21 research outputs found
Modelling Circumbinary Gas Flows in Close T Tauri Binaries
Young close binaries open central gaps in the surrounding circumbinary
accretion disc, but the stellar components may still gain mass from gas
crossing through the gap. It is not well understood how this process operates
and how the stellar components are affected by such inflows. Our main goal is
to investigate how gas accretion takes place and evolves in close T Tauri
binary systems. In particular, we model the accretion flows around two close T
Tauri binaries, V4046 Sgr and DQ Tau, both showing periodic changes in emission
lines, although their orbital characteristics are very different. In order to
derive the density and velocity maps of the circumbinary material, we employ
two-dimensional hydrodynamic simulations with a locally isothermal equation of
state. The flow patterns become quasi-stable after a few orbits in the frame
co-rotating with the system. Gas flows across the circumbinary gap through the
co-rotating Lagrangian points, and local circumstellar discs develop around
both components. Spiral density patterns develop in the circumbinary disc that
transport angular momentum efficiently. Mass is preferentially channelled
towards the primary and its circumstellar disc is more massive than the disc
around the secondary. We also compare the derived density distribution to
observed line profile variability. The line profile variability tracing the gas
flows in the central cavity shows clear similarities with the corresponding
observed line profile variability in V4046 Sgr, but only when the local
circumstellar disc emission was excluded. Closer to the stars normal
magnetospheric accretion may dominate while further out the dynamic accretion
process outlined here dominates. Periodic changes in the accretion rates onto
the stars can explain the outbursts of line emission observed in eccentric
systems such as DQ Tau.Comment: Accepted for publication in MNRA
Giant planet migration during FU Orionis outbursts: 1D disc models
I present the results of semi-analytic calculations of migrating planets in young, outbursting circumstellar discs. Formed far out in the disc via gravitational fragmentation early on in its lifetime, these planets typically migrate at very slow rates and are therefore mostly expected to remain at large radii (such as is the case in HR 8799). I show that changes in the disc structure during FUor outbursts affect the planet’s ability to maintain a gap and can allow a massive giant planet’s semimajor axis to reduce by almost 5 per cent in a single outburst under the most optimistic conditions. Given that a single disc will likely undergo ∼ 10 such outbursts this process can significantly alter the expected radial distribution for GI-formed planets
Modelling Circumbinary Gas Flows in Close T Tauri Binaries
Young close binaries open central gaps in the surrounding circumbinary
accretion disc, but the stellar components may still gain mass from gas
crossing through the gap. It is not well understood how this process operates
and how the stellar components are affected by such inflows. Our main goal is
to investigate how gas accretion takes place and evolves in close T Tauri
binary systems. In particular, we model the accretion flows around two close T
Tauri binaries, V4046 Sgr and DQ Tau, both showing periodic changes in emission
lines, although their orbital characteristics are very different. In order to
derive the density and velocity maps of the circumbinary material, we employ
two-dimensional hydrodynamic simulations with a locally isothermal equation of
state. The flow patterns become quasi-stable after a few orbits in the frame
co-rotating with the system. Gas flows across the circumbinary gap through the
co-rotating Lagrangian points, and local circumstellar discs develop around
both components. Spiral density patterns develop in the circumbinary disc that
transport angular momentum efficiently. Mass is preferentially channelled
towards the primary and its circumstellar disc is more massive than the disc
around the secondary. We also compare the derived density distribution to
observed line profile variability. The line profile variability tracing the gas
flows in the central cavity shows clear similarities with the corresponding
observed line profile variability in V4046 Sgr, but only when the local
circumstellar disc emission was excluded. Closer to the stars normal
magnetospheric accretion may dominate while further out the dynamic accretion
process outlined here dominates. Periodic changes in the accretion rates onto
the stars can explain the outbursts of line emission observed in eccentric
systems such as DQ Tau.Comment: Accepted for publication in MNRA
Planet migration: self-gravitating radiation hydrodynamical models of protoplanets with surfaces
We calculate radial migration rates of protoplanets in laminar minimum mass
solar nebula discs using three-dimensional self-gravitating radiation
hydrodynamical (RHD) models. The protoplanets are free to migrate, whereupon
their migration rates are measured. For low mass protoplanets (10-50 M_\oplus)
we find increases in the migration timescales of up to an order of magnitude
between locally-isothermal and RHD models. In the high-mass regime the
migration rates are changed very little. These results are arrived at by
calculating migration rates in locally-isothermal models, before sequentially
introducing self-gravity, and radiative transfer, allowing us to isolate the
effects of the additional physics. We find that using a locally-isothermal
equation of state, without self-gravity, we reproduce the migration rates
obtained by previous analytic and numerical models. We explore the impact of
different protoplanet models, and changes to their assumed radii, upon
migration. The introduction of self-gravity gives a slight reduction of the
migration rates, whilst the inertial mass problem, which has been proposed for
high mass protoplanets with circumplanetary discs, is reproduced. Upon
introducing radiative transfer to models of low mass protoplanets (\approx 10
M_\oplus), modelled as small radius accreting point masses, we find outward
migration with a rate of approximately twice the analytic inward rate. However,
when modelling such a protoplanet in a more realistic manner, with a surface
which enables the formation of a deep envelope, this outward migration is not
seen.Comment: 21 pages, 21 figure
Comparative statistics and origin of triple and quadruple stars
The statistics of catalogued quadruple stars consisting of two binaries
(hierarchy 2+2) is studied in comparison with triple stars, with respective
sample sizes of 81 and 724. Seven representative quadruple systems are
discussed in greater detail. The properties of multiple stars do not correspond
to the products of dynamical decay of small clusters, hence the N-body dynamics
is not the dominant process of their formation. On the other hand,
rotationally-driven (cascade) fragmentation possibly followed by migration of
inner and/or outer orbits to shorter periods is a promising scenario to explain
the origin of triple and quadruple stars. Our main results are: (i) Quadruple
systems of Epsilon Lyr type with similar masses and inner periods are common.
(ii) The distributions of the inner periods in triple and quadruple stars are
similar and bimodal. The inner mass ratios do not correlate with the inner
periods. (iii) The statistics of outer periods and mass ratios in triples and
quadruples are different. The median outer mass ratio in triples is 0.39
independently of the outer period, which has a smooth distribution. In
contrast, the outer periods of 25% quadruples concentrate in the narrow range
from 10yr to 100yr, the outer mass ratios of these tight quadruples are above
0.6 and their two inner periods are similar to each other. (iv) The outer and
inner mass ratios in triple and quadruple stars are not mutually correlated.
(v) The inner and outer orbital angular momenta and periods in triple and
quadruple systems with inner periods above 30d show some correlation, the ratio
of outer-to-inner periods is mostly comprised between 5 and 10^4. In the
systems with small period ratios the directions of the orbital spins are
correlated, while in the systems with large ratios they are not.Comment: Accepted by MNRAS, 14 pages, 12 figures. Two electronic tables at
http://www.ctio.noao.edu/ftp/pub/tokovinin/quadruples
Numerical simulations of type III planetary migration: III. Outward migration of massive planets
We present a numerical study of rapid, so called type III migration for
Jupiter-sized planets embedded in a protoplanetary disc. We limit ourselves to
the case of outward migration, and study in detail its evolution and physics,
concentrating on the structure of the co-rotation and circumplanetary regions,
and processes for stopping migration. We also consider the dependence of the
migration behaviour on several key parameters. We perform this study using
global, two-dimensional hydrodynamical simulations with adaptive mesh
refinement. We find that the outward directed type III migration can be started
if the initial conditions support , that corresponds to initial value
M_\rmn{\Delta} \ga 1.5. Unlike the inward directed migration, in the outward
migration the migration rate increases due to the growing of the volume of the
co-orbital region. We find the migration to be strongly dependent on the rate
of the mass accumulation in the circumplanetary disc, leading to two possible
regimes of migration, fast and slow. The structure of the co-orbital region and
the stopping mechanism differ between these two regimes.Comment: 18 pages, 13 figures, submitted to MNRA
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
A torque formula for non-isothermal Type I planetary migration - I. Unsaturated horseshoe drag
We study the torque on low-mass planets embedded in protoplanetary discs in
the two-dimensional approximation, incorporating non-isothermal effects. We
couple linear estimates of the Lindblad (or wave) torque to a simple, but
non-linear, model of adiabatic corotation torques (or horseshoe drag),
resulting in a simple formula that governs Type I migration in non-isothermal
discs. This formula should apply in optically thick regions of the disc, where
viscous and thermal diffusion act to keep the horseshoe drag unsaturated. We
check this formula against numerical hydrodynamical simulations, using three
independent numerical methods, and find good agreement.Comment: 17 pages, 17 figures, accepted for publication in MNRA
From Grains to Planetesimals: Les Houches Lecture
This pedagogical review covers an unsolved problem in the theory of
protoplanetary disks: the growth of dust grains into planetesimals, solids at
least a kilometer in size. I summarize timescale constraints imposed on
planetesimal formation by circumstellar disk observations, analysis of
meteorites, and aerodynamic radial migration. The infall of ~meter-sized solids
in a hundred years is the most stringent constraint. I review proposed
mechanisms for planetesimal formation. Collisional coagulation models are
informed by laboratory studies of microgravity collisions. The gravitational
collapse (or Safronov-Goldreich-Ward) hypothesis involves detailed study of the
interaction between solid particles and turbulent gas. I cover the basics of
aerodynamic drag in protoplanetary disks, including radial drift and vertical
sedimentation. I describe various mechanisms for particle concentration in gas
disks -- including turbulent pressure maxima, drag instabilities and long-lived
anticylonic vortices. I derive a general result for the minimum size for a
vortex to trap particles in a sub-Keplerian disk. Recent numerical simulations
demonstrate that particle clumping in turbulent protoplanetary disks can
trigger gravitational collapse. I discuss several outstanding issues in the
field.Comment: 20 pages, 3 figures, to appear in the proceedings of the Les Houches
Winter School "Physics and Astrophysics of Planetary Systems" (EDP Sciences:
EAS Publications Series). Version 2 is the same paper, simply adds above
publisher inf
Two phase, inward-then-outward migration of Jupiter and Saturn in the gaseous Solar Nebula
It has recently been shown that the terrestrial planets and asteroid belt can
be reproduced if the giant planets underwent an inward-then-outward migration
(the "Grand Tack"; Walsh et al 2011). Inward migration occurs when Jupiter
opens a gap and type II migrates inward. The planets "tack" and migrate outward
when Saturn reaches the gap-opening mass and is caught in the 3:2 resonance
with Jupiter. The aim is to test the viability of the Grand Tack model and to
study the dynamical evolution of Jupiter and Saturn during their growth from 10
Earth masses cores. We have performed numerical simulations using a grid-based
hydrodynamical code. Most of our simulations assume an isothermal equation of
state for the disk but a subset use a fully-radiative version of the code. For
an isothermal disk the two phase migration of Jupiter and Saturn is very robust
and independent of the mass-growth history of these planets provided the disk
is cool enough. For a radiative disk the we find some outcomes with two phase
migrations and others with more complicated behavior. We construct a simple,
1-D model of an evolving viscous disk to calculate the evolution of the disk's
radiative properties: the disk transitions from radiative to isothermal from
its outermost regions inward in time. We show that a two-phase migration is a
natural outcome at late times even under the limiting assumption that
isothermal conditions are required. Thus, our simulations provide strong
support for the Grand Tack scenario.Comment: 16 pages, 21 figures, accepted in Astronomy and Astrophysic