150 research outputs found
The evolution of a binary in a retrograde circular orbit embedded in an accretion disk
Supermassive black hole binaries may form as a consequence of galaxy mergers.
Both prograde and retrograde orbits have been proposed. We study a binary of a
small mass ratio, q, in a retrograde orbit immersed in and interacting with a
gaseous accretion disk in order to estimate time scales for inward migration
leading to coalescence and the accretion rate to the secondary component. We
employ both semi-analytic methods and two dimensional numerical simulations,
focusing on the case where the binary mass ratio is small but large enough to
significantly perturb the disk. We develop the theory of type I migration for
this case and determine conditions for gap formation finding that then inward
migration occurs on a time scale equal to the time required for one half of the
secondary mass to be accreted through the unperturbed disk, with accretion onto
the secondary playing only a minor role. The semi-analytic and fully numerical
approaches are in good agreement, the former being applicable over long time
scales. Inward migration induced by interaction with the disk alleviates the
final parsec problem. Accretion onto the secondary does not significantly
affect the orbital evolution, but may have observational consequences for high
accretion efficiency. The binary may then appear as two sources of radiation
rotating around each other. This study should be extended to consider orbits
with significant eccentricity and the effects of gravitational radiation at
small length scales. Note too that torques acting between a circumbinary disk
and a retrograde binary orbit may cause the mutual inclination to increase on a
timescale that can be similar to, or smaller than that for orbital evolution,
depending on detailed parameters. This is also an aspect for future study
(abridged).Comment: 24 pages, 18 figures, accepted for publication in A&A. For movies of
the simulations see
http://astro.qmul.ac.uk/people/sijme-jan-paardekooper/publication
On type-I migration near opacity transitions. A generalized Lindblad torque formula for planetary population synthesis
We give an expression for the Lindblad torque acting on a low-mass planet
embedded in a protoplanetary disk that is valid even at locations where the
surface density or temperature profile cannot be approximated by a power law,
such as an opacity transition. At such locations, the Lindblad torque is known
to suffer strong deviation from its standard value, with potentially important
implications for type I migration, but the full treatment of the tidal
interaction is cumbersome and not well suited to models of planetary population
synthesis. The expression that we propose retains the simplicity of the
standard Lindblad torque formula and gives results that accurately reproduce
those of numerical simulations, even at locations where the disk temperature
undergoes abrupt changes. Our study is conducted by means of customized
numerical simulations in the low-mass regime, in locally isothermal disks, and
compared to linear torque estimates obtained by summing fully analytic torque
estimates at each Lindblad resonance. The functional dependence of our modified
Lindblad torque expression is suggested by an estimate of the shift of the
Lindblad resonances that mostly contribute to the torque, in a disk with sharp
gradients of temperature or surface density, while the numerical coefficients
of the new terms are adjusted to seek agreement with numerics. As side results,
we find that the vortensity related corotation torque undergoes a boost at an
opacity transition that can counteract migration, and we find evidence from
numerical simulations that the linear corotation torque has a non-negligible
dependency upon the temperature gradient, in a locally isothermal disk.Comment: Appeared in special issue of "Celestial Mechanics and Dynamical
Astronomy" on Extrasolar Planetary System
Low-mass planet migration in three-dimensional wind-driven inviscid discs: a negative corotation torque
We present simulations of low-mass planet–disc interactions in inviscid three-dimensional discs. We show that a wind-driven laminar accretion flow through the surface layers of the disc does not significantly modify the migration torque experienced by embedded planets. More importantly, we find that 3D effects lead to a dramatic change in the behaviour of the dynamical corotation torque compared to earlier 2D theory and simulations. Although it was previously shown that the dynamical corotation torque could act to slow and essentially stall the inward migration of a low-mass planet, our results in 3D show that the dynamical corotation torque has the complete opposite effect and speeds up inward migration. Our numerical experiments implicate buoyancy resonances as the cause. These have two effects: (i) they exert a direct torque on the planet, whose magnitude relative to the Lindblad torque is measured in our simulations to be small; (ii) they torque the gas librating on horseshoe orbits in the corotation region and drive evolution of its vortensity, leading to the negative dynamical corotation torque. This indicates that at low turbulent viscosity, the detailed vertical thermal structure of the protoplanetary disc plays an important role in determining the migration behaviour of embedded planets. If this result holds up under a more refined treatment of disc thermal evolution, then it has important implications for understanding the formation and early evolution of planetary systems
Modelling circumbinary protoplanetary disks II. Gas disk feedback on planetesimal dynamical and collisional evolution in the circumbinary systems Kepler-16 and 34
Aims. We investigate the feasibility of planetesimal growth in circumbinary
protoplanetary disks around the observed systems Kepler- 16 and Kepler-34 under
the gravitational influence of a precessing eccentric gas disk. Methods. We
embed the results of our previous hydrodynamical simulations of protoplanetary
disks around binaries into an N-body code to perform 3D, high-resolution,
inter-particle gravity-enabled simulations of planetesimal growth and dynamics
that include the gravitational force imparted by the gas. Results. Including
the full, precessing asymmetric gas disk generates high eccentricity orbits for
planetesimals orbiting at the edge of the circumbinary cavity, where the gas
surface density and eccentricity have their largest values. The gas disk is
able to efficiently align planetesimal pericenters in some regions leading to
phased, non-interacting orbits. Outside of these areas eccentric planetesimal
orbits become misaligned and overlap leading to crossing orbits and high
relative velocities during planetesimal collisions. This can lead to an
increase in the number of erosive collisions that far outweighs the number of
collisions that result in growth. Gravitational focusing from the static
axisymmetric gas disk is weak and does not significantly alter collision
outcomes from the gas free case. Conclusions. Due to asymmetries in the gas
disk, planetesimals are strongly perturbed onto highly eccentric orbits. Where
planetesimals orbits are not well aligned, orbit crossings lead to an increase
in the number of erosive collisions. This makes it difficult for sustained
planetesimal accretion to occur at the location of Kepler-16b and Kepler-34b
and we therefore rule out in-situ growth. This adds further support to our
initial suggestions that most circumbinary planets should form further out in
the disk and migrate inwards.Comment: 12 pages and 12 figure
Porosity measurements of interstellar ice mixtures using optical laser interference and extended effective medium approximations
Aims. This article aims to provide an alternative method of measuring the
porosity of multi-phase composite ices from their refractive indices and of
characterising how the abundance of a premixed contaminant (e.g., CO2) affects
the porosity of water-rich ice mixtures during omni-directional deposition.
Methods. We combine optical laser interference and extended effective medium
approximations (EMAs) to measure the porosity of three astrophysically relevant
ice mixtures: H2O:CO2=10:1, 4:1, and 2:1. Infrared spectroscopy is used as a
benchmarking test of this new laboratory-based method. Results. By
independently monitoring the O-H dangling modes of the different water-rich ice
mixtures, we confirm the porosities predicted by the extended EMAs. We also
demonstrate that CO2 premixed with water in the gas phase does not
significantly affect the ice morphology during omni-directional deposition, as
long as the physical conditions favourable to segregation are not reached. We
propose a mechanism in which CO2 molecules diffuse on the surface of the
growing ice sample prior to being incorporated into the bulk and then fill the
pores partly or completely, depending on the relative abundance and the growth
temperature.Comment: 9 pages, 6 figures, 1 table. Accepted for publication in A&
Debris discs in binaries: a numerical study
Debris disc analysis and modelling provide crucial information about the
structure and the processes at play in extrasolar planetary systems. In binary
systems, this issue is more complex because the disc should in addition respond
to the companion star's perturbations. We explore the dynamical evolution of a
collisionally active debris disc for different initial parent body populations,
diverse binary configurations and optical depths. We focus on the radial extent
and size distribution of the disc at a stationary state. We numerically follow
the evolution of massless small grains, initially produced from a
circumprimary disc of parent bodies following a size distribution in ds . Grains are submitted to both stars' gravity as well as
radiation pressure. In addition, particles are assigned an empirically derived
collisional lifetime. For all the binary configurations the disc extends far
beyond the critical semimajor axis for orbital stability. This is due
to the steady production of small grains, placed on eccentric orbits reaching
beyond by radiation pressure. The amount of matter beyond acrit
depends on the balance between collisional production and dynamical removal
rates: it increases for more massive discs as well as for eccentric binaries.
Another important effect is that, in the dynamically stable region, the disc is
depleted from its smallest grains. Both results could lead to observable
signatures. We have shown that a companion star can never fully truncate a
collisionally active disc. For eccentric companions, grains in the unstable
regions can significantly contribute to the thermal emission in the mid-IR.
Discs with sharp outer edges, especially bright ones such as HR4796A, are
probably shaped by other mechanisms.Comment: accepted for publication in A&
Recent developments in planet migration theory
Planetary migration is the process by which a forming planet undergoes a
drift of its semi-major axis caused by the tidal interaction with its parent
protoplanetary disc. One of the key quantities to assess the migration of
embedded planets is the tidal torque between the disc and planet, which has two
components: the Lindblad torque and the corotation torque. We review the latest
results on both torque components for planets on circular orbits, with a
special emphasis on the various processes that give rise to additional, large
components of the corotation torque, and those contributing to the saturation
of this torque. These additional components of the corotation torque could help
address the shortcomings that have recently been exposed by models of planet
population syntheses. We also review recent results concerning the migration of
giant planets that carve gaps in the disc (type II migration) and the migration
of sub-giant planets that open partial gaps in massive discs (type III
migration).Comment: 52 pages, 18 figures. Review article to be published in "Tidal
effects in Astronomy and Astrophysics", Lecture Notes in Physic
Migrating super-Earths in low-viscosity discs: unveiling the roles of feedback, vortices, and laminar accretion flows
We present the highest resolution study to date of super-Earths migrating in
inviscid and low-viscosity discs, motivated by the connection to laminar,
wind-driven models of protoplanetary discs. Our models unveil the critical role
of vortices in determining the migration behaviour for partial gap-opening
planets. Vortices form in pressure maxima at gap edges, and prevent the
disc-feedback stopping of migration for intermediate planets in low-viscosity
and inviscid discs, contrary to the concept of the `inertial limit' or `disc
feedback' halting predicted from analytical models. Vortices may also form in
the corotation region, and can dramatically modify migration behaviour through
direct gravitational interaction with the planet. These features become
apparent at high resolution, and for all but the highest viscosities there
exist significant difficulties in obtaining numerically converged results. The
migration of partial gap-opening planets, however, clearly becomes chaotic for
sufficiently low viscosities. At moderate viscosity, a smooth disc-feedback
regime is found in which migration can slow substantially, and the migration
time-scale observed corresponds to migration being driven by diffusive
relaxation of the gap edges. At high viscosity classical Type I migration is
recovered. For Jupiter-analogue planets in inviscid discs, a wide, deep gap is
formed. Transient Type II migration occurs over radial length-scales
corresponding to the gap width, beyond which migration can stall. Finally, we
examine the particle trapping driven by structures left in inviscid discs by a
migrating planet, and find that particle traps in the form of multiple rings
and vortices can persist long after the planet has passed. In this case, the
observation of particle traps by submillimetre interferometers such as ALMA
cannot be used to infer the current presence of an adjacent planet.Comment: 24 pages, 22 figures, MNRAS accepte
- …