1,048 research outputs found
Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields
As a massive star evolves through multiple stages of nuclear burning on its
way to becoming a supernova, a complex, differentially rotating structure is
set up. Angular momentum is transported by a variety of classic instabilities,
and also by magnetic torques from fields generated by the differential
rotation. We present the first stellar evolution calculations to follow the
evolution of rotating massive stars including, at least approximately, all
these effects, magnetic and non-magnetic, from the zero-age main sequence until
the onset of iron-core collapse. The evolution and action of the magnetic
fields is as described by Spruit 2002 and a range of uncertain parameters is
explored. In general, we find that magnetic torques decrease the final rotation
rate of the collapsing iron core by about a factor of 30 to 50 when compared
with the non-magnetic counterparts. Angular momentum in that part of the
presupernova star destined to become a neutron star is an increasing function
of main sequence mass. That is, pulsars derived from more massive stars will
rotate faster and rotation will play a more dominant role in the star's
explosion. The final angular momentum of the core is determined - to within a
factor of two - by the time the star ignites carbon burning. For the lighter
stars studied, around 15 solar masses, we predict pulsar periods at birth near
15 ms, though a factor of two range is easily tolerated by the uncertainties.
Several mechanisms for additional braking in a young neutron star, especially
by fall back, are also explored.Comment: 32 pages, 3 figures (8 eps files), submitted to Ap
Differential Rotation in Neutron Stars: Magnetic Braking and Viscous Damping
Diffferentially rotating stars can support significantly more mass in
equilibrium than nonrotating or uniformly rotating stars, according to general
relativity. The remnant of a binary neutron star merger may give rise to such a
``hypermassive'' object. While such a star may be dynamically stable against
gravitational collapse and bar formation, the radial stabilization due to
differential rotation is likely to be temporary. Magnetic braking and viscosity
combine to drive the star to uniform rotation, even if the seed magnetic field
and the viscosity are small. This process inevitably leads to delayed collapse,
which will be accompanied by a delayed gravitational wave burst and, possibly,
a gamma-ray burst. We provide a simple, Newtonian, MHD calculation of the
braking of differential rotation by magnetic fields and viscosity. The star is
idealized as a differentially rotating, infinite cylinder consisting of a
homogeneous, incompressible conducting gas. We solve analytically the simplest
case in which the gas has no viscosity and the star resides in an exterior
vacuum. We treat numerically cases in which the gas has internal viscosity and
the star is embedded in an exterior, low-density, conducting medium. Our
evolution calculations are presented to stimulate more realistic MHD
simulations in full 3+1 general relativity. They serve to identify some of the
key physical and numerical parameters, scaling behavior and competing
timescales that characterize this important process.Comment: 11 pages. To appear in ApJ (November 20, 2000
Solar differential rotation and meridional flow: The role of a subadiabatic tachocline for the Taylor-Proudman balance
We present a simple model for the solar differential rotation and meridional
circulation based on a mean field parameterization of the Reynolds stresses
that drive the differential rotation. We include the subadiabatic part of the
tachocline and show that this, in conjunction with turbulent heat conductivity
within the convection zone and overshoot region, provides the key physics to
break the Taylor-Proudman constraint, which dictates differential rotation with
contour lines parallel to the axis of rotation in case of an isentropic
stratification. We show that differential rotation with contour lines inclined
by 10 - 30 degrees with respect to the axis of rotation is a robust result of
the model, which does not depend on the details of the Reynolds stress and the
assumed viscosity, as long as the Reynolds stress transports angular momentum
toward the equator. The meridional flow is more sensitive with respect to the
details of the assumed Reynolds stress, but a flow cell, equatorward at the
base of the convection zone and poleward in the upper half of the convection
zone, is the preferred flow pattern.Comment: 15 pages, 7 figure
Magnetic fields generated by r-modes in accreting millisecond pulsars
In millisecond pulsars the existence of the Coriolis force allows the
development of the so-called Rossby oscillations (r-modes) which are know to be
unstable to emission of gravitational waves. These instabilities are mainly
damped by the viscosity of the star or by the existence of a strong magnetic
field. A fraction of the observed millisecond pulsars are known to be inside
Low Mass X-ray Binaries (LMXBs), systems in which a neutron star (or a black
hole) is accreting from a donor whose mass is smaller than 1 . Here we
show that the r-mode instabilities can generate strong toroidal magnetic fields
by inducing differential rotation. In this way we also provide an alternative
scenario for the origin of the magnetars.Comment: 6 pages, 3 figures, Proceedings conference "Theoretical Nuclear
Physics", Cortona October 200
Generation of strong magnetic fields by r-modes in millisecond accreting neutron stars: induced deformations and gravitational wave emission
Differential rotation induced by the r-mode instability can generate very
strong toroidal fields in the core of accreting, millisecond spinning neutron
stars. We introduce explicitly the magnetic damping term in the evolution
equations of the r-modes and solve them numerically in the Newtonian limit, to
follow the development and growth of the internal magnetic field. We show that
the strength of the latter can reach large values, G, in the
core of the fastest accreting neutron stars. This is strong enough to induce a
significant quadrupole moment of the neutron star mass distribution,
corresponding to an ellipticity |\epsilon_B}| \sim 10^{-8}. If the symmetry
axis of the induced magnetic field is not aligned with the spin axis, the
neutron star radiates gravitational waves. We suggest that this mechanism may
explain the upper limit of the spin frequencies observed in accreting neutron
stars in Low Mass X-Ray Binaries. We discuss the relevance of our results for
the search of gravitational waves.Comment: 11 pages, 8 figure
General Relativistic Rossby-Haurwitz waves of a slowly and differentially rotating fluid shell
We show that, at first order in the angular velocity, the general
relativistic description of Rossby-Haurwitz waves (the analogues of r-waves on
a thin shell) can be obtained from the corresponding Newtonian one after a
coordinate transformation. As an application, we show that the results recently
obtained by Rezzolla and Yoshida (2001) in the analysis of Newtonian
Rossby-Haurwitz waves of a slowly and differentially rotating, fluid shell
apply also in General Relativity, at first order in the angular velocity.Comment: 4 pages. Comment to Class. Quantum Grav. 18(2001)L8
A new model for mixing by double-diffusive convection (semi-convection): I. The conditions for layer formation
The process referred to as "semi-convection" in astrophysics and
"double-diffusive convection in the diffusive regime" in Earth and planetary
sciences, occurs in stellar and planetary interiors in regions which are stable
according to the Ledoux criterion but unstable according to the Schwarzschild
criterion. In this series of papers, we analyze the results of an extensive
suite of 3D numerical simulations of the process, and ultimately propose a new
1D prescription for heat and compositional transport in this regime which can
be used in stellar or planetary structure and evolution models.
In a preliminary study of the phenomenon, Rosenblum et al. (2011) showed
that, after saturation of the primary instability, a system can evolve in one
of two possible ways: the induced turbulence either remains homogeneous, with
very weak transport properties, or transitions into a thermo-compositional
staircase where the transport rate is much larger (albeit still smaller than in
standard convection).
In this paper, we show that this dichotomous behavior is a robust property of
semi-convection across a wide region of parameter space. We propose a simple
semi-analytical criterion to determine whether layer formation is expected or
not, and at what rate it proceeds, as a function of the background
stratification and of the diffusion parameters (viscosity, thermal diffusivity
and compositional diffusivity) only. The theoretical criterion matches the
outcome of our numerical simulations very adequately in the numerically
accessible "planetary" parameter regime, and can easily be extrapolated to the
stellar parameter regime.
Subsequent papers will address more specifically the question of quantifying
transport in the layered case and in the non-layered case.Comment: Submitted to Ap
Comparison of the thin flux tube approximation with 3D MHD simulations
The structure and dynamics of small vertical photospheric magnetic flux
concentrations has been often treated in the framework of an approximation
based upon a low-order truncation of the Taylor expansions of all quantities in
the horizontal direction, together with the assumption of instantaneous total
pressure balance at the boundary to the non-magnetic external medium. Formally,
such an approximation is justified if the diameter of the structure (a flux
tube or a flux sheet) is small compared to all other relevant length scales
(scale height, radius of curvature, wavelength, etc.). The advent of realistic
3D radiative MHD simulations opens the possibility of checking the consistency
of the approximation with the properties of the flux concentrations that form
in the course of a simulation.
We carry out a comparative analysis between the thin flux tube/sheet models
and flux concentrations formed in a 3D radiation-MHD simulation. We compare the
distribution of the vertical and horizontal components of the magnetic field in
a 3D MHD simulation with the field distribution in the case of the thin flux
tube/sheet approximation. We also consider the total (gas plus magnetic)
pressure in the MHD simulation box. Flux concentrations with
super-equipartition fields are reasonably well reproduced by the second-order
thin flux tube/sheet approximation. The differences between approximation and
simulation are due to the asymmetry and the dynamics of the simulated
structures
Properties of r modes in rotating magnetic neutron stars. II. Evolution of the r modes and stellar magnetic field
The evolution of the r-mode instability is likely to be accompanied by
secular kinematic effects which will produce differential rotation with large
scale drifts of fluid elements, mostly in the azimuthal direction. As first
discussed by Rezzolla, Lamb and Shapiro 2000, the interaction of these secular
velocity fields with a pre-existing neutron star magnetic field could result in
the generation of intense and large scale toroidal fields. Following their
derivation in the companion paper, we here discuss the numerical solution of
the evolution equations for the magnetic field. The values of the magnetic
fields obtained in this way are used to estimate the conditions under which the
r-mode instability might be prevented or suppressed. We also assess the impact
of the generation of large magnetic fields on the gravitational wave
detectability of r-mode unstable neutron stars. Our results indicate that the
signal to noise ratio in the detection of gravitational waves from the r-mode
instability might be considerably decreased if the latter develops in neutron
stars with initial magnetic fields larger than 10^10 G.Comment: 16 pages, 12 figures. To appear in Phys. Rev.
Relationships between magnetic foot points and G-band bright structures
Magnetic elements are thought to be described by flux tube models, and are
well reproduced by MHD simulations. However, these simulations are only
partially constrained by observations. We observationally investigate the
relationship between G-band bright points and magnetic structures to clarify
conditions, which make magnetic structures bright in G-band. The G-band
filtergrams together with magnetograms and dopplergrams were taken for a plage
region covered by abnormal granules as well as ubiquitous G-band bright points,
using the Swedish 1-m Solar Telescope (SST) under very good seeing conditions.
High magnetic flux density regions are not necessarily associated with G-band
bright points. We refer to the observed extended areas with high magnetic flux
density as magnetic islands to separate them from magnetic elements. We
discover that G-band bright points tend to be located near the boundary of such
magnetic islands. The concentration of G-band bright points decreases with
inward distance from the boundary of the magnetic islands. Moreover, G-band
bright points are preferentially located where magnetic flux density is higher,
given the same distance from the boundary. There are some bright points located
far inside the magnetic islands. Such bright points have higher minimum
magnetic flux density at the larger inward distance from the boundary.
Convective velocity is apparently reduced for such high magnetic flux density
regions regardless of whether they are populated by G-band bright points or
not. The magnetic islands are surrounded by downflows.These results suggest
that high magnetic flux density, as well as efficient heat transport from the
sides or beneath, are required to make magnetic elements bright in G-band.Comment: 9 pages, 14 figures, accepted for publication in A&
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