8,833 research outputs found
MHD disc winds
This is a doctorate level lecture on the physics of accretion discs driving
magnetically self-confined jets, usually referred to in the literature as disc
winds. I will first review the governing magnetohydrodynamic equations and then
discuss their physical content. At that level, necessary conditions to drive
jets from keplerian accretion discs can already be derived. These conditions
are validated with self-similar calculations of accretion-ejection structures.
In a second part, I will critically discuss the biases introduced when using
self-similarity as well as some other questions such as: Are these systems
really unstable? Can a standard accretion disc provide the conditions to launch
jets in its innermost parts? What is the difference between X-winds and
disc-winds? Finally, the magnetic interaction between a protostar and its
circumstellar disc will be discussed with a focus on stellar spin down.Comment: 25 pages, 11 figures to be published in Lecture Notes in Physics,
"Jets from Young Stars: Models and Constraints", J. Ferreira, C. Dougados and
E. Whelan (eds), Springer Verla
Absorption lines from magnetically-driven winds in X-ray binaries
High resolution X-ray spectra of black hole X-ray binaries (BHBs) show
blueshifted absorption lines from disk winds which seem to be equatorial. Winds
occur in the Softer (disk-dominated) states of the outburst and are less
prominent or absent in the Harder (power-law dominated) states. We use
self-similar magneto-hydrodynamic (MHD) accretion-ejection models to explain
the disk winds in BHBs. In our models, the density at the base of the outflow
from the accretion disk is not a free parameter, but is determined by solving
the full set of dynamical MHD equations. Thus the physical properties of the
outflow are controlled by the global structure of the disk. We studied
different MHD solutions characterized by different values of (a) the disk
aspect ratio () and (b) the ejection efficiency (). We use two
kinds of MHD solutions depending on the absence (cold solution) or presence
(warm solution) of heating at the disk surface. Such heating could be from e.g.
dissipation of energy due to MHD turbulence in the disk or from illumination.
We use each of these MHD solutions to predict the physical parameters of an
outflow; put limits on the ionization parameter (), column density and
timescales, motivated by observational results; and thus select regions within
the outflow which are consistent with the observed winds. The cold MHD
solutions cannot account for winds due to their low ejection efficiency. But
warm solutions can explain the observed physical quantities in the wind because
they can have sufficiently high values of (, implying larger
mass loading at the base of the outflow). Further from our thermodynamic
equilibrium curve analysis for the outflowing gas, we found that in the Hard
state a range of is thermodynamically unstable, and had to be excluded.
This constrain made it impossible to have any wind at all, in the Hard state.Comment: 16 Pages, 10 figures in the main body and 4 figures in the appendix.
Accepted for publication in A&
Winds from Luminous Late-Type Stars: II. Broadband Frequency Distribution of Alfv\'en Waves
We present the numerical simulations of winds from evolved giant stars using
a fully non-linear, time dependent 2.5-dimensional magnetohydrodynamic (MHD)
code. This study extends our previous fully non-linear MHD wind simulations to
include a broadband frequency spectrum of Alfv\'en waves that drive winds from
red giant stars. We calculated four Alfv\'en wind models that cover the whole
range of Alfv\'en wave frequency spectrum to characterize the role of freely
propagated and reflected Alfv\'en waves in the gravitationally stratified
atmosphere of a late-type giant star. Our simulations demonstrate that, unlike
linear Alfv\'en wave-driven wind models, a stellar wind model based on plasma
acceleration due to broadband non-linear Alfv\'en waves, can consistently
reproduce the wide range of observed radial velocity profiles of the winds,
their terminal velocities and the observed mass loss rates. Comparison of the
calculated mass loss rates with the empirically determined mass loss rate for
alpha Tau suggests an anisotropic and time-dependent nature of stellar winds
from evolved giants.Comment: accepted by Ap
MHD Disc Winds and Linewidth Distributions
We study AGN emission line profiles combining an improved version of the
accretion disc-wind model of Murray & Chiang with the magneto-hydrodynamic
model of Emmering et al. We show how the shape, broadening and shift of the C
IV line depend not only on the viewing angle to the object but also on the wind
launching angle, especially for small launching angles. We have compared the
dispersions in our model C IV linewidth distributions to observational upper
limit on that dispersion, considering both smooth and clumpy torus models. As
the torus half-opening angle (measured from the polar axis) increases above
about 18? degrees, increasingly larger wind launching angles are required to
match the observational constraints. Above a half-opening angle of about 47?
degrees, no wind launch angle (within the maximum allowed by the MHD solutions)
can match the observations. Considering a model that replaces the torus by a
warped disc yields the same constraints obtained with the two other models
On fan-shaped cold MHD winds from Keplerian accretion discs
We investigate under which conditions cold, fan-shaped winds can be steadily
launched from thin (Keplerian) accretion discs. Such winds are
magneto-centrifugal winds launched from a thin annulus in the disc, along open
magnetic field lines that fan out above the disc. In principle, such winds
could be found in two situations: (1) at the interface between an inner Jet
Emitting Disc, which is itself powering magneto-centrifugally driven winds, and
an outer standard accretion disc; (2) at the interface between an inner closed
stellar magnetosphere and the outer standard accretion disc. We refer to
Terminal or T-winds to the former kind and to Magnetospheric or M-winds to the
latter.
The full set of resistive and viscous steady state MHD equations are analyzed
for the disc (the annulus), which allow us to derive general expressions valid
for both configurations. We find that, under the framework of our analysis, the
only source of energy able to power any kind of fan-shaped winds is the viscous
transport of rotational energy coming below the inner radii. Using standard
local prescriptions for the anomalous (turbulent) transport of angular
momentum and magnetic fields in the disc, we derive the strength of the
transport coefficients that are needed to steadily sustain the global
configuration. It turns out that, in order for these winds to be dynamically
relevant and explain observed jets, the disc coefficients must be far much
larger than values expected from current knowledge of turbulence occurring
inside proto-stellar discs.
Either the current view on MHD turbulence must be deeply reconsidered or
steady-state fan-shaped winds are never realized in Nature. The latter
hypothesis seems to be consistent with current numerical simulations.Comment: Among several possibilites, this paper addresses also the case of the
X-wind Accepted for publication in MNRA
Magnetic Nested-wind Scenarios for Bipolar Outflows: Pre-planetary and YSO nebular shaping
We present results of a series of magnetohydrodynamic (MHD) and hydro-
dynamic (HD) 2.5D simulations of the morphology of outflows driven by nested
wide-angle winds - i.e. winds which eminate from a central star as well as from
an orbiting accretion disk. While our results are broadly relevent to nested
wind systems we have tuned the parameters of the simulations to touch on issues
in both Young Stellar Objects and Planetary Nebula studies. In particular our
studies connect to open issues in the early evolution of Planetary Nebulae. We
find that nested MHD winds exhibit marked morphological differences from the
single MHD wind case along both dimensions of the flow. Nested HD winds on the
other hand give rise mainly to geometric distortions of an outflow that is
topologically similar to the flow arising from a single stellar HD wind. Our
MHD results are insensitive to changes in ambient temperature between ionized
and un-ionized circumstellar environments. The results are sensitive to the
relative mass-loss rates, and to the relative speeds of the stellar and disk
winds. We also present synthetic emission maps of both nested MHD and HD
simulations. We find that nested MHD winds show knots of emission appearing
on-axis that do not appear in the HD case.Comment: 28 pages, 8 figure
Stratified Magnetically-Driven Accretion-Disk Winds and Their Relations to Jets
We explore the poloidal structure of two-dimensional (2D) MHD winds in
relation to their potential association with the X-ray warm absorbers (WAs) and
the highly-ionized ultra-fast outflows (UFOs) in AGN, in a single unifying
approach. We present the density , ionization parameter
, and velocity structure of such ionized winds for
typical values of their fluid-to-magnetic flux ratio, , and specific angular
momentum, , for which wind solutions become super-\Alfvenic. We explore the
geometrical shape of winds for different values of these parameters and
delineate the values that produce the widest and narrowest opening angles of
these winds, quantities necessary in the determination of the statistics of AGN
obscuration. We find that winds with smaller show a poloidal geometry of
narrower opening angles with their \Alfven\ surface at lower inclination angles
and therefore they produce the highest line of sight (LoS) velocities for
observers at higher latitudes with the respect to the disk plane. We further
note a physical and spatial correlation between the X-ray WAs and UFOs that
form along the same LoS to the observer but at different radii, , and
distinct values of , and consistent with the latest spectroscopic
data of radio-quiet Seyfert galaxies. We also show that, at least in the case
of 3C 111, the winds' pressure is sufficient to contain the relativistic plasma
responsible for its radio emission. Stratified MHD disk-winds could therefore
serve as a unique means to understand and unify the diverse AGN outflows.Comment: version 2 (modified), 27 pages, 5 figures, accepted to Ap
Stellar Outflows Driven by Magnetized Wide-Angle Winds
We present two-dimensional, cylindrically symmetric simulations of
hydrodynamic and magnetohydrodynamic (MHD) wide-angle winds interacting with a
collapsing environment. These simulations have direct relevance to young
stellar objects (YSOs). The results may also be of use in the study of
collimated outflows from proto-planetary and planetary nebulae. We study a
range of wind configurations consistent with asymptotic MHD wind collimation.
The degree of collimation is parameterized by the ratio of the wind density at
the pole to that of the equator. We find that a toroidal magnetic field can
have a significant influence on the resulting outflow, giving rise to a very
dense, jet-like flow in the post-shock region. The properties of the flow in
this region are similar to the asymptotic state of a collimated MHD wind. We
conclude that wide-angle MHD winds are quite likely capable of driving
molecular outflows. Due to difficulty in treating MHD winds ab-initio in
simulations we choose magnetic field strengths in the wind consistent slow
magnetic rotators. While MHD launched winds will be in the fast rotator regime
we discuss how our results, which rely on toroidal pinch effects, will hold for
stronger field strengths
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