25 research outputs found
Global 3D Simulations of Disc Accretion onto the classical T Tauri Star V2129 Oph
The magnetic field of the classical T Tauri star V2129 Oph can be modeled
approximately by superposing slightly tilted dipole and octupole moments, with
polar magnetic field strengths of 0.35kG and 1.2kG respectively (Donati et al.
2007). Here we construct a numerical model of V2129 Oph incorporating this
result and simulate accretion onto the star. Simulations show that the disk is
truncated by the dipole component and matter flows towards the star in two
funnel streams. Closer to the star, the flow is redirected by the octupolar
component, with some of the matter flowing towards the high-latitude poles, and
the rest into the octupolar belts. The shape and position of the spots differ
from those in a pure dipole case, where crescent-shaped spots are observed at
the intermediate latitudes. Simulations show that if the disk is truncated at
the distance of 6.2 R_* which is comparable with the co-rotation radius, 6.8
R_*, then the high-latitude polar spots dominate, but the accretion rate
obtained from the simulations is about an order of magnitude lower than the
observed one. The accretion rate matches the observed one if the disk is
disrupted much closer to the star, at 3.4 R_*. However, the octupolar belt
spots strongly dominate. Better match has been obtained in experiments with a
dipole field twice as strong. The torque on the star from the
disk-magnetosphere interaction is small, and the time-scale of spin evolution,
2 x10^7-10^9 years is longer than the 2x10^6 years age of V2129 Oph. The
external magnetic flux of the star is strongly influenced by the disk: the
field lines connecting the disk and the star inflate and form magnetic towers
above and below the disk. The potential (vacuum) approximation is still valid
inside the Alfv\'en (magnetospheric) surface where the magnetic stress
dominates over the matter stress.Comment: 15 pages, 15 figures, after major revision, added 3 figures, 2
tables. Accepted to MNRA
Global 3D Simulations of Disc Accretion onto the classical T Tauri Star BP Tauri
The magnetic field of the classical T Tauri star BP Tau can be approximated
as a superposition of dipole and octupole moments with respective strengths of
the polar magnetic fields of 1.2 kG and 1.6 kG (Donati et al. 2008). We adopt
the measured properties of BP Tau and model the disc accretion onto the star.
We observed in simulations that the disc is disrupted by the dipole component
and matter flows towards the star in two funnel streams which form two
accretion spots below the dipole magnetic poles. The octupolar component
becomes dynamically important very close to the star and it redirects the
matter flow to higher latitudes. The spots are meridionally elongated and are
located at higher latitudes, compared with the pure dipole case, where
crescent-shaped, latitudinally elongated spots form at lower latitudes. The
position and shape of the spots are in good agreement with observations. The
disk-magnetosphere interaction leads to the inflation of the field lines and to
the formation of magnetic towers above and below the disk. The magnetic field
of BP Tau is close to the potential only near the star, inside the
magnetospheric surface, where magnetic stress dominates over the matter stress.
A series of simulation runs were performed for different accretion rates. They
show that an accretion rate is lower than obtained in many observations, unless
the disc is truncated close to the star. The torque acting on the star is about
an order of magnitude lower than that which is required for the rotational
equilibrium. We suggest that a star could lose most of its angular momentum at
earlier stages of its evolution.Comment: 11 pages, 13 figures, submitted to MNRA
Modeling interaction of relativistic and nonrelativistic winds in binary system PSR 1259-63/SS2883. I.Hydrodynamical limit
In this paper, we present a detailed hydrodynamical study of the properties
of the flow produced by the collision of a pulsar wind with the surrounding in
a binary system. This work is the first attempt to simulate interaction of the
ultrarelativistic flow (pulsar wind) with the nonrelativistic stellar wind.
Obtained results show that the wind collision could result in the formation of
an "unclosed" (at spatial scales comparable to the binary system size) pulsar
wind termination shock even when the stellar wind ram pressure exceeds
significantly the pulsar wind kinetical pressure. Moreover, the post-shock flow
propagates in a rather narrow region, with very high bulk Lorentz factor
(). This flow acceleration is related to adiabatical losses,
which are purely hydrodynamical effects. Interestingly, in this particular
case, no magnetic field is required for formation of the ultrarelativistic bulk
outflow. The obtained results provide a new interpretation for the orbital
variability of radio, X-ray and gamma-ray signals detected from binary pulsar
system PSR 1259-63/SS2883.Comment: 11 pages, 13 figures, submitted to MNRA
Magneto-centrifugally driven winds: comparison of MHD simulations with theory
Stationary magnetohydrodynamic (MHD) outflows from a rotating, conducting
Keplerian accretion disk threaded by B-field are investigated numerically by
time-dependent, axisymmetric (2.5D) simulations using a Godunov-type code. A
large class of stationary magneto-centrifugally driven winds are found where
matter is accelerated from a thermal speed at the disk to much larger velocity,
greater than the fast magnetosonic speed and larger than the escape speed. The
flows are approximately spherical outflows with only small collimation within
the simulation region. Numerical results are shown to coincide with the
theoretical predictions of ideal, axisymmetric MHD to high accuracy.
Investigation of the influence of outer boundary conditions, particularly that
on the toroidal component of magnetic field shows that the commonly used
``free'' boundary condition leads to artificial magnetic forces which can act
to give spurious collimation. New boundary conditions are proposed which do not
generate artificial forces. Artificial results may also arise for cases where
the Mach cones on the outer boundaries are partially directed into the
simulation region.Comment: 19 pages, 18 figures, emulapj.sty is use
Three-dimensional Simulations of Disk Accretion to an Inclined Dipole: I. Magnetospheric Flow at Different Theta
We present results of fully three-dimensional MHD simulations of disk
accretion to a rotating magnetized star with its dipole moment inclined at an
angle Theta to the rotation axis of the disk. We observed that matter accretes
from the disk to a star in two or several streams depending on Theta. Streams
may precess around the star at small Theta. The inner regions of the disk are
warped. The warping is due to the tendency of matter to co-rotate with inclined
magnetosphere. The accreting matter brings positive angular momentum to the
(slowly rotating) star tending to spin it up. The corresponding torque N_z
depends only weakly on Theta. The angular momentum flux to the star is
transported predominantly by the magnetic field; the matter component
contributes < 1 % of the total flux. Results of simulations are important for
understanding the nature of classical T Tauri stars, cataclysmic variables, and
X-ray pulsars.Comment: 26 pages, 22 figures, LaTeX, macros: emulapj.sty, avi simulations are
available at http://www.astro.cornell.edu/us-rus/inclined.ht
Possible Quasi-Periodic Oscillations from Unstable Accretion: 3D MHD Simulations
We investigate the photometric variability of magnetized stars, particularly
neutron stars, accreting through a magnetic Rayleigh-Taylor-type instability at
the disk-magnetosphere interface, and compare it with the variability during
stable accretion, with the goal of looking for possible quasi-periodic
oscillations. The lightcurves during stable accretion show periodicity at the
star's frequency and sometimes twice that, due to the presence of two funnel
streams that produce antipodal hotspots near the magnetic poles. On the other
hand, lightcurves during unstable accretion through tongues penetrating the
magnetosphere are more chaotic due to the stochastic behaviour of the tongues,
and produce noisier power spectra. However, the power spectra do show some
signs of quasi-periodic variability. Most importantly, the rotation frequency
of the tongues and the resulting hotspots is close to the inner-disk orbital
frequency, except in the most strongly unstable cases. There is therefore a
high probability of observing QPOs at that frequency in longer simulations. In
addition, the lightcurves in the unstable regime show periodicity at the star's
rotation frequency in many of the cases investigated here, again except in the
most strongly unstable cases which lack funnel flows and the resulting
antipodal hotspots. The noisier power spectra result in the fractional rms
amplitudes of the Fourier peaks being smaller.
We also study in detail the effect of the misalignment angle between the
rotation and magnetic axes of the star on the variability, and find that at
misalignment angles , the star's period always appears in the
lightcurves.Comment: 14 pages, 16 figures, accepted by MNRAS. v2 comments: significant
revision. v3 comments: after referee report. Rewrote QPO section (4.5). v4
comments: final versio
Accretion Disks and Dynamos: Toward a Unified Mean Field Theory
Conversion of gravitational energy into radiation in accretion discs and the
origin of large scale magnetic fields in astrophysical rotators have often been
distinct topics of research. In semi-analytic work on both problems it has been
useful to presume large scale symmetries, necessarily resulting in mean field
theories. MHD turbulence makes the underlying systems locally asymmetric and
nonlinear. Synergy between theory and simulations should aim for the
development of practical mean field models that capture essential physics and
can be used for observational modeling. Mean field dynamo (MFD) theory and
alpha-viscosity accretion theory exemplify such ongoing pursuits. 21st century
MFD theory has more nonlinear predictive power compared to 20th century MFD
theory, whereas accretion theory is still in a 20th century state. In fact,
insights from MFD theory are applicable to accretion theory and the two are
artificially separated pieces of what should be a single theory. I discuss
pieces of progress that provide clues toward a unified theory. A key concept is
that large scale magnetic fields can be sustained via local or global magnetic
helicity fluxes or via relaxation of small scale magnetic fluctuations, without
the kinetic helicity driver of 20th century textbooks. These concepts may help
explain the formation of large scale fields that supply non-local angular
momentum transport via coronae and jets in a unified theory of accretion and
dynamos. In diagnosing the role of helicities and helicity fluxes in disk
simulations, each disk hemisphere should be studied separately to avoid being
misled by cancelation that occurs as a result of reflection asymmetry. The
fraction of helical field energy in disks is expected to be small compared to
the total field in each hemisphere as a result of shear, but can still be
essential for large scale dynamo action.Comment: For the Proceedings of the Third International Conference and
Advanced School "Turbulent Mixing and Beyond," TMB-2011 held on 21 - 28
August 2011 at the Abdus Salam International Centre for Theoretical Physics,
Trieste, http://users.ictp.it/~tmb/index2011.html Italy, To Appear in Physica
Scripta (corrected small items to match version in print