5,270 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
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
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
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
Microwave polarization in the direction of galaxy clusters induced by the CMB quadrupole anisotropy
Electron scattering induces a polarization in the cosmic microwave background
(CMB) signal measured in the direction of a galaxy cluster due to the presence
of a quadrupole component in the CMB temperature distribution. Measuring the
polarization towards distant clusters provides the unique opportunity to
observe the evolution of the CMB quadrupole at moderate redshifts, z~0.5-3. We
demonstrate that for the local cluster population the polarization degree will
depend on the cluster celestial position. There are two extended regions in the
sky, which are opposite to each other, where the polarization is maximal,
0.1(tau/0.02) microK in the Rayleigh-Jeans part of the CMB spectrum (tau being
the Thomson optical depth across the cluster) exceeding the contribution from
the cluster transverse peculiar motion if v_t<1300 km/s. One can hope to detect
this small signal by measuring a large number of clusters, thereby effectively
removing the systematic contribution from other polarization components
produced in clusters. These polarization effects, which are of the order of
(v_t/c)^2 tau, (v_t/c) tau^2 and (kT_e/m_ec^2) tau^2, as well as the
polarization due to the CMB quadrupole, were previously calculated by Sunyaev
and Zel'dovich for the Rayleigh-Jeans region. We fully confirm their earlier
results and present exact frequency dependencies for all these effects. The
polarization is considerably higher in the Wien region of the CMB spectrum.Comment: 8 pages, 5 figures, submitted to MNRA
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
The Correlation Function of Clusters of Galaxies and the Amplitude of Mass Fluctuations in the Universe
We show that if a sample of galaxy clusters is complete above some mass
threshold, then hierarchical clustering theories for structure formation
predict its autocorrelation function to be determined purely by the cluster
abundance and by the spectrum of linear density fluctuations. Thus if the shape
of the initial fluctuation spectrum is known, its amplitude can be
estimated directly from the correlation length of a cluster sample in a way
which is independent of the value of . If the cluster mass
corresponding to the sample threshold is also known, it provides an independent
estimate of the quantity . Thus cluster data should
allow both and to be determined observationally. We
explore these questions using N-body simulations together with a simple but
accurate analytical model based on extensions of Press-Schechter theory.
Applying our results to currently available data we find that if the linear
fluctuation spectrum has a shape similar to that suggested by the APM galaxy
survey, then a correlation length in excess of 20\mpch for Abell
clusters would require , while r_0<15\mpch would require
. With conventional estimates of the relevant mass threshold
these imply \Omega_0\la 0.3 and \Omega_0\ga 1 respectively.Comment: Latex, 25 pages (including 8 PS figures). The PS-file of the paper is
also available via anonymous ftp at:
ftp://ibm-3.mpa-garching.mpg.de/pub/jing/xicc.ps . Submitted to MNRAS. In the
replaced version, a typo in Eq.(1a) is fixe
Three-dimensional simulations of type Ia supernovae
We present the results of three-dimensional hydrodynamical simulations of the
subsonic thermonuclear burning phase in type Ia supernovae. The burning front
model contains no adjustable parameters so that variations of the explosion
outcome can be linked directly to changes in the initial conditions. In
particular, we investigate the influence of the initial flame geometry on the
explosion energy and find that it appears to be weaker than in 2D. Most
importantly, our models predict global properties such as the produced nickel
masses and ejecta velocities within their observed ranges without any fine
tuning.Comment: 7 pages, 5 figures, accepted by A&
Unbiased Reconstruction of the Large Scale Structure
We present a new Unbiased Minimal Variance (UMV) estimator for the purpose of
reconstructing the large--scale structure of the universe from noisy, sparse
and incomplete data. Similar to the Wiener Filter (WF), the UMV estimator is
derived by requiring the linear minimal variance solution given the data and an
assumed prior model specifying the underlying field covariance matrix. However,
unlike the WF, the minimization is carried out with the added constraint of an
unbiased reconstructed mean field. The new estimator does not necessitate a
noise model to estimate the underlying field; however, such a model is required
for evaluating the errors at each point in space. The general application of
the UMV estimator is to predict the values of the reconstructed field in
un-sampled regions of space (e.g., interpolation in the unobserved Zone of
Avoidance), and to dynamically transform from one measured field to another
(e.g., inversion of radial peculiar velocities to over-densities). Here, we
provide two very simple applications of the method. The first, is to recover a
1D signal from noisy, convolved data with gaps, e.g., CMB time-ordered data.
The second application is a reconstruction of the density and 3D peculiar
velocity fields from mock SEcat galaxy peculiar velocity catalogs.Comment: Revised version with new section and figures. To appear in MNRA
An analytical model for the non-linear redshift-space power spectrum
We use N-body simulations to test the predictions of the redshift distortion
in the power spectrum given by the halo model in which the clustering of dark
matter particles is considered as a result both of the clustering of dark halos
in space and of the distribution of dark matter particles in individual dark
halo. The predicted redshift distortion depends sensitively on several model
parameters in a way different from the real-space power spectrum. An accurate
model of the redshift distortion can be constructed if the following properties
of the halo population are modelled accurately: the mass function of dark
halos, the velocity dispersion among dark halos, and the non-linear nature of
halo bias on small scales. The model can be readily applied to interpreting the
clustering properties and velocity dispersion of different populations of
galaxies once a cluster-weighted bias (or equivalently an halo occupation
number model) is specified for the galaxies. Some non-trivial bias features
observed from redshift surveys of optical galaxies and of IRAS galaxies
relative to the standard low-density cold dark matter model can be easily
explained in the cluster weighted bias model. The halo model further indicates
that a linear bias can be a good approximation only on for k <= 0.1 hMpc^{-1}.Comment: 10 pages, 10 figures, accepted for publication in MNRA
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