390 research outputs found
Global MRI with Braginskii viscosity in a galactic profile
We present a global-in-radius linear analysis of the axisymmetric
magnetorotational instability (MRI) in a collisional magnetized plasma with
Braginskii viscosity. For a galactic angular velocity profile we
obtain analytic solutions for three magnetic field orientations: purely
azimuthal, purely vertical and slightly pitched (almost azimuthal). In the
first two cases the Braginskii viscosity damps otherwise neutrally stable
modes, and reduces the growth rate of the MRI respectively. In the final case
the Braginskii viscosity makes the MRI up to times faster than its
inviscid counterpart, even for \emph{asymptotically small} pitch angles. We
investigate the transition between the Lorentz-force-dominated and the
Braginskii viscosity-dominated regimes in terms of a parameter \sim \Omega
\nub/B^2 where \nub is the viscous coefficient and the Alfv\'en speed.
In the limit where the parameter is small and large respectively we recover the
inviscid MRI and the magnetoviscous instability (MVI). We obtain asymptotic
expressions for the approach to these limits, and find the Braginskii viscosity
can magnify the effects of azimuthal hoop tension (the growth rate becomes
complex) by over an order of magnitude. We discuss the relevance of our results
to the local approximation, galaxies and other magnetized astrophysical
plasmas. Our results should prove useful for benchmarking codes in global
geometries.Comment: 14 pages, 5 figure
Plasma Modes Along the Open Field Lines of a Neutron Star
We consider electrostatic plasma modes along the open field lines of a
rotating neutron star. Goldreich-Julian charge density in general relativity is
analyzed for the neutron star with zero inclination. It is found that the
charge density is maximum at the polar cap and it remains almost same in
certain extended region of the pole. For a steady state Goldreich-Julian charge
density we found the usual plasma oscillation along the field lines; plasma
frequency resembles to the gravitational redshift close to the Schwarzschild
radius. We study the nonlinear plasma mode along the field lines. From the
system of equations under general relativity, a second order differential
equation is derived. The equation contains a term which describes the growing
plasma modes near Schwarzschild radius in a black hole environment. The term
vanishes with the distance far away from the gravitating object. For initially
zero potential and field on the surface of a neutron star, Goldreich-Julian
charge density is found to create the plasma mode, which is enhanced and
propagates almost without damping along the open field lines. We briefly
outline our plan to extend the work for studying soliton propagation along the
open field lines of strongly gravitating objects
The Crustal Rigidity of a Neutron Star, and Implications for PSR 1828-11 and other Precession Candidates
We calculate the crustal rigidity parameter, b, of a neutron star (NS), and
show that b is a factor 40 smaller than the standard estimate due to Baym &
Pines (1971). For a NS with a relaxed crust, the NS's free-precession frequency
is directly proportional to b. We apply our result for b to PSR 1828-11, a 2.5
Hz pulsar that appears to be precessing with period 511 d. Assuming this 511-d
period is set by crustal rigidity, we show that this NS's crust is not relaxed,
and that its reference spin (roughly, the spin for which the crust is most
relaxed) is 40 Hz, and that the average spindown strain in the crust is 5
\times 10^{-5}. We also briefly describe the implications of our b calculation
for other well-known precession candidates.Comment: 44 pages, 10 figures, submitted to Ap
Stellar models of evolved secondaries in CVs
In this paper we study the impact of chemically evolved secondaries on CV
evolution. We find that when evolved secondaries are included a spread in the
secondary mass-orbital period plane comparable to that seen in the data is
produced for either the saturated prescription for magnetic braking or the
unsaturated model commonly used for CVs. We argue that in order to explain this
spread a considerable fraction of all CVs should have evolved stars as the
secondaries. The evolved stars become fully convective at lower orbital
periods. Therefore, even if there was an abrupt decrease in magnetic braking
for fully convective stars (contrary to open cluster data) it would not be
expected to produce a sharp break in the period distribution for CVs. We also
explore recent proposed revisions to the angular momentum loss rate for single
stars, and find that only modest increases over the saturated prescription are
consistent with the overall observed spindown pattern. We compare predictions
of our models with diagnostics of the mass accretion rate in WDs and find
results intermediate between the saturated and the older braking prescription.
Taken together these suggest that the angular momentum loss rate may be higher
in CV secondaries than in single stars of the same rotation period, but is
still significantly lower than in the traditional model. Alternative
explanations for the CV period gap are discussed.Comment: 24 pages, 9 figures. Submitted to Ap
General Relativistic Effect of Gravitomagnetic Charge on Pulsar Magnetosphere and Particle Acceleration in a Polar Cap
We study magnetospheric structure surrounding rotating magnetized neutron
star with nonvanishing NUT (Newman-Tamburino-Unti) parameter. For the
simplicity of calculations Goldreich-Julian charge density is analyzed for the
aligned neutron star with zero inclination between magnetic field,
gravitomagnetic field and rotation axis. From the system of Maxwell equations
in spacetime of slowly rotating NUT star, second-order differential equation
for electrostatic potential is derived. Analytical solution of this equation
indicates the general relativistic modification of an accelerating electric
field and charge density along the open field lines by the gravitomagnetic
charge. The implication of this effect to the magnetospheric energy loss
problem is underlined. In the second part of the paper we derive the equations
of motion of test particles in magnetosphere of slowly rotating NUT star. Then
we analyze particle motion in the polar cap and show that NUT parameter can
significantly change conditions for particle acceleration.Comment: 21 pages, 6 figures, accepted for publication in Ap
An Empirical Model for the Radio Emission from Pulsars
A model for slow radio pulsars is proposed which involves the entire
magnetosphere in the production of the observed radio emission. It is argued
that observations of pulsar profiles suggest that a feedback mechanism exists
between the star surface and the null charge surface, requiring particle flow
in both directions. In their flow to and from the surface the particles execute
an azimuthal drift around the magnetic pole, thereby creating a ring of
discrete `emission nodes' close to the surface. Motion of the nodes is observed
as the well-known subpulse `drift', but is interpreted here as a small residual
component of the real particle drift. The nodes can therefore move in either
direction, or even remain stationary. A precise fit is found for the pulsar
PSR0943+10. Azimuthal interactions between different regions of the
magnetosphere depend on the angle between the magnetic and rotation axes and
influence the conal type, as observed. The requirement of intermittent weak
pair-production in an outergap suggests a natural evolutionary link between
radio and gamma-ray pulsars.Comment: 17 pages 8 figure
The effect of magnetic fields on star cluster formation
We examine the effect of magnetic fields on star cluster formation by
performing simulations following the self-gravitating collapse of a turbulent
molecular cloud to form stars in ideal MHD. The collapse of the cloud is
computed for global mass-to-flux ratios of infinity, 20, 10, 5 and 3, that is
using both weak and strong magnetic fields. Whilst even at very low strengths
the magnetic field is able to significantly influence the star formation
process, for magnetic fields with plasma beta < 1 the results are substantially
different to the hydrodynamic case. In these cases we find large-scale
magnetically-supported voids imprinted in the cloud structure; anisotropic
turbulent motions and column density structure aligned with the magnetic field
lines, both of which have recently been observed in the Taurus molecular cloud.
We also find strongly suppressed accretion in the magnetised runs, leading to
up to a 75% reduction in the amount of mass converted into stars over the
course of the calculations and a more quiescent mode of star formation. There
is also some indication that the relative formation efficiency of brown dwarfs
is lower in the strongly magnetised runs due to the reduction in the importance
of protostellar ejections.Comment: 16 pages, 9 figures, 8 very pretty movies, MNRAS, accepted. Version
with high-res figures + movies available from
http://www.astro.ex.ac.uk/people/dprice/pubs/mcluster/index.htm
Angular Momentum Transfer in Star-Discs Encounters: The Case of Low-Mass Discs
A prerequisite for the formation of stars and planetary systems is that
angular momentum is transported in some way from the inner regions of the
accretion disc. Tidal effects may play an important part in this angular
momentum transport. Here the angular momentum transfer in an star-disc
encounter is investigated numerically for a variety of encounter parameters in
the case of low mass discs. Although good agreement is found with analytical
results for the entire disc, the loss {\it inside} the disc can be up to an
order of magnitude higher than previously assumed. The differences in angular
momentum transport by secondaries on a hyperbolic, parabolic and elliptical
path are shown, and it is found that a succession of distant encounters might
be equally, if not more, successful in removing angular momentum than single
close encounter.Comment: 11pages, 8 figures, 1 tabl
Evolutionary calculations of phase separation in crystallizing white dwarf stars
We present an exploration of the significance of Carbon/Oxygen phase
separation in white dwarf stars in the context of self-consistent evolutionary
calculations. Because phase separation can potentially increase the calculated
ages of the oldest white dwarfs, it can affect the age of the Galactic disk as
derived from the downturn in the white dwarf luminosity function. We find that
the largest possible increase in ages due to phase separation is 1.5 Gyr, with
a most likely value of approximately 0.6 Gyr, depending on the parameters of
our white dwarf models.
The most important factors influencing the size of this delay are the total
stellar mass, the initial composition profile, and the phase diagram assumed
for crystallization. We find a maximum age delay in models with masses of 0.6
solar masses, which is near the peak in the observed white dwarf mass
distribution. We find that varying the opacities (via the metallicity) has
little effect on the calculated age delays.
In the context of Galactic evolution, age estimates for the oldest Galactic
globular clusters range from 11.5 to 16 Gyr, and depend on a variety of
parameters. In addition, a 4 to 6 Gyr delay is expected between the formation
of the globular clusters and that of the Galactic thin disk, while the observed
white dwarf luminosity function gives an age estimate for the thin disk of 9.5
+/-1.0 Gyr, without including the effect of phase separation. Using the above
numbers, we see that phase separation could add between 0 to 3 Gyr to the white
dwarf ages and still be consistent with the overall picture of Galaxy
formation. Our calculated maximum value of 1.5 Gyr fits within these bounds, as
does our best guess value of 0.6 Gyr.Comment: 13 total pages, 8 figures, 3 tables, accepted for publication in the
Astrophysical Journal on May 25, 199
On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: I- Analytical Approach
In the presence of rotation-induced mixing, element diffusion still occurs in
stellar radiative zones, although at a slower rate than in the case of a
complete stability of the stellar gas. As a consequence, helium settling leads
to vertical -gradients which, due to the meridional
circulation, turn into horizontal fluctuations. Up to now, the feed-back effect
of this process on the rotation-induced mixing was currently neglected in the
computations of abundance variations in stellar surfaces, or artificially
reduced. Here we analyse its consequences and derive an approximate analytical
solution in a quasi-stationary case. We also discuss the relative importance of
the various physical effects which influence the meridional circulation
velocity. In a second paper (Th\'eado and Vauclair 2002a), we will present a
complete 2D numerical simulation of this process while a third paper (Th\'eado
and Vauclair 2002b) will be devoted to special applications to Pop I stars.Comment: 7 printed pages, 1 figure. ApJ, in press (April 20, 2003
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