1,805 research outputs found
Numerical experiments of adjusted BSSN systems for controlling constraint violations
We present our numerical comparisons between the BSSN formulation widely used
in numerical relativity today and its adjusted versions using constraints. We
performed three testbeds: gauge-wave, linear wave, and Gowdy-wave tests,
proposed by the Mexico workshop on the formulation problem of the Einstein
equations. We tried three kinds of adjustments, which were previously proposed
from the analysis of the constraint propagation equations, and investigated how
they improve the accuracy and stability of evolutions. We observed that the
signature of the proposed Lagrange multipliers are always right and the
adjustments improve the convergence and stability of the simulations. When the
original BSSN system already shows satisfactory good evolutions (e.g., linear
wave test), the adjusted versions also coincide with those evolutions; while in
some cases (e.g., gauge-wave or Gowdy-wave tests) the simulations using the
adjusted systems last 10 times as long as those using the original BSSN
equations. Our demonstrations imply a potential to construct a robust evolution
system against constraint violations even in highly dynamical situations.Comment: to be published in PR
Generation of scalar-tensor gravity effects in equilibrium state boson stars
Boson stars in zero-, one-, and two-node equilibrium states are modeled
numerically within the framework of Scalar-Tensor Gravity. The complex scalar
field is taken to be both massive and self-interacting. Configurations are
formed in the case of a linear gravitational scalar coupling (the Brans-Dicke
case) and a quadratic coupling which has been used previously in a cosmological
context. The coupling parameters and asymptotic value for the gravitational
scalar field are chosen so that the known observational constraints on
Scalar-Tensor Gravity are satisfied. It is found that the constraints are so
restrictive that the field equations of General Relativity and Scalar-Tensor
gravity yield virtually identical solutions. We then use catastrophe theory to
determine the dynamically stable configurations. It is found that the maximum
mass allowed for a stable state in Scalar-Tensor gravity in the present
cosmological era is essentially unchanged from that of General Relativity. We
also construct boson star configurations appropriate to earlier cosmological
eras and find that the maximum mass for stable states is smaller than that
predicted by General Relativity, and the more so for earlier eras. However, our
results also show that if the cosmological era is early enough then only states
with positive binding energy can be constructed.Comment: 20 pages, RevTeX, 11 figures, to appear in Class. Quantum Grav.,
comments added, refs update
The Ultraviolet Radiation Environment Around M dwarf Exoplanet Host Stars
The spectral and temporal behavior of exoplanet host stars is a critical
input to models of the chemistry and evolution of planetary atmospheres. At
present, little observational or theoretical basis exists for understanding the
ultraviolet spectra of M dwarfs, despite their critical importance to
predicting and interpreting the spectra of potentially habitable planets as
they are obtained in the coming decades. Using observations from the Hubble
Space Telescope, we present a study of the UV radiation fields around nearby M
dwarf planet hosts that covers both FUV and NUV wavelengths. The combined
FUV+NUV spectra are publically available in machine-readable format. We find
that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ
832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition
region UV emission. No "UV quiet" M dwarfs are observed. The bright stellar
Ly-alpha emission lines are reconstructed, and we find that the Ly-alpha line
fluxes comprise ~37-75% of the total 1150-3100A flux from most M dwarfs; >
10^{3} times the solar value. The F(FUV)/F(NUV) flux ratio, a driver for
abiotic production of the suggested biomarkers O2 and O3, is shown to be ~0.5-3
for all M dwarfs in our sample, > 10^{3} times the solar ratio. For the four
stars with moderate signal-to-noise COS time-resolved spectra, we find UV
emission line variability with amplitudes of 50-500% on 10^{2} - 10^{3} s
timescales. Finally, we observe relatively bright H2 fluorescent emission from
four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832).
Additional modeling work is needed to differentiate between a stellar
photospheric or possible exoplanetary origin for the hot (T(H2) \approx
2000-4000 K) molecular gas observed in these objects.Comment: ApJ, accepted. 16 pages, 10 figures. On-line data at:
http://cos.colorado.edu/~kevinf/muscles.htm
P-Process Nucleosynthesis inside Supernova-Driven Supercritical Accretion Disks
We investigate p-process nucleosynthesis in a supercritical accretion disk
around a compact object of 1.4 M_solar, using the self-similar solution of an
optically thick advection dominated flow. Supercritical accretion is expected
to occur in a supernova with fallback material accreting onto a new-born
compact object. It is found that appreciable amounts of p-nuclei are
synthesized via the p-process in supernova-driven supercritical accretion disks
(SSADs) when the accretion rate m_dot = M_dot c^2/(16 L_Edd) >10^5, where L_Edd
is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs
have similar feature to those of the oxygen/neon layers in Type II supernovae
when the abundance of the fallback gas far from the compact object is that of
the oxygen/neon layers in the progenitor. The overall abundance profile is in
agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and
La, are underproduced in the SSADs as in Type II supernovae. If the fallback
gas is mixed with a small fraction of proton through Rayleigh-Taylor
instability during the explosion, significant amounts of Mo92 are produced
inside the SSADs. Ru96 and La138 are also produced when the fallback gas
contains abundant proton though the overall abundance profile of p-nuclei is
rather different from that of the solar system. The p-process nucleosynthesis
in SSADs contributes to chemical evolution of p-nuclei, in particular Mo92, if
several percents of fallback matter are ejected via jets and/or winds.Comment: 15 pages, 7 figures included, 3 tables, LaTeX emulateapj5.sty,
accepted for publication by the Astronomical Journal (March, 2003
Explosive Nucleosynthesis in Axisymmetrically Deformed Type II Supernovae
Explosive nucleosynthesis under the axisymmetric explosion in Type II
supernova has been performed by means of two dimensional hydrodynamical
calculations. We have compared the results with the observations of SN 1987A.
Our chief findings are as follows: (1) is synthesized so much as to
explain the tail of the bolometric light curve of SN 1987A. We think this is
because the alpha-rich freezeout takes place more actively under the
axisymmetric explosion. (2) and tend to be overproduced
compared with the observations. However, this tendency relies strongly on the
progenitor's model.
We have also compared the abundance of each element in the mass number range
with the solar values. We have found three outstanding features. (1)
For the nuclei in the range , their abundances are insensitive to the
initial form of the shock wave. This insensitivity is favored since the
spherical calculations thus far can explain the solar system abundances in this
mass range. (2) There is an enhancement around A=45 in the axisymmetric
explosion compared with the spherical explosion fairly well. In particular,
, which is underproduced in the present spherical calculations, is
enhanced significantly. (3) In addition, there is an enhancement around A=65.
This tendency does not rely on the form of the mass cut but of the initial
shock wave. This enhancement may be the problem of the overproduction in this
mass range, although this effect would be relatively small since Type I
supernovae are chiefly responsible for this mass number range.Comment: 32 pages, 12 figures, LaTe
Six-body Light-Front Tamm-Dancoff approximation and wave functions for the massive Schwinger model
The spectrum of the massive Schwinger model in the strong coupling region is
obtained by using the light-front Tamm-Dancoff (LFTD) approximation up to
including six-body states. We numerically confirm that the two-meson bound
state has a negligibly small six-body component. Emphasis is on the usefulness
of the information about states (wave functions). It is used for identifying
the three-meson bound state among the states below the three-meson threshold.
We also show that the two-meson bound state is well described by the wave
function of the relative motion.Comment: 19 pages, RevTeX, 7 figures are available upon request; Minor errors
have been corrected; Final version to appear in Phys.Rev.
Dynamics of the Light-Cone Zero Modes: Theta Vacuum of the Massive Schwinger Model
The massive Schwinger model is quantized on the light cone with great care on
the bosonic zero modes by putting the system in a finite (light-cone) spatial
box. The zero mode of survives Dirac's procedure for the constrained
system as a dynamical degree of freedom. After regularization and quantization,
we show that the physical space condition is consistently imposed and relates
the fermion Fock states to the zero mode of the gauge field. The vacuum is
obtained by solving a Schr\"odinger equation in a periodic potential, so that
the theta is understood as the Bloch momentum. We also construct a one-meson
state in the fermion-antifermion sector and obtained the Schr\"odinger equation
for it.Comment: 23 pages, RevTex, no figure
Dynamical evolution of boson stars in Brans-Dicke theory
We study the dynamics of a self-gravitating scalar field solitonic object
(boson star) in the Jordan-Brans-Dicke (BD) theory of gravity. We show
dynamical processes of this system such as (i) black hole formation of
perturbed equilibrium configuration on an unstable branch; (ii) migration of
perturbed equilibrium configuration from the unstable branch to stable branch;
(iii) transition from excited state to a ground state. We find that the
dynamical behavior of boson stars in BD theory is quite similar to that in
general relativity (GR), with comparable scalar wave emission. We also
demonstrate the formation of a stable boson star from a Gaussian scalar field
packet with flat gravitational scalar field initial data. This suggests that
boson stars can be formed in the BD theory in much the same way as in GR.Comment: 13 pages by RevTeX, epsf.sty, 16 figures, comments added, refs
updated, to appear in Phys. Rev.
Seismic topographic scattering in the context of GW detector site selection
In this paper, we present a calculation of seismic scattering from irregular
surface topography in the Born approximation. Based on US-wide topographic
data, we investigate topographic scattering at specific sites to demonstrate
its impact on Newtonian-noise estimation and subtraction for future
gravitational-wave detectors. We find that topographic scattering at a
comparatively flat site in Oregon would not pose any problems, whereas
scattering at a second site in Montana leads to significant broadening of wave
amplitudes in wavenumber space that would make Newtonian-noise subtraction very
challenging. Therefore, it is shown that topographic scattering should be
included as criterion in the site-selection process of future low-frequency
gravitational-wave detectors.Comment: 16 pages, 7 figure
Gravitational Waves in Brans-Dicke Theory : Analysis by Test Particles around a Kerr Black Hole
Analyzing test particles falling into a Kerr black hole, we study
gravitational waves in Brans-Dicke theory of gravity. First we consider a test
particle plunging with a constant azimuthal angle into a rotating black hole
and calculate the waveform and emitted energy of both scalar and tensor modes
of gravitational radiation. We find that the waveform as well as the energy of
the scalar gravitational waves weakly depends on the rotation parameter of
black hole and on the azimuthal angle.
Secondly, using a model of a non-spherical dust shell of test particles
falling into a Kerr black hole, we study when the scalar modes dominate. When a
black hole is rotating, the tensor modes do not vanish even for a ``spherically
symmetric" shell, instead a slightly oblate shell minimizes their energy but
with non-zero finite value, which depends on Kerr parameter . As a result,
we find that the scalar modes dominate only for highly spherical collapse, but
they never exceed the tensor modes unless the Brans-Dicke parameter
\omega_{BD} \lsim 750 for or unless \omega_{BD} \lsim 20,000
for , where is mass of black hole.
We conclude that the scalar gravitational waves with \omega_{BD} \lsim
several thousands do not dominate except for very limited situations
(observation from the face-on direction of a test particle falling into a
Schwarzschild black hole or highly spherical dust shell collapse into a Kerr
black hole). Therefore observation of polarization is also required when we
determine the theory of gravity by the observation of gravitational waves.Comment: 24 pages, revtex, 18 figures are attached with ps file
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