101,026 research outputs found
No Pulsar Kicks from Deformed Neutrinospheres
In a supernova core, magnetic fields cause a directional variation of the
neutrino refractive index so that resonant flavor oscillations would lead to a
deformation of the "neutrinosphere" for, say, tau neutrinos. The associated
anisotropic neutrino emission was proposed as a possible origin of the observed
pulsar proper motions. We argue that this effect was vastly overestimated
because the variation of the temperature over the deformed neutrinosphere is
not an adequate measure for the anisotropy of neutrino emission. The neutrino
flux is generated inside the neutron star core and is transported through the
atmosphere at a constant luminosity, forcing the temperature gradient in the
atmosphere to adjust to the inflow of energy from below. Therefore, no emission
anisotropy is caused by a deformation of the neutrinosphere to lowest order. An
estimate of the higher-order corrections must take into account the modified
atmospheric temperature profile in response to the deformation of the
neutrinosphere and the corresponding feedback on the core. We go through this
exercise in the framework of a simplified model which can be solved
analytically.Comment: Final version with minor corrections, to be published in PRD.
Includes a "Note Added" in response to astro-ph/981114
Towards a Stable Numerical Evolution of Strongly Gravitating Systems in General Relativity: The Conformal Treatments
We study the stability of three-dimensional numerical evolutions of the
Einstein equations, comparing the standard ADM formulation to variations on a
family of formulations that separate out the conformal and traceless parts of
the system. We develop an implementation of the conformal-traceless (CT)
approach that has improved stability properties in evolving weak and strong
gravitational fields, and for both vacuum and spacetimes with active coupling
to matter sources. Cases studied include weak and strong gravitational wave
packets, black holes, boson stars and neutron stars. We show under what
conditions the CT approach gives better results in 3D numerical evolutions
compared to the ADM formulation. In particular, we show that our implementation
of the CT approach gives more long term stable evolutions than ADM in all the
cases studied, but is less accurate in the short term for the range of
resolutions used in our 3D simulations.Comment: 17 pages, 15 figures. Small changes in the text, and a change in the
list of authors. One new reference adde
Mass estimation in the outer regions of galaxy clusters
We present a technique for estimating the mass in the outskirts of galaxy
clusters where the usual assumption of dynamical equilibrium is not valid. The
method assumes that clusters form through hierarchical clustering and requires
only galaxy redshifts and positions on the sky. We apply the method to
dissipationless cosmological N-body simulations where galaxies form and evolve
according to semi-analytic modelling. The method recovers the actual cluster
mass profile within a factor of two to several megaparsecs from the cluster
centre. This error originates from projection effects, sparse sampling, and
contamination by foreground and background galaxies. In the absence of velocity
biases, this method can provide an estimate of the mass-to-light ratio on
scales ~1-10 Mpc/h where this quantity is still poorly known.Comment: 14 pages, 7 figures, MN LaTeX style, MNRAS, in pres
On the Microlensing Optical Depth of the Galactic Bar
The microlensing probability (optical depth ) toward the Galactic
center carries information about the mass distribution of the Galactic
bulge/bar, so can be used to constrain the very uncertain shape parameters of
the bar. We find depends on the bar mass, radial profile, angle, axis
scale lengths and boxyness by a few simple analytical formulae, which shows:
(1) is proportional to the mass of the bar, . (2) falls along
the minor axis with a strong gradient. (3) An oblate bulge can have more
optical depth than a triaxial bar if the bar angle degress. (4)
is the largest if the angle and the axis ratio
conspires so that . (5) At a fixed field on the minor axis
but away from the center, boxy bars with a flat density profile tend to give a
larger optical depth than ellipsoidal bars with a steep profile. (6) Main
sequence sources should have a significantly lower (20-50\% lower) optical
depth than red clump giants if main sequence stars are not observed as deep as
the bright clump giants.
An application to four COBE-constrained models (Dwek et al. 1994) shows most
models produce optical depth lower than MACHO and OGLE observed
values even with both a massive bar and a full
disk. The high argues for a massive () boxy
bar with and deg and with a flat radial
profile up to corotation.Comment: 28 pages including 6 postscript figures in uuencoded compressed tar
file. Submitted to MNRA
Welfare spending and ethnic heterogeneity: Evidence from a massive immigration wave
The aim of this paper is to examine the relationship between ethnic heterogeneity and redistribution, by using the recent and massive arrival of immigrants in Spain. Specifically, we focus on the effect of changes in immigrant density, recorded between 1998 and 2006, on contemporaneous changes in municipal welfare spending. We instrument for immigrant density using established settlement patterns per country of origin so as to assign predicted flows of immigrants to municipalities. We find that welfare spending increased less in those municipalities that recorded the largest increases in immigrant density. We also provide evidence of a positive relationship between immigrant density and the vote share accruing to right-wing parties. Hence, our results are consistent with theories that predict a negative relationship between ethnic heterogeneity and redistribution.welfare spending, ethnic heterogeneity, redistribution, immigration
Dynamics of Primordial Black Hole Formation
We present a numerical investigation of the gravitational collapse of
horizon-size density fluctuations to primordial black holes (PBHs) during the
radiation-dominated phase of the Early Universe. The collapse dynamics of three
different families of initial perturbation shapes, imposed at the time of
horizon crossing, is computed. The perturbation threshold for black hole
formation, needed for estimations of the cosmological PBH mass function, is
found to be rather than the generally employed
, if is defined as \Delta M/\mh, the
relative excess mass within the initial horizon volume. In order to study the
accretion onto the newly formed black holes, we use a numerical scheme that
allows us to follow the evolution for long times after formation of the event
horizon. In general, small black holes (compared to the horizon mass at the
onset of the collapse) give rise to a fluid bounce that effectively shuts off
accretion onto the black hole, while large ones do not. In both cases, the
growth of the black hole mass owing to accretion is insignificant. Furthermore,
the scaling of black hole mass with distance from the formation threshold,
known to occur in near-critical gravitational collapse, is demonstrated to
apply to primordial black hole formation.Comment: 10 pages, 8 figures, revtex style, submitted to PR
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