391 research outputs found
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
Effects of Rotation and Relativistic Charge Flow on Pulsar Magnetospheric Structure
We propose an analytical 3-D model of the open field-line region of a neutron
star (NS) magnetosphere. We construct an explicit analytic solution for
arbitrary obliquity (angle between the rotation and magnetic axes)
incorporating the effects of magnetospheric rotation, relativistic flow of
charges (e.g. primary electron beam) along the open field lines, and E X B
drift of these charges. Our solution employs the space-charge-limited
longitudinal current calculated in the electrodynamic model of Muslimov &
Tsygan (1992) and is valid up to very high altitudes nearly approaching the
light cylinder. We assume that in the innermost magnetosphere, the NS magnetic
field can be well represented by a static magnetic dipole configuration. At
high altitudes the open magnetic field lines significantly deviate from those
of a static dipole and tend to focus into a cylindrical bundle, swept back in
the direction opposite to the rotation, and also bent towards the rotational
equator. We briefly discuss some implications of our study to spin-powered
pulsars.Comment: 24 pages, 3 figures, accepted for publication in Ap
Magnetar Spindown, Hyper-Energetic Supernovae, and Gamma Ray Bursts
The Kelvin-Helmholtz cooling epoch, lasting tens of seconds after the birth
of a neutron star in a successful core-collapse supernova, is accompanied by a
neutrino-driven wind. For magnetar-strength ( G) large scale
surface magnetic fields, this outflow is magnetically-dominated during the
entire cooling epoch.Because the strong magnetic field forces the wind to
co-rotate with the protoneutron star,this outflow can significantly effect the
neutron star's early angular momentum evolution, as in analogous models of
stellar winds (e.g. Weber & Davis 1967). If the rotational energy is large in
comparison with the supernova energy and the spindown timescale is short with
respect to the time required for the supernova shockwave to traverse the
stellar progenitor, the energy extracted may modify the supernova shock
dynamics significantly. This effect is capable of producing hyper-energetic
supernovae and, in some cases, provides conditions favorable for gamma ray
bursts. We estimate spindown timescales for magnetized, rotating protoneutron
stars and construct steady-state models of neutrino-magnetocentrifugally driven
winds. We find that if magnetars are born rapidly rotating, with initial spin
periods () of millisecond, that of order erg of
rotational energy can be extracted in seconds. If magnetars are born
slowly rotating ( ms) they can spin down to periods of
second on the Kelvin-Helmholtz timescale.Comment: 16 pages, 5 figures, emulateap
Fragmentation Instability of Molecular Clouds: Numerical Simulations
We simulate fragmentation and gravitational collapse of cold, magnetized
molecular clouds. We explore the nonlinear development of an instability
mediated by ambipolar diffusion, in which the collapse rate is intermediate to
fast gravitational collapse and slow quasistatic collapse. Initially uniform
stable clouds fragment into elongated clumps with masses largely determined by
the cloud temperature, but substantially larger than the thermal Jeans mass.
The clumps are asymmetric, with significant rotation and vorticity, and lose
magnetic flux as they collapse. The clump shapes, intermediate collapse rates,
and infall profiles may help explain observations not easily fit by
contemporary slow or rapid collapse models.Comment: 25pp, 20 small eps figures, in press ApJ, April 1, 200
Charged Condensate and Helium Dwarf Stars
White dwarf stars composed of carbon, oxygen or heavier elements are expected
to crystallize as they cool down below certain temperatures. Yet, simple
arguments suggest that the helium white dwarf cores may not solidify, mostly
because of zero-point oscillations of the helium ions that would dissolve the
crystalline structure. We argue that the interior of the helium dwarfs may
instead form a macroscopic quantum state in which the charged helium-4 nuclei
are in a Bose-Einstein condensate, while the relativistic electrons form a
neutralizing degenerate Fermi liquid. We discuss the electric charge screening,
and the spectrum of this substance, showing that the bosonic long-wavelength
fluctuations exhibit a mass gap. Hence, there is a suppression at low
temperatures of the boson contribution to the specific heat -- the latter being
dominated by the specific heat of the electrons near the Fermi surface. This
state of matter may have observational signatures.Comment: 10 pages; v2: to appear in JCAP, brief comments and section titles
added, typos correcte
Optimal battery charge/discharge strategies for prosumers and suppliers
We discuss the application of classical variational methods to optimal charging/discharging strategies for a prosumer or storage supplier, where the price of electrical power is known in advance. We outline how a classical calculus of variations approach can be applied to two related problems: (i) how can a prosumer minimise the cost of charging/discharging a battery, when the price of electrical power is known throughout the charging/discharging period? and (ii) how can an electricity supplier incentivise desired prosumer/storage supplier behaviour by adjusting the price
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
The Maximum Optical Depth Towards Bulge Stars From Axisymmetric Models of the Milky Way
It has been known that recent microlensing results towards the bulge imply
mass densities that are surprisingly high given dynamical constraints on the
Milky Way mass distribution. We derive the maximum optical depth towards the
bulge that may be generated by axisymmetric structures in the Milky Way, and
show that observations are close to surpassing these limits. This result argues
in favor of a bar as a source of significantly enhanced microlensing. Several
of the bar models in the literature are discussed.Comment: Latex, 6 pages, 4 figures, uses aas2pp4 and epsf style files.
Accepted for publication in ApJ Letter
Cluster-assisted accretion for massive stars
Gravitational interactions in very young high-density stellar clusters can to
some degree change the angular momentum in the circumstellar discs surrounding
initially the majority of stars. However, for most stars the cluster
environment alters the angular momentum only slightly. For example, in
simulations of the Orion Nebula cluster (ONC) encounters reduce the angular
momentum of the discs on average at most by 3-5% and in the higher density
region of the Trapezium %where encounters are more likely, the disc angular
momentum is on average lowered by 15-20% - still a minor loss process. However,
in this paper it is demonstrated that the situation is very different if one
considers high-mass stars (M* > 10 M(solar) only. Assuming an age of 2 Myr for
the ONC, their discs have on average a 50-90% lower angular momentum than
primordially. This enormous loss in angular momentum in the disc should result
in an equivalent increase in accretion, implying that the cluster environment
boosts accretion for high-mass stars, thus %in the cluster center, making them
even more massive.Comment: 10 pages including 2 figures, accepted for publication in ApJ
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
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