter size in the slow transport models
and efficiently drifts in the CSC, where it accumulates reaching the limit when
streaming instability becomes operational. We argue that gravitational
instability, together with streaming instability likely operating in the inner
disk regions, constitute two concurrent planet-forming mechanisms, which may
explain the observed diversity of exoplanetary orbits (Abridged).Comment: Accepted for publication in Astronomy \& Astrophysic
Episodic excursions of low-mass protostars on the Hertzsprung-Russell diagram
Following our recent work devoted to the effect of accretion on the
pre-main-sequence evolution of low-mass stars, we perform a detailed analysis
of episodic excursions of low-mass protostars in the Hertzsprung-Russell (H-R)
diagram triggered by strong mass accretion bursts typical of FU Orionis-type
objects (FUors). These excursions reveal themselves as sharp increases in the
stellar total luminosity and/or effective temperature of the protostar and can
last from hundreds to a few thousands of years, depending on the burst strength
and characteristics of the protostar. During the excursions, low-mass
protostars occupy the same part of the H-R diagram as young intermediate-mass
protostars in the quiescent phase of accretion. Moreover, the time spent by
low-mass protostars in these regions is on average a factor of several longer
than that spent by the intermediate-mass stars in quiescence. During the
excursions, low-mass protostars pass close to the position of most known FUors
in the H-R diagram, but owing to intrinsic ambiguity the model stellar
evolutionary tracks are unreliable in determining the FUor properties. We find
that the photospheric luminosity in the outburst state may dominate the
accretion luminosity already after a few years after the onset of the outburst,
meaning that the mass accretion rates of known FUors inferred from the
bolometric luminosity may be systematically overestimated, especially in the
fading phase.Comment: 15 pages, 12 figure
Distinguishing between different mechanisms of FU-Orionis-type luminosity outbursts
Aims. Accretion and luminosity bursts triggered by three distinct mechanisms:
the magnetorotational instability in the inner disk regions, clump infall in
gravitationally fragmented disks and close encounters with an intruder star,
were studied to determine the disk kinematic characteristics that can help to
distinguish between these burst mechanisms. Methods. Numerical hydrodynamics
simulations in the thin-disk limit were employed to model the bursts in disk
environments that are expected for each burst mechanism. Results. We found that
the circumstellar disks featuring accretion bursts can bear kinematic features
that are distinct for different burst mechanisms, which can be useful when
identifying the burst origin. The disks in the stellar encounter and
clump-infall models are characterized by tens of percent deviations from the
Keplerian rotation, whie the disks in the MRI models are characterized only a
few percent deviation, which is mostly caused by the gravitational instability
that fuels the MRI bursts. Velocity channel maps also show distinct kinks and
wiggles, which are caused by gas disk flows that are peculiar to each
considered burst mechanism. The deviations of velocity channels in the
burst-hosting disks from a symmetric pattern typical of Keplerian disks are
strongest for the clump-infall and collision models, and carry individual
features that may be useful for the identification of the corresponding burst
mechanism. The considered burst mechanisms produce a variety of light curves
with the burst amplitudes varying in the \Delta m=2.5-3.7 limits, except for
the clump-infall model where \Delta m can reach 5.4, although the derived
numbers may be affected by a small sample and boundary conditions. Conclusions.
Burst triggering mechanisms are associated with distinct kinematic features in
the burst-hosting disks that may be used for their identification. Abridged.Comment: Accepted for publication by Astronomy & Astrophysic
Episodic accretion and mergers during growth of massive protostars
3D simulations of high mass young stellar object (HMYSO) growth show that
their circumstellar discs fragment onto multiple self-gravitating objects.
Accretion of these by HMYSO may explain episodic accretion bursts discovered
recently. We post-process results of a previous 3D simulation of a HMYSO disc
with a 1D code that resolves the disc and object dynamics down to the stellar
surface. We find that burst-like deposition of material into the inner disc
seen in 3D simulations by itself does not always signify powerful accretion
bursts. Only high density post-collapse clumps crossing the inner computational
boundary may result in observable bursts. The rich physics of the inner disc
has a significant impact on the expected accretion bursts: (1) In the standard
turbulent viscosity discs, migrating objects can stall at a migration trap at
the distance of a few au from the star. However, in discs powered by magnetised
winds, the objects are able to cross the trap and produce bursts akin to those
observed so far. (2) Migrating objects may interact with and modify the thermal
(hydrogen ionisation) instability of the inner disc, which can be responsible
for longer duration and lower luminosity bursts in HMYSOs. (3) If the central
star is bloated to a fraction of an au by a previous episode of high accretion
rate, or if the migrating object is particularly dense, a merger rather than a
disc-mediated accretion burst results; (4) Object disruption bursts may be
super-Eddington, leading to episodic feedback on HMYSO surroundings via
powerful outflows.Comment: 19 pages, 16 figures, accepted to MNRA
Accretion bursts in high-mass protostars: a new testbed for models of episodic accretion
It is well known that low mass young stellar objects (LMYSOs) gain a
significant portion of their final mass through episodes of very rapid
accretion, with mass accretion rates up to ~yr. Recent observations of high mass young stellar objects
(HMYSO) with masses uncovered outbursts with
accretion rates exceeding ~yr. Here we
examine which scenarios proposed in the literature so far to explain accretion
bursts of LMYSOs can apply to the episodic accretion in HMYSOs. We utilise a 1D
time dependent models of protoplanetary discs around HMYSOs to study burst
properties. We find that discs around HMYSOs are much hotter than those around
their low mass cousins. As a result, much more extended regions of the disc are
prone to the thermal hydrogen ionisation and MRI activation instabilities. The
former in particular is found to be ubiquitous in a very wide range of
accretion rates and disc viscosity parameters. The outbursts triggered by these
instabilities, however, always have too low , and are one to several
orders of magnitude too long compared to those observed from HMYSOs so far. On
the other hand, bursts generated by tidal disruptions of gaseous giant planets
formed by the gravitational instability of the protoplanetary discs yield
properties commensurate with observations, provided that the clumps are in the
post-collapse configuration with planet radius Jupiter
radii. Furthermore, if observed bursts are caused by disc ionisation
instabilities then they should be periodic phenomena with the duration of the
quiescent phase comparable to that of the bursts. This may yield potentially
observable burst periodicity signatures in the jets, the outer disc, or the
surrounding diffuse material of massive HMYSOs. (abridged)Comment: 8 pages, 6 figures, Accepted to A&A Letter
Bacteria Hunt: A multimodal, multiparadigm BCI game
Brain-Computer Interfaces (BCIs) allow users to control applications by brain activity. Among their possible applications for non-disabled people, games are promising candidates. BCIs can enrich game play by the mental and affective state information they contain. During the eNTERFACE’09 workshop we developed the Bacteria Hunt game which can be played by keyboard and BCI, using SSVEP and relative alpha power. We conducted experiments in order to investigate what difference positive vs. negative neurofeedback would have on subjects’ relaxation states and how well the different BCI paradigms can be used together. We observed no significant difference in mean alpha band power, thus relaxation, and in user experience between the games applying positive and negative feedback. We also found that alpha power before SSVEP stimulation was significantly higher than alpha power during SSVEP stimulation indicating that there is some interference between the two BCI paradigms
Formation of pebbles in (gravito-)viscous protoplanetary disks with various turbulent strengths
Aims. Dust plays a crucial role in the evolution of protoplanetary disks. We
study the dynamics and growth of initially sub- dust particles in
self-gravitating young protoplanetary disks with various strengths of turbulent
viscosity. We aim to understand the physical conditions that determine the
formation and spatial distribution of pebbles when both disk self-gravity and
turbulent viscosity can be concurrently at work. Methods. We perform the
thin-disk hydrodynamics simulations of self-gravitating protoplanetary disks
over an initial time period of 0.5 Myr using the FEOSAD code. Turbulent
viscosity is parameterized in terms of the spatially and temporally constant
-parameter, while the effects of gravitational instability on dust
growth is accounted for by calculating the effective parameter . We consider the evolution of dust component including momentum exchange
with gas, dust self-gravity, and also a simplified model of dust growth.
Results. We find that the level of turbulent viscosity strongly affects the
spatial distribution and total mass of pebbles in the disk. The
model is viscosity-dominated, pebbles are completely absent,
and dust-to-gas mass ratio deviates from the reference 1:100 value no more than
by 30\% throughout the disk extent. On the contrary, the model
and, especially, the model are dominated by gravitational
instability. The effective parameter is now a strongly
varying function of radial distance. As a consequence, a bottle neck effect
develops in the innermost disk regions, which makes gas and dust accumulate in
a ring-like structure. Abridged.Comment: Accepted for publication in Astronomy and Astrophysic
Gravitoviscous protoplanetary disks with a dust component: III. Evolution of gas, dust, and pebbles
Aims. We study the dynamics and growth of dust particles in circumstellar disks of different masses that are prone to gravitational instability during the critical first Myr of their evolution. Methods. We solved the hydrodynamics equations for a self-gravitating and viscous circumstellar disk in a thin-disk limit using the FEOSAD numerical hydrodynamics code. The dust component is made up of two different components: micron-sized dust and grown dust of evolving size. For the dust component, we considered the dust coagulation, fragmentation, momentum exchange with the gas, and dust self-gravity. Results. We found that the micron-sized dust particles grow rapidly in the circumstellar disk, reaching a few cm in size in the inner 100 au of the disk during less than 100 kyr after the disk formation, provided that fragmentation velocity is 30 ms-1. Due to the accretion of micron dust particles from the surrounding envelope, which serves as a micron dust reservoir, the approximately cm-sized dust particles continue to be present in the disk for more than 900 kyr after the disk formation and maintain a dust-to-gas ratio close to 0.01. We show that a strong correlation exists between the gas and pebble fluxes in the disk. We find that radial surface density distribution of pebbles in the disk shows power-law distribution with an index similar to that of the Minimum-mass solar nebula regardless the disk mass. We also show that the gas surface density in our models agrees well with measurements of dust in protoplanetary disks of AS 209, HD 163296, and DoAr 25 systems. Conclusions. Pebbles are formed during the very early stages of protoplanetary disk evolution. They play a crucial role in the planet formation process. Our disc simulations reveal the early onset (<105 yr) of an inwards-drifting flux of pebble-sized particles that makes up approximately between one hundredth and one tenth of the gas mass flux, which appears consistent with mm-observations of discs. Such a pebble flux would allow for the formation of planetesimals by streaming instability and the early growth of embryos by pebble accretion. We conclude that unlike the more common studies of isolated steady-state protoplanetary disks, more sophisticated global numerical simulations of circumstellar disk formation and evolution, including the pebble formation from the micron dust particles, are needed for performing realistic planet formation studies. © ESO 2020.We thank the anonymous referee for a insightful report, which helped to improve this paper. Research was financially supported by the Ministry of Science and Higher Education of the Russian Federation (State assignment in the field of scientific activity, Southern Federal University, 2020). V.G.E. acknowledges the Swedish Institute for a visitor grant allowing to conduct research at Lund University. A.J. was supported by the Swedish Research Council (grant 2018-04867), the Knut and Alice Wallenberg Foundation (grant 2012.0150) and the European Research Council (ERC Consolidator Grant 724687-PLANETESYS). M.L. was supported by the Knut and Alice Wallenberg Foundation (grant 2012.0150). V.A. was supported by RFBR grant 18-52-52006