21 research outputs found
Formation of Giant Planets by Concurrent Accretion of Solids and Gas inside an Anti-Cyclonic Vortex
We study the formation of a giant gas planet by the core--accretion
gas--capture process, with numerical simulations, under the assumption that the
planetary core forms in the center of an anti-cyclonic vortex. The presence of
the vortex concentrates particles of centimeter to meter size from the
surrounding disk, and speeds up the core formation process. Assuming that a
planet of Jupiter mass is forming at 5 AU from the star, the vortex enhancement
results in considerably shorter formation times than are found in standard
core--accretion gas--capture simulations. Also, formation of a gas giant is
possible in a disk with mass comparable to that of the minimum mass solar
nebula.Comment: 27 pages, 4 figures, ApJ in pres
Tracing large-scale structures in circumstellar disks with ALMA
Planets are supposed to form in circumstellar disks. The gravitational
potential of a planet perturbs the disk and leads to characteristic structures,
i.e. spiral waves and gaps, in the disk's density profile. We perform a
large-scale parameter study of the observability of these planet-induced
structures in circumstellar disks with ALMA. On the basis of HD and MHD
simulations, we calculated the disk temperature structure and (sub)mm images of
these systems. These were used to derive simulated ALMA images. Because
appropriate objects are frequent in Taurus, we focused on a distance of 140pc
and a declination of 20{\deg}. The explored range of star-disk-planet
configurations consists of 6 HD simulations (including magnetic fields and
different planet masses), 9 disk sizes, 15 total disk masses, 6 different
central stars, and two different grain size distributions. On almost all scales
and in particular down to a scale of a few AU, ALMA is able to trace disk
structures induced by planet-disk interaction or by the influence of magnetic
fields on the wavelength range between 0.4 and 2.0mm. In most cases, the
optimum angular resolution is limited by the sensitivity. However, within the
range of typical masses of protoplanetary disks (0.1-0.001Msun) the disk mass
has a minor impact on the observability. It is possible to resolve disks down
to 2.67e-6Msun and trace gaps induced by a planet with M_p/M_s = 0.001 in disks
with 2.67e-4Msun with a signal-to-noise ratio greater than three. The central
star has a major impact on the observability of gaps, as well as the considered
maximum grainsize of the dust in the disk. In general, it is more likely to
trace planet-induced gaps in our MHD models, because gaps are wider in the
presence of magnetic fields. We also find that zonal flows resulting from MRI
create gap-like structures in the disk's re-emission radiation, which are
observable with ALMA.Comment: 17 pages, 21 figure
Planet-induced disk structures: A comparison between (sub)mm and infrared radiation
Young giant planets, which are embedded in a circumstellar disk, will
significantly perturb the disk density distribution. This effect can
potentially be used as an indirect tracer for planets. We investigate the
feasibility of observing planet-induced gaps in circumstellar disks in
scattered light. We perform 3D hydrodynamical disk simulations combined with
subsequent radiative transfer calculations in scattered light for different
star, disk, and planet configurations. The results are compared to those of a
corresponding study for the (sub)mm thermal re-emission. The feasibility of
detecting planet-induced gaps in scattered light is mainly influenced by the
optical depth of the disk and therefore by the disk size and mass.
Planet-induced gaps are in general only detectable if the photosphere of the
disks is sufficiently disturbed. Within the limitations given by the parameter
space here considered, we find that gap detection is possible in the case of
disks with masses below . Compared to the
disk mass that marks the lower Atacama Large (Sub)Millimeter Array (ALMA)
detection limit for the thermal radiation re-emitted by the disk, it is
possible to detect the same gap both in re-emission and scattered light only in
a narrow range of disk masses around ,
corresponding to of cases considered in our study.Comment: 4 pages, 6 figure
Dust sedimentation and self-sustained Kelvin-Helmholtz turbulence in protoplanetary disk mid-planes. I. Radially symmetric simulations
We perform numerical simulations of the Kelvin-Helmholtz instability in the
mid-plane of a protoplanetary disk. A two-dimensional corotating slice in the
azimuthal--vertical plane of the disk is considered where we include the
Coriolis force and the radial advection of the Keplerian rotation flow. Dust
grains, treated as individual particles, move under the influence of friction
with the gas, while the gas is treated as a compressible fluid. The friction
force from the dust grains on the gas leads to a vertical shear in the gas
rotation velocity. As the particles settle around the mid-plane due to gravity,
the shear increases, and eventually the flow becomes unstable to the
Kelvin-Helmholtz instability. The Kelvin-Helmholtz turbulence saturates when
the vertical settling of the dust is balanced by the turbulent diffusion away
from the mid-plane. The azimuthally averaged state of the self-sustained
Kelvin-Helmholtz turbulence is found to have a constant Richardson number in
the region around the mid-plane where the dust-to-gas ratio is significant.
Nevertheless the dust density has a strong non-axisymmetric component. We
identify a powerful clumping mechanism, caused by the dependence of the
rotation velocity of the dust grains on the dust-to-gas ratio, as the source of
the non-axisymmetry. Our simulations confirm recent findings that the critical
Richardson number for Kelvin-Helmholtz instability is around unity or larger,
rather than the classical value of 1/4Comment: Accepted for publication in ApJ. Some minor changes due to referee
report, most notably that the clumping mechanism has been identified as the
streaming instability of Youdin & Goodman (2005). Movies of the simulations
are still available at http://www.mpia.de/homes/johansen/research_en.ph
Dust diffusion in protoplanetary discs by magnetorotational turbulence
We measure the turbulent diffusion coefficient of dust grains embedded in
magnetorotational turbulence in a protoplanetary disc directly from numerical
simulations and compare it to the turbulent viscosity of the flow. The
simulations are done in a local coordinate frame comoving with the gas in
Keplerian rotation. Periodic boundary conditions are used in all directions,
and vertical gravity is not applied to the gas. Using a two-fluid approach,
small dust grains of various sizes (with friction times up to ) are allowed to move under the influence of friction with
the turbulent gas. We measure the turbulent diffusion coefficient of the dust
grains by applying an external sinusoidal force field acting in the vertical
direction on the dust component only. This concentrates the dust around the
mid-plane of the disc, and an equilibrium distribution of the dust density is
achieved when the vertical settling is counteracted by the turbulent diffusion
away from the mid-plane. Comparing with analytical expressions for the
equilibrium concentration we deduce the vertical turbulent diffusion
coefficient. The vertical diffusion coefficient is found to be lower than the
turbulent viscosity and to have an associated vertical diffusion Prandtl number
of about 1.5. A similar radial force field also allows us to measure the radial
turbulent diffusion coefficient. We find a radial diffusion Prandtl number of
about 0.85 and also find that the radial turbulent diffusion coefficient is
around 70% higher than the vertical. We also find evidence for trapping of dust
grains of intermediate friction time in turbulent eddies.Comment: Accepted for publication in ApJ. An additional MPEG movie can be
downloaded at http://www.mpia.de/homes/johansen
Rapid planetesimal formation in turbulent circumstellar discs
The initial stages of planet formation in circumstellar gas discs proceed via
dust grains that collide and build up larger and larger bodies (Safronov 1969).
How this process continues from metre-sized boulders to kilometre-scale
planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick
together poorly (Benz 2000), and spiral into the protostar in a few hundred
orbits due to a head wind from the slower rotating gas (Weidenschilling 1977).
Gravitational collapse of the solid component has been suggested to overcome
this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even
low levels of turbulence, however, inhibit sedimentation of solids to a
sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al.
2007), but turbulence must be present to explain observed gas accretion in
protostellar discs (Hartmann 1998). Here we report the discovery of efficient
gravitational collapse of boulders in locally overdense regions in the
midplane. The boulders concentrate initially in transient high pressures in the
turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are
augmented a further order of magnitude by a streaming instability (Youdin &
Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven
by the relative flow of gas and solids. We find that gravitationally bound
clusters form with masses comparable to dwarf planets and containing a
distribution of boulder sizes. Gravitational collapse happens much faster than
radial drift, offering a possible path to planetesimal formation in accreting
circumstellar discs.Comment: To appear in Nature (30 August 2007 issue). 18 pages (in referee
mode), 3 figures. Supplementary Information can be found at 0708.389
Tracing Planets in Circumstellar Discs
Planets are assumed to form in circumstellar discs around young stellar objects. The additional gravitational potential of a planet perturbs the disc and leads to characteristic structures, i.e. spiral waves and gaps, in the disc density profile. We perform a large-scale parameter study on the observability of these planet-induced structures in circumstellar discs in the (sub)mm wavelength range for the Atacama Large (Sub)Millimeter Array (ALMA). On the basis of hydrodynamical and magneto-hydrodynamical simulations of star-disc-planet models we calculate the disc temperature structure and (sub)mm images of these systems. These are used to derive simulated ALMA maps. Because appropriate objects are frequent in the Taurus-Auriga region, we focus on a distance of 140 pc and a declination of ≈ 20°. The explored range of star-disc-planet configurations consists of six hydrodynamical simulations (including magnetic fields and different planet masses), nine disc sizes with outer radii ranging from 9 AU to 225 AU, 15 total disc masses in the range between 2.67·10-7 M⊙ and 4.10·10-2 M⊙, six different central stars and two different grain size distributions, resulting in 10 000 disc models. At almost all scales and in particular down to a scale of a few AU, ALMA is able to trace disc structures induced by planet-disc interaction or the influence of magnetic fields in the wavelength range between 0.4...2.0 mm. In most cases, the optimum angular resolution is limited by the sensitivity of ALMA. However, within the range of typical masses of protoplane tary discs (0.1 M⊙...0.001 M⊙) the disc mass has a minor impact on the observability. At the distance of 140 pc it is possible to resolve discs down to 2.67·10-6 M⊙ and trace gaps in discs with 2.67·10-4 M⊙ with a signal-to-noise ratio greater than three. In general, it is more likely to trace planet-induced gaps in magneto-hydrodynamical disc models, because gaps are wider in the presence of magnetic fields [1]. We also find, that zonal flows resulting from magneto-rotational instability (MRI) create gap-like structures in the disc re-emission radiation which are observable with ALMA. Through the unprecedented resolution and sensitivity of ALMA in the (sub)mm wavelength range the expected detailed observations of planet-disc interaction and global disc structures will deepen our understanding of the planet formation and disc evolution process. This article presents a summary of the study published by [2]
Tracing Planets in Circumstellar Discs
Planets are assumed to form in circumstellar discs around young stellar objects. The
additional gravitational potential of a planet perturbs the disc and leads to
characteristic structures, i.e. spiral waves and gaps, in the disc density profile. We
perform a large-scale parameter study on the observability of these planet-induced
structures in circumstellar discs in the (sub)mm wavelength range for the Atacama Large
(Sub)Millimeter Array (ALMA). On the basis of hydrodynamical and magneto-hydrodynamical
simulations of star-disc-planet models we calculate the disc temperature structure and
(sub)mm images of these systems. These are used to derive simulated ALMA maps. Because
appropriate objects are frequent in the Taurus-Auriga region, we focus on a distance of
140 pc and a declination of ≈ 20°. The explored range of star-disc-planet configurations
consists of six hydrodynamical simulations (including magnetic fields and different planet
masses), nine disc sizes with outer radii ranging from 9 AU to 225 AU, 15 total disc
masses in the range between 2.67·10-7 M⊙ and 4.10·10-2 M⊙, six different central stars
and two different grain size distributions, resulting in 10 000 disc models. At almost all
scales and in particular down to a scale of a few AU, ALMA is able to trace disc
structures induced by planet-disc interaction or the influence of magnetic fields in the
wavelength range between 0.4...2.0 mm. In most cases, the optimum angular resolution
is limited by the sensitivity of ALMA. However, within the range of typical masses of
protoplane tary discs (0.1 M⊙...0.001 M⊙) the disc mass has a minor impact on the
observability. At the distance of 140 pc it is possible to resolve discs down to 2.67·10-6 M⊙ and trace gaps in discs with 2.67·10-4 M⊙ with a signal-to-noise ratio greater
than three. In general, it is more likely to trace planet-induced gaps in
magneto-hydrodynamical disc models, because gaps are wider in the presence of magnetic
fields [1]. We also find, that zonal flows resulting from magneto-rotational instability
(MRI) create gap-like structures in the disc re-emission radiation which are observable
with ALMA. Through the unprecedented resolution and sensitivity of ALMA in the (sub)mm
wavelength range the expected detailed observations of planet-disc interaction and global
disc structures will deepen our understanding of the planet formation and disc evolution
process. This article presents a summary of the study published by [2]