53 research outputs found
On the energy dissipation rate at the inner edge of circumbinary discs
We study, by means of numerical simulations and analysis, the details of the
accretion process from a disc onto a binary system. We show that energy is
dissipated at the edge of a circumbinary disc and this is associated with the
tidal torque that maintains the cavity: angular momentum is transferred from
the binary to the disc through the action of compressional shocks and viscous
friction. These shocks can be viewed as being produced by fluid elements which
drift into the cavity and, before being accreted, are accelerated onto
trajectories that send them back to impact the disc. The rate of energy
dissipation is approximately equal to the product of potential energy per unit
mass at the disc's inner edge and the accretion rate, estimated from the disc
parameters just beyond the cavity edge, that would occur without the binary.
For very thin discs, the actual accretion rate onto the binary may be
significantly less. We calculate the energy emitted by a circumbinary disc
taking into account energy dissipation at the inner edge and also irradiation
arising there from reprocessing of light from the stars. We find that, for
tight PMS binaries, the SED is dominated by emission from the inner edge at
wavelengths between 1-4 and 10 m. This may apply to systems like CoRoT
223992193 and V1481 Ori.This is the final version of the article. It first appeared from Oxford University Press via https://doi.org/10.1093/mnras/stw248
On the orbital evolution of a pair of giant planets in mean motion resonance
Pairs of extrasolar giant planets in a mean motion commensurability are
common with 2:1 resonance occurring most frequently. Disc-planet interaction
provides a mechanism for their origin. However, the time scale on which this
could operate in particular cases is unclear. We perform 2D and 3D numerical
simulations of pairs of giant planets in a protoplanetary disc as they form and
maintain a mean motion commensurability. We consider systems with current
parameters similar to those of HD 155358, 24 Sextantis and HD 60532, and disc
models of varying mass, decreasing mass corresponding to increasing age. For
the lowest mass discs, systems with planets in the Jovian mass range migrate
inwards maintaining a 2:1 commensurability. Systems with the inner planet
currently at around 1 au from the central star could have originated at a few
au and migrated inwards on a time scale comparable to protoplanetary disc
lifetimes. Systems of larger mass planets such as HD 60532 attain 3:1 resonance
as observed. For a given mass accretion rate, results are insensitive to the
disc model for the range of viscosity prescriptions adopted, there being good
agreement between 2D and 3D simulations. However, in a higher mass disc a pair
of Jovian mass planets passes through 2:1 resonance before attaining a
temporary phase lasting a few thousand orbits in an unstable 5:3 resonance
prior to undergoing a scattering. Thus finding systems in this commensurability
is unlikely.ENS CachanThis is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw157
Consequences of tidal interaction between disks and orbiting protoplanets for the evolution of multi-planet systems with architecture resembling that of Kepler 444
We study orbital evolution of multi-planet systems with masses in the
terrestrial planet regime induced through tidal interaction with a
protoplanetary disk assuming that this is the dominant mechanism for producing
orbital migration and circularization. We develop a simple analytic model for a
system that maintains consecutive pairs in resonance while undergoing orbital
circularization and migration. Migration times for each planet may be estimated
once planet masses, circularization times and the migration time for the
innermost planet are given. We applied it to a model system with the current
architecture of Kepler 444 interacting with a protoplanetary disk, the
evolution time for the system as a whole being comparable to current
protoplanetary disk lifetimes.
In addition we performed numerical simulations with input data obtained from
this model. These indicate that although the analytic model is inexact,
relatively small corrections to estimated migration rates yield systems for
which period ratios vary by a minimal extent. Because of relatively large
deviations from exact resonance in the observed system of up to the
migration times obtained in this way indicate only weak convergent migration
such that a system for which the planets did not interact would contract by
only although undergoing significant inward migration as a whole. We
performed additional simulations to investigate how the system could undergo
significant convergent migration before reaching its final state. These
indicate migration times have to be significantly shorter and resonances
significantly closer. Relative migration rates would then have to decrease
allowing period ratios to increase to become more distant from resonances as
the system approached its final state in the inner regions of the
protoplanetary disk (abridged).This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by Springer
On the formation of a quasi-stationary twisted disc after a tidal disruption event
We investigate misaligned accretion discs formed after tidal disruption
events that occur when a star encounters a supermassive black hole. We employ
the linear theory of warped accretion discs to find the shape of a disc for
which the stream arising from the disrupted star provides a source of angular
momentum that is misaligned with that of the black hole. For quasi-steady
configurations we find that when the warp diffusion or propagation time is
large compared to the local mass accretion time and/or the natural disc
alignment radius is small, misalignment is favoured. These results have been
verified using SPH simulations. We also simulated 1D model discs including gas
and radiation pressure. As accretion rates initially exceed the Eddington limit
the disc is initially advection dominated. Assuming the model for the
disc, where it can be thermally unstable it subsequently undergoes cyclic
transitions between high and low states. During these transitions the aspect
ratio varies from to which is reflected in changes in
the degree of disc misalignment at the stream impact location. For maximal
black hole rotation and sufficiently large values of viscosity parameter
the ratio of the disc inclination to that of the
initial stellar orbit is estimated to be in the advection dominated
state, while reaching of order unity in the low state. Misalignment descreases
with decrease of , but increases as the black hole rotation parameter
decreases. Thus, it is always significant when the latter is small.MXG acknowledges support through Leopoldina fellowship programme (fellowship number LPDS 2009-50). Simulations were performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing Service, provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council. MXG also acknowledges the computing time granted (NIC project number 8163) on the supercomputer JUROPA at Jülich Supercomputing Centre (JSC). PBI was supported in part by RFBR grants 15-02-08476 and 16-02-01043 and also by Grant of the President of the Russian Federation for Support of the Leading Scientific Schools NSh-6595.2016.2.This is the final version of the article. It first appeared from Oxford University Pressvia https://doi.org/10.1093/mnras/stw213
Dynamical tides in exoplanetary systems containing hot Jupiters: Confronting theory and observations
We study the effect of dynamical tides associated with the excitation of gravity waves in an interior radiative region of the central star on orbital evolution in observed systems containing hot Jupiters. We consider WASP-43, OGLE-TR-113, WASP-12 and WASP-18 that contain stars on the main sequence (MS). For these systems there are observational estimates regarding the rate of change of the orbital period.We also investigate Kepler-91 that contains an evolved giant star.We adopt the formalism of Ivanov et al. for calculating the orbital evolution. For the MS stars we determine expected rates of orbital evolution under different assumptions about the amount of dissipation acting on the tides, estimate the effect of stellar rotation for the two most rapidly rotating stars and compare results with observations. All cases apart from possibly WASP-43 are consistent with a regime in which gravity waves are damped during their propagation over the star. However, at present this is not definitive as observational errors are large. We find that although it is expected to apply to Kepler-91, linear radiative damping cannot explain this dissipation regime applying to MS stars. Thus, a non-linear mechanism may be needed. Kepler-91 is found to be such that the time-scale for evolution of the star is comparable to that for the orbit. This implies that significant orbital circularization may have occurred through tides acting on the star. Quasi-static tides, stellar winds, hydrodynamic drag and tides acting on the planet have likely played a minor role.We are grateful to G. I. Ogilvie for his important remarks and suggestions. SVC and PBI were supported in part by RFBR grants 15-02-08476 and 16-02-01043, by programme 7 of the Presidium of Russian Academy of Sciences and also by Grant of the President of the Russian Federation for Support of the Leading Scientific Schools NSh-6595.2016.2
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Torsional Alfvén resonances as an efficient damping mechanism for non-radial oscillations in red giant stars
Stars are self-gravitating fluids in which pressure, buoyancy, rotation and magnetic fields provide the restoring forces for global modes of oscillation. Pressure and buoyancy energetically dominate, while rotation and magnetism are generally assumed to be weak perturbations and often ignored. However, observations of anomalously weak dipole mode amplitudes in red giant stars suggest that a substantial fraction of these are subject to an additional source of damping localized to their core region, with indirect evidence pointing to the role of a deeply buried magnetic field. It is also known that in many instances, the gravity-mode character of affected modes is preserved, but so far, no effective damping mechanism has been proposed that accommodates this aspect. Here we present such a mechanism, which damps the oscillations of stars harbouring magnetised cores via resonant interactions with standing Alfvén modes of high harmonic index. The damping rates produced by this mechanism are quantitatively on par with those associated with turbulent convection, and in the range required to explain observations, for realistic stellar models and magnetic field strengths. Our results suggest that magnetic fields can provide an efficient means of damping stellar oscillations without needing to disrupt the internal structure of the modes, and lay the groundwork for an extension of the theory of global stellar oscillations that incorporates these effects
The evolution of a supermassive retrograde binary embedded in an accretion disk
In this note we discuss the main results of a study of a massive binary with
unequal mass ratio, q, embedded in an accretion disk, with its orbital rotation
being opposed to that of the disk. When the mass ratio is sufficiently large, a
gap opens in the disk, but the mechanism of gap formation is very different
from the prograde case. Inward migration occurs on a timescale of t_ev ~
M_p/(dot M), where M_p is the mass of the less massive component (the
perturber), and dot M is the accretion rate. When q<< 1, the accretion takes
place mostly onto the more massive component, with the accretion rate onto the
perturber being smaller than, or of order of, q^(1/3)M. However, this rate
increases when supermassive binary black holes are considered and gravitational
wave emission is important. We estimate a typical duration of time for which
the accretion onto the perturber and gravitational waves could be detected
Black Hole Models of Quasars
Observations of active galactic nuclei are interpreted in terms of a theoretical model involving accretion onto a massive black hole. Optical quasars and Seyfert galaxies are associated with holes accreting near the Eddington rate and radio galaxies with sub-critical accretion. It is argued that magnetic fields are largely responsible for extracting energy and angular momentum from black holes and disks. Recent studies of electron-positron pair plasmas and their possible role in establishing the emergent X-ray spectrum are reviewed. The main evolutionary properties of active galactic nuclei can be interpreted in terms of a simple model in which black holes accrete gas at a rate dictated by the rate of gas supply which decreases with cosmic time. It may be worth searching for eclipsing binary black holes in lower power Seyferts
Enhanced Angular Momentum Transport in Accretion Disks
The status of our current understanding of angular momentum transport in
accretion disks is reviewed. The last decade has seen a dramatic increase both
in the recognition of key physical processes and in our ability to carry
through direct numerical simulations of turbulent flow. Magnetic fields have at
once powerful and subtle influences on the behavior of (sufficiently) ionized
gas, rendering them directly unstable to free energy gradients. Outwardly
decreasing angular velocity profiles are unstable. The breakdown of Keplerian
rotation into MHD turbulence may be studied in some numerical detail, and key
transport coefficients may be evaluated. Chandra observations of the Galactic
Center support the existence of low luminosity accretion, which may ultimately
prove amenable to global three-dimensional numerical simulation.Comment: 43 pages, 2 figures, to appear v.43 A.R.A.A. October 200
Theory of disk accretion onto supermassive black holes
Accretion onto supermassive black holes produces both the dramatic phenomena
associated with active galactic nuclei and the underwhelming displays seen in
the Galactic Center and most other nearby galaxies. I review selected aspects
of the current theoretical understanding of black hole accretion, emphasizing
the role of magnetohydrodynamic turbulence and gravitational instabilities in
driving the actual accretion and the importance of the efficacy of cooling in
determining the structure and observational appearance of the accretion flow.
Ongoing investigations into the dynamics of the plunging region, the origin of
variability in the accretion process, and the evolution of warped, twisted, or
eccentric disks are summarized.Comment: Mostly introductory review, to appear in "Supermassive black holes in
the distant Universe", ed. A.J. Barger, Kluwer Academic Publishers, in pres
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