123 research outputs found
A magnetic thrust action on small bodies orbiting a pulsar
We investigate the electromagnetic interaction of a relativistic stellar wind
with small bodies in orbit around the star. Based on our work on the theory of
Alfv\'en wings to relativistic winds presented in a companion paper, we
estimate the force exerted by the associated current system on orbiting bodies
and evaluate the resulting orbital drift. This Alfv\'enic structure is found to
have no significant influence on planets or smaller bodies orbiting a
millisecond pulsar. %influence on the orbit of bodies around a millisecond
pulsar. On the timescale of millions of years, it can however affect the orbit
of bodies with a diameter of 100 kilometres around standard pulsars with a
period 1 s and a magnetic field T. Kilometer-sized
bodies experience drastic orbital changes on a timescale of years.Comment: accepted for publication in "Astronomy and Astrophysics
Thermal Instability in a Cooling and Expanding Medium Including Self-Gravity and Conduction
A systematic study of the linear thermal stability of a medium subject to
cooling, self-gravity and thermal conduction is carried out for the case when
the unperturbed state is subject to global cooling and expansion. A general,
recursive WKB solution for the perturbation problem is obtained which can be
applied to a large variety of situations in which there is a separation of
time-scales for the different physical processes. Solutions are explicitly
given and discussed for the case when sound propagation and/or self-gravity are
the fastest processes, with cooling, expansion and thermal conduction operating
on slower time-scales. A brief discussion is also added for the solutions in
the cases in which cooling or conduction operate on the fastest time-scale. The
general WKB solution obtained in this paper permits solving the problem of the
effect of thermal conduction and self-gravity on the thermal stability of a
globally cooling and expanding medium. As a result of the analysis, the
critical wavelength (often called Field length) above which cooling makes the
perturbations unstable against the action of thermal conduction is generalized
to the case of an unperturbed background with net cooling. As an astrophysical
application, the generalized Field length is calculated for a hot (10^4 - 10^8
K), optically thin medium (as pertains, for instance, for the hot interstellar
medium of SNRs or superbubbles) using a realistic cooling function and
including a weak magnetic field. The stability domains are compared with the
predictions made on the basis of models for which the background is in thermal
equilibrium. The instability domain of the sound waves, in particular, is seen
to be much larger in the case with net global cooling.Comment: 36 pages, 6 figures, accepted by ApJ, probable publication date:
April 20, 200
Alfven Wave-Driven Supernova Explosion
We investigate the role of Alfven waves in the core-collapse supernova (SN)
explosion. We assume that Alfven waves are generated by convections inside a
proto-neutron star (PNS) and emitted from its surface. Then these waves
propagate outwards, dissipate via nonlinear processes, and heat up matter
around a stalled prompt shock. To quantitatively assess the importance of this
process for the revival of the stalled shock, we perform 1D time-dependent
hydrodynamical simulations, taking into account the heating via the dissipation
of Alfven waves that propagate radially outwards along open flux tubes. We show
that the shock revival occurs if the surface field strength is larger than
~2e15 G and if the amplitude of velocity fluctuation at the PNS surface is
larger than 20% of the local sound speed. Interestingly, the Alfven wave
mechanism is self-regulating in the sense that the explosion energy is not very
sensitive to the surface field strength and initial amplitude of Alfven waves
as long as they are larger than the threshold values given above.Comment: 7 pages, 3 figures embedded, submitted to Ap
An Ab Initio Approach to the Solar Coronal Heating Problem
We present an ab initio approach to the solar coronal heating problem by
modelling a small part of the solar corona in a computational box using a 3D
MHD code including realistic physics. The observed solar granular velocity
pattern and its amplitude and vorticity power spectra, as reproduced by a
weighted Voronoi tessellation method, are used as a boundary condition that
generates a Poynting flux in the presence of a magnetic field. The initial
magnetic field is a potential extrapolation of a SOHO/MDI high resolution
magnetogram, and a standard stratified atmosphere is used as a thermal initial
condition. Except for the chromospheric temperature structure, which is kept
fixed, the initial conditions are quickly forgotten because the included
Spitzer conductivity and radiative cooling function have typical timescales
much shorter than the time span of the simulation. After a short initial start
up period, the magnetic field is able to dissipate 3-4 10^6 ergs cm^{-2} s^{-1}
in a highly intermittent corona, maintaining an average temperature of K, at coronal density values for which emulated images of the Transition
Region And Coronal Explorer(TRACE) 171 and 195 pass bands reproduce observed
photon count rates.Comment: 12 pages, 14 figures. Submitted to Ap
Statistical Description of a Magnetized Corona above a Turbulent Accretion Disk
We present a physics-based statistical theory of a force-free magnetic field
in the corona above a turbulent accretion disk. The field is represented by a
statistical ensemble of loops tied to the disk. Each loop evolves under several
physical processes: Keplerian shear, turbulent random walk of the disk
footpoints, and reconnection with other loops. To build a statistical
description, we introduce the distribution function of loops over their sizes
and construct a kinetic equation that governs its evolution. This loop kinetic
equation is formally analogous to Boltzmann's kinetic equation, with loop-loop
reconnection described by a binary collision integral. A dimensionless
parameter is introduced to scale the (unknown) overall rate of reconnection
relative to Keplerian shear. After solving for the loop distribution function
numerically, we calculate self-consistently the distribution of the mean
magnetic pressure and dissipation rate with height, and the equilibrium shapes
of loops of different sizes. We also compute the energy and torque associated
with a given loop, as well as the total magnetic energy and torque in the
corona. We explore the dependence of these quantities on the reconnection
parameter and find that they can be greatly enhanced if reconnection between
loops is suppressed.Comment: 22 pages, 15 figures. Submitted to the Astrophysical Journa
AGN disks and black holes on the weighting scales
We exploit our formula for the gravitational potential of finite size,
power-law disks to derive a general expression linking the mass of the black
hole in active galactic nuclei (AGN), the mass of the surrounding disk, its
surface density profile (through the power index s), and the differential
rotation law. We find that the global rotation curve v(R) of the disk in
centrifugal balance does not obey a power law of the cylindrical radius R
(except in the confusing case s = -2 that mimics a Keplerian motion), and
discuss the local velocity index. This formula can help to understand how, from
position-velocity diagrams, mass is shared between the disk and the black hole.
To this purpose, we have checked the idea by generating a sample of synthetic
data with different levels of Gaussian noise, added in radius. It turns out
that, when observations are spread over a large radial domain and exhibit low
dispersion (standard deviation less than 10% typically), the disk properties
(mass and s-parameter) and black hole mass can be deduced from a non linear fit
of kinematic data plotted on a (R, Rv 2)-diagram. For a deviation higher than
10%, masses are estimated fairly well from a linear regression (corresponding
to the zeroth-order treatment of the formula), but the power index s is no
longer accessible. We have applied the model to 7 AGN disks whose rotation has
already been probed through water maser emission. For NGC3393 and UGC3789, the
masses seem well constrained through the linear approach. For IC1481, the
power-law exponent s can even be deduced. Because the model is scale-free, it
applies to any kind of star/disk system. Extension to disks around young stars
showing deviation from Keplerian motion is thus straightforward.Comment: accepted for publication in A&
Magneto-Acoustic Waves of Small Amplitude in Optically Thin Quasi-Isentropic Plasmas
The evolution of quasi-isentropic magnetohydrodynamic waves of small but
finite amplitude in an optically thin plasma is analyzed. The plasma is assumed
to be initially homogeneous, in thermal equilibrium and with a straight and
homogeneous magnetic field frozen in. Depending on the particular form of the
heating/cooling function, the plasma may act as a dissipative or active medium
for magnetoacoustic waves, while Alfven waves are not directly affected. An
evolutionary equation for fast and slow magnetoacoustic waves in the single
wave limit, has been derived and solved, allowing us to analyse the wave
modification by competition of weakly nonlinear and quasi-isentropic effects.
It was shown that the sign of the quasi-isentropic term determines the scenario
of the evolution, either dissipative or active. In the dissipative case, when
the plasma is first order isentropically stable the magnetoacoustic waves are
damped and the time for shock wave formation is delayed. However, in the active
case when the plasma is isentropically overstable, the wave amplitude grows,
the strength of the shock increases and the breaking time decreases. The
magnitude of the above effects depends upon the angle between the wave vector
and the magnetic field. For hot (T > 10^4 K) atomic plasmas with solar
abundances either in the interstellar medium or in the solar atmosphere, as
well as for the cold (T < 10^3 K) ISM molecular gas, the range of temperature
where the plasma is isentropically unstable and the corresponding time and
length-scale for wave breaking have been found.Comment: 14 pages, 10 figures. To appear in ApJ January 200
Thin accretion disc with a corona in a central magnetic field
We study the steady-state structure of an accretion disc with a corona
surrounding a central, rotating, magnetized star. We assume that the
magneto-rotational instability is the dominant mechanism of angular momentum
transport inside the disc and is responsible for producing magnetic tubes above
the disc. In our model, a fraction of the dissipated energy inside the disc is
transported to the corona via these magnetic tubes. This energy exchange from
the disc to the corona which depends on the disc physical properties is
modified because of the magnetic interaction between the stellar magnetic field
and the accretion disc. According to our fully analytical solutions for such a
system, the existence of a corona not only increases the surface density but
reduces the temperature of the accretion disc. Also, the presence of a corona
enhances the ratio of gas pressure to the total pressure. Our solutions show
that when the strength of the magnetic field of the central neutron star is
large or the star is rotating fast enough, profiles of the physical variables
of the disc significantly modify due to the existence of a corona.Comment: Accepted for publication in Astrophysics & Space Scienc
Dynamics of Magnetic Flux Elements in the Solar Photosphere
The interaction of magnetic fields and convection is investigated in the
context of the coronal heating problem. We study the motions of photospheric
magnetic elements using filtergrams obtained at the Swedish Vacuum Solar
Telescope at La Palma. We use potential-field modeling to extrapolate the
magnetic and velocity fields to larger height. We find that the velocity in the
chromosphere can be locally enhanced at the separatrix surfaces between
neighboring flux tubes. The predicted velocities are several km/s,
significantly larger than those of the photospheric flux tubes, which may have
important implications for coronal heating. sComment: submitted to ApJ, 21 pages, 10 figure
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