586 research outputs found
Accretion disc dynamics in extreme mass ratio compact binaries
An analysis is presented of a numerical investigation of the dynamics and
geometry of accretion discs in binary systems with mass ratios q < 0.1,
applicable to ultra-compact X-ray binaries, AM CVn stars and very short period
cataclysmic variables. The steady-state geometry of the disc in the binary
reference frame is found to be quite different from that expected at higher
mass ratios. For q ~ 0.1, the disc takes on the usual elliptical shape, with
the major axis aligned perpendicular to the line of centres of the two stars.
However, at smaller mass ratios the elliptical gaseous orbits in the outer
regions of the disc are rotated in the binary plane. The angle of rotation
increases with gas temperature, but is found to vary inversely with q. At q =
0.01, the major axis of these orbits is aligned almost parallel to the line of
centres of the two stars. These effects may be responsible for the similar disc
structure inferred from Doppler tomography of the AM CVn star GP Com
(Morales-Rueda et al. 2003), which has q = 0.02. The steady-state geometry at
low mass ratios is not predicted by an inviscid, restricted three-body model of
gaseous orbits; it is related to the effects of tidal-viscous truncation of the
disc near the Roche lobe boundary. Since the disc geometry can be inferred
observationally for some systems, it is proposed that this may offer a useful
diagnostic for the determination of mass ratios in ultra-compact binaries.Comment: 17 pages, 9 figures, 7 in colour. Accepted for publication in MNRAS.
Plain article formatting to get round arXiv problems with mn2e.st
Large Scale B-Field in Stationary Accretion Disks
We reconsider the problem of the formation of a large-scale magnetic field in
the accretion disks around black holes. In contrast with previous work we take
into account the nonuniform vertical structure of the disk. The high electrical
conductivity of the outer layers of the disk prevents the outward diffusion of
the magnetic field. This implies a stationary state with a strong magnetic
field in the inner parts of the accretion disk close to the black hole.Comment: 5 pages, 2 figure
Evolution of Migrating Planets Undergoing Gas Accretion
We analyze the orbital and mass evolution of planets that undergo run-away
gas accretion by means of 2D and 3D hydrodynamic simulations. The disk torque
distribution per unit disk mass as a function of radius provides an important
diagnostic for the nature of the disk-planet interactions. We first consider
torque distributions for nonmigrating planets of fixed mass and show that there
is general agreement with the expectations of resonance theory. We then present
results of simulations for mass-gaining, migrating planets. For planets with an
initial mass of 5 Earth masses, which are embedded in disks with standard
parameters and which undergo run-away gas accretion to one Jupiter mass (Mjup),
the torque distributions per unit disk mass are largely unaffected by migration
and accretion for a given planet mass. The migration rates for these planets
are in agreement with the predictions of the standard theory for planet
migration (Type I and Type II migration). The planet mass growth occurs through
gas capture within the planet's Bondi radius at lower planet masses, the Hill
radius at intermediate planet masses, and through reduced accretion at higher
planet masses due to gap formation. During run-away mass growth, a planet
migrates inwards by only about 20% in radius before achieving a mass of ~1
Mjup. For the above models, we find no evidence of fast migration driven by
coorbital torques, known as Type III migration. We do find evidence of Type III
migration for a fixed mass planet of Saturn's mass that is immersed in a cold
and massive disk. In this case the planet migration is assumed to begin before
gap formation completes. The migration is understood through a model in which
the torque is due to an asymmetry in density between trapped gas on the leading
side of the planet and ambient gas on the trailing side of the planet.Comment: 26 pages, 29 figures. To appear in The Astrophysical Journal vol.684
(September 20, 2008 issue
V407 Vul: a direct impact accretor
V407 Vul (= RXJ1914.4+2457) shows pulsations in X-ray flux on a period of 9.5
minutes, which have been ascribed to accretion onto a magnetic white dwarf,
with the X-ray pulses seen as the accreting pole moves into and out of view.
The X-ray flux drops to zero between pulses, and no other periods are seen,
suggesting that V407 Vul is a type of system known as a ``polar'' in which the
white dwarf has a strong enough field to lock to the orbit of its companion. If
so, then V407 Vul has the shortest orbital period known for any binary star.
However, unlike other polars, V407 Vul shows neither polarization nor line
emission. In this paper we propose that V407 Vul is the first example of a new
type of X-ray emitting binary in which the mass transfer stream directly hits a
non-magnetic white dwarf as a result of the very compact orbit. Our model
naturally explains the X-ray and optical pulsations, as well as the absence of
polarization and line emission. We show that direct impact will occur for
plausible masses of the accreting star and its companion, e.g. M1 ~ 0.5, M2 ~
0.1 Msun. In our model V407 Vul retains its status as the binary star with the
shortest known orbital period, and is therefore a strong source of
low-frequency gravitational waves. V407 Vul is representative of an early phase
of the evolution of the AM CVn class of binary stars and will evolve into the
normal disc-accretion phase on a timescale of 10^6 to 10^7 yr. The existence of
V407 Vul supports the double-degenerate route for the formation of AM CVn
stars.Comment: 5 pages, 2 figures, in press, MNRAS pink page
Magnetically warped discs in close binaries
We demonstrate that measurable vertical structure can be excited in the
accretion disc of a close binary system by a dipolar magnetic field centred on
the secondary star. We present the first high resolution hydrodynamic
simulations to show the initial development of a uniform warp in a tidally
truncated accretion disc. The warp precesses retrogradely with respect to the
inertial frame. The amplitude depends on the phase of the warp with respect to
the binary frame. A warped disc is the best available explanation for negative
superhumps.Comment: 11 pages, 10 figures, MNRAS accepte
Vertical Structure of Stationary Accretion Disks with a Large-Scale Magnetic Field
In earlier works we pointed out that the disk's surface layers are
non-turbulent and thus highly conducting (or non-diffusive) because the
hydrodynamic and/or magnetorotational (MRI) instabilities are suppressed high
in the disk where the magnetic and radiation pressures are larger than the
plasma thermal pressure. Here, we calculate the vertical profiles of the {\it
stationary} accretion flows (with radial and azimuthal components), and the
profiles of the large-scale, magnetic field taking into account the turbulent
viscosity and diffusivity and the fact that the turbulence vanishes at the
surface of the disk.
Also, here we require that the radial accretion speed be zero at the disk's
surface and we assume that the ratio of the turbulent viscosity to the
turbulent magnetic diffusivity is of order unity. Thus at the disk's surface
there are three boundary conditions. As a result, for a fixed dimensionless
viscosity -value, we find that there is a definite relation between the
ratio of the accretion power going into magnetic disk winds to the
viscous power dissipation and the midplane plasma-, which is the ratio
of the plasma to magnetic pressure in the disk. For a specific disk model with
of order unity we find that the critical value required for a
stationary solution is , where the disk's
half thickness. For weaker magnetic fields, , we argue that
the poloidal field will advect outward while for it will
advect inward. Alternatively, if the disk wind is negligible (), there are stationary solutions with .Comment: 5 pages, 3 figure
Ring Formation in Magnetically Subcritical Clouds and Multiple Star Formation
We study numerically the ambipolar diffusion-driven evolution of
non-rotating, magnetically subcritical, disk-like molecular clouds, assuming
axisymmetry. Previous similar studies have concentrated on the formation of
single magnetically supercritical cores at the cloud center, which collapse to
form isolated stars. We show that, for a cloud with many Jeans masses and a
relatively flat mass distribution near the center, a magnetically supercritical
ring is produced instead. The supercritical ring contains a mass well above the
Jeans limit. It is expected to break up, through both gravitational and
possibly magnetic interchange instabilities, into a number of supercritical
dense cores, whose dynamic collapse may give rise to a burst of star formation.
Non-axisymmetric calculations are needed to follow in detail the expected ring
fragmentation into multiple cores and the subsequent core evolution.
Implications of our results on multiple star formation in general and the
northwestern cluster of protostars in the Serpens molecular cloud core in
particular are discussed.Comment: 25 pages, 4 figures, to appear in Ap
Trapping of magnetic flux by the plunge region of a black hole accretion disk
The existence of the radius of marginal stability means that accretion flows
around black holes invariably undergo a transition from a MHD turbulent
disk-like flow to an inward plunging flow. We argue that the plunging inflow
can greatly enhance the trapping of large scale magnetic field on the black
hole, and therefore may increase the importance of the Blandford-Znajek (BZ)
effect relative to previous estimates that ignore the plunge region. We support
this hypothesis by constructing and analyzing a toy-model of the dragging and
trapping of a large scale field by a black hole disk, revealing a strong
dependence of this effect on the effective magnetic Prandtl number of the MHD
turbulent disk. Furthermore, we show that the enhancement of the BZ effect
depends on the geometric thickness of the accretion disk. This may be, at least
in part, the physical underpinnings of the empirical relation between the
inferred geometric thickness of a black hole disk and the presence of a radio
jet.Comment: 18 pages, 3 figures, accepted for publication in the Astrophysical
Journal. See
http://www.astro.umd.edu/~chris/publications/movies/flux_trapping.html for
animation
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