46 research outputs found
Resistive jet simulations extending radially self-similar magnetohydrodynamic models
Numerical simulations with self-similar initial and boundary conditions
provide a link between theoretical and numerical investigations of jet
dynamics. We perform axisymmetric resistive magnetohydrodynamic (MHD)
simulations for a generalised solution of the Blandford & Payne type, and
compare them with the corresponding analytical and numerical ideal-MHD
solutions. We disentangle the effects of the numerical and physical
diffusivity. The latter could occur in outflows above an accretion disk, being
transferred from the underlying disk into the disk corona by MHD turbulence
(anomalous turbulent diffusivity), or as a result of ambipolar diffusion in
partially ionized flows. We conclude that while the classical magnetic Reynolds
number measures the importance of resistive effects in the
induction equation, a new introduced number, \rbeta=(\beta/2)R_{\rm m} with
the plasma beta, measures the importance of the resistive effects in
the energy equation. Thus, in magnetised jets with , when \rbeta \la
1 resistive effects are non-negligible and affect mostly the energy equation.
The presented simulations indeed show that for a range of magnetic
diffusivities corresponding to \rbeta \ga 1 the flow remains close to the
ideal-MHD self-similar solution.Comment: Accepted for publication in MNRA
A Physical Limit to the Magnetic Fields of T Tauri Stars
Recent estimates of magnetic field strengths in T Tauri stars yield values
--. In this paper, I present an upper limit to the
photospheric values of by computing the equipartition values for different
surface gravities and effective temperatures. The values of derived from
the observations exceed this limit, and I examine the possible causes for this
discrepancy
On the Saturation of Astrophysical Dynamos: Numerical Experiments with the No-cosines flow
In the context of astrophysical dynamos we illustrate that the no-cosines
flow, with zero mean helicity, can drive fast dynamo action and study the
dynamo's mode of operation during both the linear and non-linear saturation
regime: It turns out that in addition to a high growth rate in the linear
regime, the dynamo saturates at a level significantly higher than normal
turbulent dynamos, namely at exact equipartition when the magnetic Prandtl
number is on the order of unity. Visualization of the magnetic and velocity
fields at saturation will help us to understand some of the aspects of the
non-linear dynamo problem.Comment: 8 pages, 5 figures, submitted to the proceedings of "Space Climate 1"
to be peer-reviewed to Solar Physic
A Critique of Current Magnetic-Accretion Models for Classical T-Tauri Stars
Current magnetic-accretion models for classical T-Tauri stars rely on a
strong, dipolar magnetic field of stellar origin to funnel the disk material
onto the star, and assume a steady-state. In this paper, I critically examine
the physical basis of these models in light of the observational evidence and
our knowledge of magnetic fields in low-mass stars, and find it lacking.
I also argue that magnetic accretion onto these stars is inherently a
time-dependent problem, and that a steady-state is not warranted.
Finally, directions for future work towards fully-consistent models are
pointed out.Comment: 2 figure
Modeling the Halpha line emission around classical T Tauri stars using magnetospheric accretion and disk wind models
Spectral observations of classical T Tauri stars show a wide range of line
profiles, many of which reveal signs of matter inflow and outflow. Halpha is
the most commonly observed line profile due to its intensity, and it is highly
dependent on the characteristics of the surrounding environment of these stars.
Our aim is to analyze how the Halpha line profile is affected by the various
parameters of our model which contains both the magnetospheric and disk wind
contributions to the Halpha flux. We used a dipolar axisymmetric stellar
magnetic field to model the stellar magnetosphere and a modified Blandford &
Payne model was used in our disk wind region. A three-level atom with continuum
was used to calculate the required Hydrogen level populations. We use the
Sobolev approximation and a ray-by-ray method to calculate the integrated line
profile. Through an extensive study of the model parameter space, we have
investigated the contribution of many of the model parameters on the calculated
line profiles. Our results show that the Halpha line is strongly dependent on
the densities and temperatures inside the magnetosphere and the disk wind
region. The bulk of the flux comes, most of the time, from the magnetospheric
component for standard classical T Tauri stars parameters, but the disk wind
contribution becomes more important as the mass accretion rate, the
temperatures and densities inside the disk wind increase. We have also found
that most of the disk wind contribution to the Halpha line is emitted at the
innermost region of the disk wind. Models that take into consideration both
inflow and outflow of matter are a necessity to fully understand and describe
classical T Tauri stars.Comment: 15 pages, 9 figures, accepted for publication in Astronomy &
Astrophysics. Revised version with English correction
Star Formation in Cold, Spherical, Magnetized Molecular Clouds
We present an idealized, spherical model of the evolution of a magnetized
molecular cloud due to ambipolar diffusion. This model allows us to follow the
quasi-static evolution of the cloud's core prior to collapse and the subsequent
evolution of the remaining envelope. By neglecting the thermal pressure
gradients in comparison with magnetic stresses and by assuming that the ion
velocity is small compared with the neutral velocity, we are able to find exact
analytic solutions to the MHD equations. We show that, in the case of a
centrally condensed cloud, a core of finite mass collapses into the origin
leaving behind a quasi-static envelope, whereas initially homogeneous clouds
never develop any structure in the absence of thermal stresses, and collapse as
a whole. Prior to the collapse of the core, the cloud's evolution is
characterized by two phases: a long, quasi-static phase where the relevant
timescale is the ambipolar diffusion time (treated in this paper), and a short,
dynamical phase where the characteristic timescale is the free-fall time. The
collapse of the core is an "outside-in" collapse. The quasi-static evolution
terminates when the cloud becomes magnetically supercritical; thereafter its
evolution is dynamical, and a singularity develops at the origin-a protostar.
After the initial formation of the protostar, the outer envelope continues to
evolve quasi-statically, while the region of dynamical infall grows with
time-an "inside-out" collapse. We use our solution to estimate the magnetic
flux trapped in the collapsing core and the mass accretion rate onto the newly
formed protostar. Our results agree, within factors of order unity, with the
numerical results of Fiedler & Mouschovias (1992) for the physical quantities
in the midplane ofComment: 18 postscript figures Accepted by The Astrophysical Journa
The Disk Wind in the Young Binaries and the Origin of the Cyclic Activity of Young Stars
We present results of numerical modeling of the cyclic brightness modulation
in the young binary systems with the eccentric orbits and low-mass secondary
components. Brightness variations of the primary is due to the periodical
extinction variations on the line-of-sight caused by the disk wind of the
secondary and a common envelope it produces. A matter distribution in the
envelope has been calculated in the ballistic approach. Calculations showed
that for the young binaries with the elliptic orbits parameters of the
photometric minima (their depth, duration and the shape of light curves) depend
not only on the disk wind parameters and an inclination of the binary orbit to
the line-of-sight but also on the longitude of the periastron. A modulation of
the scattered radiation of the common envelope with a phase of the orbital
period has been investigated in the single scattering approach. It is shown
that an amplitude of the modulation is maximal when the system is seen edge-on
and has also a non-zero value in the binaries observed pole-on. Possible
applications of the theory to the young stellar objects are discussed. In
particular, an attention is payed to a resemblance of the light curves in some
models with light curves of the objects suspected as candidates to FUORs.Comment: 18 pages, 9 figures, accepted by Astronomy Letter
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Multisite observations of SU Aurigae
We present results from the 1996 MUSICOS (MUlti-SIte COntinuous Spectroscopy)
campaign on the T Tauri star SU Aurigae. We find a 2.7-d periodicity in the He
I (587.6 nm) line and somewhat longer, less well-pronounced periodicities in
the Balmer lines and in Na D. Our observations support the suggestion that the
wind and infall signatures are out of phase on SU Aur. We present Doppler
images of SU Aur that have been obtained from least-squares deconvolved
profiles. Images taken about one rotation apart show only limited overlap, in
particular at low latitudes. This is in part due to limitations in
signal-to-noise, and in part due to line profile deformations that arise from
short-lived and/or non-surface features. The agreement at high latitudes is
better and suggests that at least some longer-lived features are present. The
analysis of Stokes V profiles yields a marginal magnetic field detection during
one of the phases.Comment: 22 pages, 20 figures, accepted for publication in MNRA