493 research outputs found
Influence of viscosity and the adiabatic index on planetary migration
The strength and direction of migration of low mass embedded planets depends
on the disk's thermodynamic state, where the internal dissipation is balanced
by radiative transport, and the migration can be directed outwards, a process
which extends the lifetime of growing embryos. Very important parameters
determining the structure of disks, and hence the direction of migration, are
the viscosity and the adiabatic index. In this paper we investigate the
influence of different viscosity prescriptions (alpha-type and constant) and
adiabatic indices on disk structures and how this affects the migration rate of
planets embedded in such disks. We perform 3D numerical simulations of
accretion disks with embedded planets. We use the explicit/implicit
hydrodynamical code NIRVANA that includes full tensor viscosity and radiation
transport in the flux-limited diffusion approximation, as well as a proper
equation of state for molecular hydrogen. The migration of embedded 20Earthmass
planets is studied. Low-viscosity disks have cooler temperatures and the
migration rates of embedded planets tend toward the isothermal limit. In these
disks, planets migrate inwards even in the fully radiative case. The effect of
outward migration can only be sustained if the viscosity in the disk is large.
Overall, the differences between the treatments for the equation of state seem
to play a more important role in disks with higher viscosity. A change in the
adiabatic index and in the viscosity changes the zero-torque radius that
separates inward from outward migration. For larger viscosities, temperatures
in the disk become higher and the zero-torque radius moves to larger radii,
allowing outward migration of a 20 Earth-mass planet to persist over an
extended radial range. In combination with large disk masses, this may allow
for an extended period of the outward migration of growing protoplanetary
cores
Migration of Earth-size planets in 3D radiative discs
In this paper, we address the migration of small mass planets in 3D radiative
disks. Indeed, migration of small planets is known to be too fast inwards in
locally isothermal conditions. However, thermal effects could reverse its
direction, potentially saving planets in the inner, optically thick parts of
the protoplanetary disc. This effect has been seen for masses larger than 5
Earth masses, but the minimum mass for this to happen has never been probed
numerically, although it is of crucial importance for planet formation
scenarios. We have extended the hydro-dynamical code FARGO to 3D, with thermal
diffusion. With this code, we perform simulations of embedded planets down to 2
Earth masses. For a set of discs parameters for which outward migration has
been shown in the range of Earth masses, we find that the transition
to inward migration occurs for masses in the range Earth masses. The
transition appears to be due to an unexpected phenomenon: the formation of an
asymmetric cold and dense finger of gas driven by circulation and libration
streamlines. We recover this phenomenon in 2D simulations where we control the
cooling effects of the gas through a simple modeling of the energy equation.Comment: 17 pages, 20 figures, accepted. MNRAS, 201
The great dichotomy of the Solar System: small terrestrial embryos and massive giant planet cores
The basic structure of the solar system is set by the presence of low-mass
terrestrial planets in its inner part and giant planets in its outer part. This
is the result of the formation of a system of multiple embryos with
approximately the mass of Mars in the inner disk and of a few multi-Earth-mass
cores in the outer disk, within the lifetime of the gaseous component of the
protoplanetary disk. What was the origin of this dichotomy in the mass
distribution of embryos/cores? We show in this paper that the classic processes
of runaway and oligarchic growth from a disk of planetesimals cannot explain
this dichotomy, even if the original surface density of solids increased at the
snowline. Instead, the accretion of drifting pebbles by embryos and cores can
explain the dichotomy, provided that some assumptions hold true. We propose
that the mass-flow of pebbles is two-times lower and the characteristic size of
the pebbles is approximately ten times smaller within the snowline than beyond
the snowline (respectively at heliocentric distance and
, where is the snowline heliocentric distance), due to ice
sublimation and the splitting of icy pebbles into a collection of
chondrule-size silicate grains. In this case, objects of original sub-lunar
mass would grow at drastically different rates in the two regions of the disk.
Within the snowline these bodies would reach approximately the mass of Mars
while beyond the snowline they would grow to Earth masses. The
results may change quantitatively with changes to the assumed parameters, but
the establishment of a clear dichotomy in the mass distribution of protoplanets
appears robust, provided that there is enough turbulence in the disk to prevent
the sedimentation of the silicate grains into a very thin layer.Comment: In press in Icaru
Surface waves in protoplanetary disks induced by outbursts: Concentric rings in scattered light
Context: Vertically hydrostatic protoplanetary disk models are based on the
assumption that the main heating source, stellar irradiation, does not vary
much with time. However, it is known that accreting young stars are variable
sources of radiation. This is particularly evident for outbursting sources such
as EX Lupi and FU Orionis stars. Aim: We investigate how such outbursts affect
the vertical structure of the outer regions of the protoplanetary disk, in
particular their appearance in scattered light at optical and near-infrared
wavelengths. Methods: We employ the 3D FARGOCA radiation-hydrodynamics code, in
polar coordinates, to compute the time-dependent behavior of the axisymmetric
disk structure. The outbursting inner disk region is not included explicitly.
Instead, its luminosity is added to the stellar luminosity and is thus included
in the irradiation of the outer disk regions. For time snapshots of interest we
insert the density structure into the RADMC-3D radiative transfer code and
compute the appearance of the disk at optical/near-infrared wavelengths.
Results: We find that, depending on the amplitude of the outbursts, the
vertical structure of the disk can become highly dynamic, featuring circular
surface waves of considerable amplitude. These "hills" and "valleys" on the
disk's surface show up in the scattered light images as bright and dark
concentric rings. Initially these rings are small and act as standing waves,
but they subsequently lead to outward propagating waves, like the waves
produced by a stone thrown into a pond. These waves continue long after the
actual outburst has died out. Conclusions: We propose that some of the
multi-ringed structures seen in optical/infrared images of several
protoplanetary disks may have their origin in outbursts that occurred decades
or centuries ago.Comment: Accepted for publication in A&A Letter
RISK IN HUMAN RESOURCE MANAGEMENT AND IMPLICATIONS FOR EXTENSION PROGRAMMING - RESULTS OF FOCUS GROUP DISCUSSIONS WITH DAIRY AND GREEN INDUSTRY MANAGERS
Employees are both a source of risk and means of addressing risk, and good employee management practices can increase risk resilience. Forty green industry managers and 22 dairy managers discussed personnel issues related to their industry. Influx of Hispanic labor has changed personnel management and the focus of risk management.Teaching/Communication/Extension/Profession,
Forming super-Mercuries: The role of stellar abundances
Super-Mercuries, rocky exoplanets with bulk iron mass fraction of more than
60 per cent, appear to be preferentially hosted by stars with higher iron mass
fraction than the Earth. It is unclear whether these iron-rich planets can form
in the disc, or if giant impacts are necessary. Here we investigate the
formation of super-Mercuries in their natal protoplanetary discs by taking into
account their host stars' abundances (Fe, Mg, Si, S). We employ a disc
evolution model which includes the growth, drift, evaporation and
recondensation of pebbles to compute the pebble iron mass fraction. The
recondensation of outward-drifting iron vapour near the iron evaporation front
is the key mechanism that facilitates an increase in the pebble iron mass
fraction. We also simulate the growth of planetary seeds around the iron
evaporation front using a planet formation model which includes pebble
accretion and planet migration, and compute the final composition of the
planets. Our simulations are able to reproduce the observed iron compositions
of the super-Mercuries provided that all the iron in the disc are locked in
pure Fe grains and that the disc viscosity is low. The combined effects of slow
orbital migration of planets and long retention time of iron vapour in
low-viscosity discs makes it easier to form iron-rich planets. Furthermore, we
find that decreasing the stellar Mg/Si ratio results in an increase in the iron
mass fraction of the planet due to a reduction in the abundance of Mg2SiO4,
which has a very similar condensation temperature as iron, in the disc. Our
results thus imply that super-Mercuries are more likely to form around stars
with low Mg/Si, in agreement with observational data.Comment: 9 pages, 6 figures, accepted for publication in A&
Meridional circulation of gas into gaps opened by giant planets in three-dimensional low-viscosity disks
We examine the gas circulation near a gap opened by a giant planet in a
protoplanetary disk. We show with high resolution 3D simulations that the gas
flows into the gap at high altitude over the mid-plane, at a rate dependent on
viscosity. We explain this observation with a simple conceptual model. From
this model we derive an estimate of the amount of gas flowing into a gap opened
by a planet with Hill radius comparable to the scale-height of a layered disk
(i. e. a disk with viscous upper layer and inviscid midplane). Our estimate
agrees with modern MRI simulations(Gressel et al., 2013). We conclude that gap
opening in a layered disk can not slow down significantly the runaway gas
accretion of Saturn to Jupiter-mass planets.Comment: in press as a Note in Icaru
Close-in ice lines and the super-stellar C/O ratio in discs around very low-mass stars
The origin of the elevated C/O ratios in discs around late M dwarfs compared
to discs around solar-type stars is not well understood. Here we endeavour to
reproduce the observed differences in the disc C/O ratios as a function of
stellar mass using a viscosity-driven disc evolution model and study the
corresponding atmospheric composition of planets that grow inside the water-ice
line in these discs. We carried out simulations using a coupled disc evolution
and planet formation code that includes pebble drift and evaporation. We used a
chemical partitioning model for the dust composition in the disc midplane.
Inside the water-ice line, the disc's C/O ratio initially decreases to
sub-stellar due to the inward drift and evaporation of water-ice-rich pebbles
before increasing again to super-stellar values due to the inward diffusion of
carbon-rich vapour. We show that this process is more efficient for very
low-mass stars compared to solar-type stars due to the closer-in ice lines and
shorter disc viscous timescales. In high-viscosity discs, the transition from
sub-stellar to super-stellar takes place faster due to the fast inward
advection of carbon-rich gas. Our results suggest that planets accreting their
atmospheres early (when the disc C/O is still sub-stellar) will have low
atmospheric C/O ratios, while planets that accrete their atmospheres late (when
the disc C/O has become super-stellar) can obtain high C/O ratios. Our model
predictions are consistent with observations, under the assumption that all
stars have the same metallicity and chemical composition, and that the vertical
mixing timescales in the inner disc are much shorter than the radial advection
timescales. This further strengthens the case for considering stellar
abundances alongside disc evolution in future studies that aim to link planet
(atmospheric) composition to disc composition.Comment: Accepted for publication in A&
The solar abundance problem and eMSTOs in clusters
We study the impact of accretion from protoplanetary discs on stellar
evolution of AFG-type stars. We use a simplified disc model computed using the
Two-Pop-Py code that contains the growth and drift of dust particles in the
protoplanetary disc. It is used to model the accretion scenarios for a range of
physical conditions of protoplanetary discs. Two limiting cases are combined
with the evolution of stellar convective envelopes computed using the Garstec
stellar evolution code. We find that the accretion of metal-poor (gas) or
metal-rich (dust) material has a significant impact on the chemical composition
of the stellar convective envelope. As a consequence, the evolutionary track of
the star diverts from the standard scenario predicted by canonical stellar
evolution models, which assume a constant and homogeneous chemical composition
after the assembly of the star has finished. In the case of the Sun, we find a
modest impact on the solar chemical composition. Accretion of metal-poor
material indeed reduces the overall metallicity of the solar atmosphere, and it
is consistent, within the uncertainty, with the solar Z reported by Caffau et
al. (2011), but our model is not consistent with the measurement by Asplund et
al. (2009). Another effect is the change of the position of the star in the
colour-magnitude diagram. We compare our predictions to a set of open clusters
from the Gaia DR2 and show that it is possible to produce a scatter close to
the turn-off of young clusters that could contribute to explain the observed
scatter in CMDs. Detailed measurements of metallicities and abundances in the
nearby open clusters will provide a stringent observational test of our
proposed scenario.Comment: 10 pages, 7 figures, 1 table. Accepted for publication in A&
Evolution of inclined planets in three-dimensional radiative discs
While planets in the solar system only have a low inclination with respect to
the ecliptic there is mounting evidence that in extrasolar systems the
inclination can be very high, at least for close-in planets. One process to
alter the inclination of a planet is through planet-disc interactions. Recent
simulations considering radiative transport have shown that the evolution of
migration and eccentricity can strongly depend on the thermodynamic state of
the disc. We extend previous studies to investigate the planet-disc
interactions of fixed and moving planets on inclined and eccentric orbits. We
also analyse the effect of the disc's thermodynamic properties on the orbital
evolution of embedded planets in detail. The protoplanetary disc is modelled as
a viscous gas where the internally produced dissipation is transported by
radiation. For locally isothermal discs, we confirm previous results and find
inclination damping and inward migration for planetary cores. For low
inclinations i < 2 H/r, the damping is exponential, while di/dt is proportional
to i^-2 for larger i. For radiative discs, the planetary migration is very
limited, as long as their inclination exceeds a certain threshold. If the
inclination is damped below this threshold, planetary cores with a mass up to
approximately 33 Earth masses start to migrate outwards, while larger cores
migrate inwards right from the start. The inclination is damped for all
analysed planet masses. In a viscous disc an initial inclination of embedded
planets will be damped for all planet masses. This damping occurs on timescales
that are shorter than the migration time. If the inclination lies beneath a
certain threshold, the outward migration in radiative discs is not handicapped.
Outward migration is strongest for circular and non-inclined orbits
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