60 research outputs found
Hydrodynamical Non-radiative Accretion Flows in Two-Dimensions
Two-dimensional (axially symmetric) numerical hydrodynamical calculations of
accretion flows which cannot cool through emission of radiation are presented.
The calculations begin from an equilibrium configuration consisting of a thick
torus with constant specific angular momentum. Accretion is induced by the
addition of a small anomalous azimuthal shear stress which is characterized by
a function \nu. We study the flows generated as the amplitude and form of \nu
are varied. A spherical polar grid which spans more than two orders of
magnitude in radius is used to resolve the flow over a wide range of spatial
scales. We find that convection in the inner regions produces significant
outward mass motions that carry away both the energy liberated by, and a large
fraction of the mass participating in, the accretion flow. Although the
instantaneous structure of the flow is complex and dominated by convective
eddies, long time averages of the dynamical variables show remarkable
correspondence to certain steady-state solutions. Near the equatorial plane,
the radial profiles of the time-averaged variables are power-laws with an index
that depends on the radial scaling of the shear stress. We find that regardless
of the adiabatic index of the gas, or the form or magnitude of the shear
stress, the mass inflow rate is a strongly increasing function of radius, and
is everywhere nearly exactly balanced by mass outflow. The net mass accretion
rate through the disc is only a fraction of the rate at which mass is supplied
to the inflow at large radii, and is given by the local, viscous accretion rate
associated with the flow properties near the central object.Comment: 33 pages, 12 figures, accepted by MNRA
Design and Manufacture of the Superconducting Bus-bars for the LHC Main Magnets
The main magnets of the LHC are series-connected electrically in different powering circuits by means of superconducting bus-bars, carrying a maximum current of 13 kA. These superconducting bus-bars consist of a superconducting cable thermally and electrically coupled to a copper profile all along the length. The function of the copper profile is essentially to provide an alternative path for the current in case the superconducting cable loses its superconducting state and returns to normal state because of a transient disturbance or of a normal zone propagation coming from the neighbouring magnets. When a superconducting bus-bar quenches to normal state its temperature must always stay below a safe values of about 100°C while the copper is conducting. When a resistive transition is detected, the protection systems triggers the ramping down of the current from 13000 A to 0. The ramp rate must not exceed a maximum value to avoid the transition of magnets series-connected in the circuit. This paper concerns the design and the manufacture of the high current superconducting bus-bars needed to interconnect the magnetic elements of the main dipoles, the main quadrupoles of the arcs and of the dispersion suppressors of the LHC
Diffusive Nuclear Burning on Neutron Star Envelopes
We calculate the rate of hydrogen burning for neutron stars (NSs) with
hydrogen atmospheres and an underlying reservoir of nuclei capable of proton
capture. This burning occurs in the exponentially suppressed diffusive tail of
H that extends to the hotter depths of the envelope where protons are rapidly
captured. This process, which we call diffusive nuclear burning (DNB), can
change the H abundance at the NS photosphere on timescales as short as
years. In the absence of diffusion, the hydrogen at the photosphere
(where and ) would last
for far longer than a Hubble time. Our work impacts the understanding of the
evolution of surface abundances of isolated NSs, which is important to their
thermal spectrum and their effective temperature-core temperature relation. In
this paper, we calculate the rate of H burning when the overall consumption
rate is controlled by the nuclear timescales, rather than diffusion timescales.
The immediate application is for H burning on millisecond radio pulsars and in
quiescence for the accreting NS Cen X-4. We will apply this work to young radio
pulsars and magnetars once we have incorporated the effects of strong
magnetic fields.Comment: 18 pages, 8 figures, accepted for publication by Ap
Large scale magnetic fields in viscous resistive accretion disks. I. Ejection from weakly magnetized disks
Cold steady-state disk wind theory from near Keplerian accretion disks
requires a large scale magnetic field at near equipartition strength. However
the minimum magnetization has never been tested. We investigate the time
evolution of an accretion disk threaded by a weak vertical magnetic field. The
strength of the field is such that the disk magnetization falls off rapidly
with radius. Four 2.5D numerical simulations of viscous resistive accretion
disk are performed using the magnetohydrodynamic code PLUTO. In these
simulations, a mean field approach is used and turbulence is assumed to give
rise to anomalous transport coefficients (alpha prescription). The large scale
magnetic field introduces only a small perturbation to the disk structure, with
accretion driven by the dominant viscous torque. A super fast magnetosonic jet
is observed to be launched from the innermost regions and remains stationary
over more than 953 Keplerian orbits. The self-confined jet is launched from a
finite radial zone in the disk which remains constant over time. Ejection is
made possible because the magnetization reaches unity at the disk surface, due
to the steep density decrease. However, no ejection is reported when the
midplane magnetization becomes too small. The asymptotic jet velocity remains
nevertheless too low to explain observed jets due to the negligible power
carried away by the jet. Astrophysical disks with superheated surface layers
could drive analogous outflows even if their midplane magnetization is low.
Sufficient angular momentum would be extracted by the turbulent viscosity to
allow the accretion process to continue. The magnetized outflows would be no
more than byproducts, rather than a fundamental driver of accretion. However,
if the midplane magnetization increases towards the center, a natural
transition to an inner jet dominated disk could be achieved.Comment: Accepted by Astronomy and Astrophysic
Bondi Accretion and the Problem of the Missing Isolated Neutron Stars
A large number of neutron stars (NSs), ~10^9, populate the Galaxy, but only a
tiny fraction of them is observable during the short radio pulsar lifetime. The
majority of these isolated NSs, too cold to be detectable by their own thermal
emission, should be visible in X-rays as a result of accretion from the
interstellar medium. The ROSAT all sky survey has however shown that such
accreting isolated NSs are very elusive: only a few tentative candidates have
been identified, contrary to theoretical predictions that up to several
thousands should be seen. We suggest that the fundamental reason for this
discrepancy lies in the use of the standard Bondi formula to estimate the
accretion rates. We compute the expected source counts using updated estimates
of the pulsar velocity distribution, realistic hydrogen atmosphere spectra, and
a modified expression for the Bondi accretion rate as suggested by recent MHD
simulations, and supported by direct observations in the case of accretion
around supermassive black holes in nearby galaxies and in our own. We find
that, whereas the inclusion of atmospheric spectra partly compensates for the
reduction in the counts due to the higher mean velocities of the new
distribution, the modified Bondi formula dramatically suppresses the source
counts. The new predictions are consistent with a null detection at the ROSAT
sensitivity.Comment: accepted to ApJ; 19 pages, 4 figure
Dust crystallinity in protoplanetary disks: the effect of diffusion/viscosity ratio
The process of turbulent radial mixing in protoplanetary disks has strong
relevance to the analysis of the spatial distribution of crystalline dust
species in disks around young stars and to studies of the composition of
meteorites and comets in our own solar system.
A debate has gone on in the recent literature on the ratio of the effective
viscosity coefficient (responsible for accretion) to the turbulent
diffusion coefficient (responsible for mixing). Numerical
magneto-hydrodynamic simulations have yielded values between
(Carballido, Stone & Pringle, 2005) and (Johansen & Klahr,
2005}). Here we present two analytic arguments for the ratio which
are based on elegant, though strongly simplified assumptions. We argue that
whichever of these numbers comes closest to reality may be determined {\em
observationally} by using spatially resolved mid-infrared measurements of
protoplanetary disks around Herbig stars. If meridional flows are present in
the disk, then we expect less abundance of crystalline dust in the surface
layers, a prediction which can likewise be observationally tested with
mid-infrared interferometers.Comment: 9 pages, 5 figures, accepted for publication in A&
Restrictions on parameters of power-law magnetic field decay for accreting isolated neutron stars
In this short note we discuss the influence of power-law magnetic field decay
on the evolution of old accreting isolated neutron stars. We show, that,
contrary to exponential field decay (Popov & Prokhorov 2000), no additional
restrictions can be made for the parameters of power-law decay from the
statistics of isolated neutron star candidates in ROSAT observations.
We also briefly discuss the fate of old magnetars with and without field
decay, and describe parameters of old accreting magnetars.Comment: 8 pages including 3 PostScript figure
Radial Flow of Dust Particles in Accretion Disks
We study the radial migration of dust particles in accreting protostellar
disks analogous to the primordial solar nebula. This study takes account of the
two dimensional (radial and normal) structure of the disk gas, including the
effects of the variation in the gas velocity as a function of distance from the
midplane. It is shown that the dust component of disks accretes slower than the
gas component. At high altitude from the disk midplane, the gas rotates faster
than particles because of the inward pressure gradient force, and its drag
force causes particles to move outward in the radial direction. Viscous torque
induces the gas within a scale height from the disk midplane to flow outward,
carrying small (size < 100 micron at 10 AU) particles with it. Only particles
at intermediate altitude or with sufficiently large sizes (> 1 mm at 10 AU)
move inward. When the particles' radial velocities are averaged over the entire
vertical direction, particles have a net inward flux. At large distances from
the central star, particles migrate inward with a velocity much faster than the
gas accretion velocity. However, their inward velocity is reduced below that of
the gas in the inner regions of the disk. The rate of velocity decrease is a
function of the particles' size. While larger particles retain fast accretion
velocity until they approach closer to the star, 10 micron particles have
slower velocity than the gas in the most part of the disk (r < 100 AU). This
differential migration of particles causes the size fractionation. Dust disks
composed mostly of small particles (size < 10 micron) accrete slower than gas
disks, resulting in the increase in the dust-gas ratio during the gas accretion
phase.Comment: ApJ, accepted, 17 pages, 14 figure
The Neutron Stars Census
The paucity of old isolated accreting neutron stars in ROSAT observations is
used to derive a lower limit on the mean velocity of neutron stars at birth.
The secular evolution of the population is simulated following the paths of a
statistical sample of stars for different values of the initial kick velocity,
drawn from an isotropic Gaussian distribution with mean velocity . The spin--down, induced by dipole losses and the
interaction with the ambient medium, is tracked together with the dynamical
evolution in the Galactic potential, allowing for the determination of the
fraction of stars which are, at present, in each of the four possible stages:
Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All Sky
Survey an upper limit of accreting neutron stars within pc
from the Sun, we infer a lower bound for the mean kick velocity, corresponding to a velocity dispersion
km s. The same conclusion is reached for both
a constant magnetic field ( G) and a magnetic field decaying
exponentially with a timescale yr. Such high velocities are
consistent with those derived from radio pulsar observations. Present results,
moreover, constrain the fraction of low velocity stars, which could have
escaped pulsar statistics, to less than 1%.Comment: 13 pages, 6 PostScript figures, accepted to Ap
Turning Points in the Evolution of Isolated Neutron Stars' Magnetic Fields
During the life of isolated neutron stars (NSs) their magnetic field passes
through a variety of evolutionary phases. Depending on its strength and
structure and on the physical state of the NS (e.g. cooling, rotation), the
field looks qualitatively and quantitatively different after each of these
phases. Three of them, the phase of MHD instabilities immediately after NS's
birth, the phase of fallback which may take place hours to months after NS's
birth, and the phase when strong temperature gradients may drive thermoelectric
instabilities, are concentrated in a period lasting from the end of the
proto--NS phase until 100, perhaps 1000 years, when the NS has become almost
isothermal. The further evolution of the magnetic field proceeds in general
inconspicuous since the star is in isolation. However, as soon as the product
of Larmor frequency and electron relaxation time, the so-called magnetization
parameter, locally and/or temporally considerably exceeds unity, phases, also
unstable ones, of dramatic changes of the field structure and magnitude can
appear. An overview is given about that field evolution phases, the outcome of
which makes a qualitative decision regarding the further evolution of the
magnetic field and its host NS.Comment: References updated, typos correcte
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