17,468 research outputs found
Comparison of primordial tensor power spectra from the deformed algebra and dressed metric approaches in loop quantum cosmology
Loop quantum cosmology tries to capture the main ideas of loop quantum
gravity and to apply them to the Universe as a whole. Two main approaches
within this framework have been considered to date for the study of
cosmological perturbations: the dressed metric approach and the deformed
algebra approach. They both have advantages and drawbacks. In this article, we
accurately compare their predictions. In particular, we compute the associated
primordial tensor power spectra. We show -- numerically and analytically --
that the large scale behavior is similar for both approaches and compatible
with the usual prediction of general relativity. The small scale behavior is,
the other way round, drastically different. Most importantly, we show that in a
range of wavenumbers explicitly calculated, both approaches do agree on
predictions that, in addition, differ from standard general relativity and do
not depend on unknown parameters. These features of the power spectrum at
intermediate scales might constitute a universal loop quantum cosmology
prediction that can hopefully lead to observational tests and constraints. We
also present a complete analytical study of the background evolution for the
bouncing universe that can be used for other purposes.Comment: 15 pages, 7 figure
Multipole Expansion of Bremsstrahlung in Intermediate Energy Heavy Ion Collisions
Using a multipole expansion of the radiated field generated by a classical
electric current, we present a way to interprete the bremsstrahlung spectra of
low energy heavy ion collisions. We perform the calculation explicitely for the
system ^{12}C+ ^{12}C at 84AMeV and compare the result with the experimental
data of E. Grosse et al. Using simple model assumptions for the electromagnetic
source current we are able to describe the measured data in terms of coherent
photon emission. In this context, the information contained in the measured
data is discussed.Comment: LaTex, 4 Figure
General relativistic gravitational field of a rigidly rotating disk of dust: Solution in terms of ultraelliptic functions
In a recent paper we presented analytic expressions for the axis potential,
the disk metric, and the surface mass density of the global solution to
Einstein's field equations describing a rigidly rotating disk of dust. Here we
add the complete solution in terms of ultraelliptic functions and quadratures.Comment: 5 pages, published in 1995 [Phys. Rev. Lett. 75 (1995) 3046
Spectroscopic and photometric oscillatory envelope variability during the S Doradus outburst of the Luminous Blue Variable R71
To better understand the LBV phenomenon, we analyze multi-epoch and
multi-wavelength spectra and photometry of R71. Pre-outburst spectra are
analyzed with the radiative transfer code CMFGEN to determine the star's
fundamental stellar parameters. During quiescence, R71 has an effective
temperature of and a luminosity of
log = 5.78 and is thus a classical LBV, but at the lower
luminosity end of this group. We determine its mass-loss rate to yr. We present R71's spectral energy distribution
from the near-ultraviolet to the mid-infrared during its present outburst.
Mid-infrared observations suggest that we are witnessing dust formation and
grain evolution. Semi-regular oscillatory variability in the star's light curve
is observed during the current outburst. Absorption lines develop a second blue
component on a timescale twice that length. The variability may consist of one
(quasi-)periodic component with P ~ 425/850 d with additional variations
superimposed. During its current S Doradus outburst, R71 occupies a region in
the HR diagram at the high-luminosity extension of the Cepheid instability
strip and exhibits similar irregular variations as RV Tau variables. LBVs do
not pass the Cepheid instability strip because of core evolution, but they
develop comparable cool, low-mass, extended atmospheres in which convective
instabilities may occur. As in the case of RV Tau variables, the occurrence of
double absorption lines with an apparent regular cycle may be due to shocks
within the atmosphere and period doubling may explain the factor of two in the
lengths of the photometric and spectroscopic cycles.Comment: 18 pages, 14 figures, submitted to A&
Extended Magnetic Dome Induced by Low Pressures in Superconducting FeSeS
We report muon spin rotation (SR) and magnetization measurements under
pressure on FeSeS with x
.Above GPa we find microscopic coexistence of
superconductivity with an extended dome of long range magnetic order that spans
a pressure range between previously reported separated magnetic phases. The
magnetism initially competes on an atomic scale with the coexisting
superconductivity leading to a local maximum and minimum of the superconducting
. The maximum of corresponds to the onset of
magnetism while the minimum coincides with the pressure of strongest
competition. A shift of the maximum of for a series of single
crystals with x up to 0.14 roughly extrapolates to a putative magnetic and
superconducting state at ambient pressure for x .Comment: 10 pages, 6 figures, including supplemental materia
A Keck High Resolution Spectroscopic Study of the Orion Nebula Proplyds
We present the results of spectroscopy of four bright proplyds in the Orion
Nebula obtained at a velocity resolution of 6 km/s. After careful isolation of
the proplyd spectra from the confusing nebular radiation, the emission line
profiles are compared with those predicted by realistic dynamic/photoionization
models of the objects. The spectral line widths show a clear correlation with
ionization potential, which is consistent with the free expansion of a
transonic, ionization-stratified, photoevaporating flow. Fitting models of such
a flow simultaneously to our spectra and HST emission line imaging provides
direct measurements of the proplyd size, ionized density and outflow velocity.
These measurements confirm that the ionization front in the proplyds is
approximately D-critical and provide the most accurate and robust estimate to
date of the proplyd mass loss rate. Values of 0.7E-6 to 1.5E-6 Msun/year are
found for our spectroscopic sample, although extrapolating our results to a
larger sample of proplyds implies that 0.4E-6 Msun/year is more typical of the
proplyds as a whole. In view of the reported limits on the masses of the
circumstellar disks within the proplyds, the length of time that they can have
been exposed to ionizing radiation should not greatly exceed 10,000 years - a
factor of 30 less than the mean age of the proplyd stars. We review the various
mechanisms that have been proposed to explain this situation, and conclude that
none can plausibly work unless the disk masses are revised upwards by a
substantial amount.Comment: 23 pages, 8 figures, uses emulateapj.sty, accepted for publication in
The Astronomical Journal (scheduled November 1999
Strong asymptotics for Jacobi polynomials with varying nonstandard parameters
Strong asymptotics on the whole complex plane of a sequence of monic Jacobi
polynomials is studied, assuming that with and satisfying , , . The
asymptotic analysis is based on the non-Hermitian orthogonality of these
polynomials, and uses the Deift/Zhou steepest descent analysis for matrix
Riemann-Hilbert problems. As a corollary, asymptotic zero behavior is derived.
We show that in a generic case the zeros distribute on the set of critical
trajectories of a certain quadratic differential according to the
equilibrium measure on in an external field. However, when either
, or are geometrically close to ,
part of the zeros accumulate along a different trajectory of the same quadratic
differential.Comment: 31 pages, 12 figures. Some references added. To appear in Journal
D'Analyse Mathematiqu
Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood
We examine 20 diffuse and translucent Galactic sight lines and extract the
column densities of the ^12CO and ^13CO isotopologues from their ultraviolet
A--X absorption bands detected in archival Space Telescope Imaging Spectrograph
data with lambda/Deltalambda geq 46,000. Five more targets with Goddard
High-Resolution Spectrograph data are added to the sample that more than
doubles the number of sight lines with published Hubble Space Telescope
observations of ^13CO. Most sight lines have 12-to-13 isotopic ratios that are
not significantly different from the local value of 70 for ^12C/^13C, which is
based on mm-wave observations of rotational lines in emission from CO and H_2CO
inside dense molecular clouds, as well as on results from optical measurements
of CH^+. Five of the 25 sight lines are found to be fractionated toward lower
12-to-13 values, while three sight lines in the sample are fractionated toward
higher ratios, signaling the predominance of either isotopic charge exchange or
selective photodissociation, respectively. There are no obvious trends of the
^12CO-to-^13CO ratio with physical conditions such as gas temperature or
density, yet ^12CO/^13CO does vary in a complicated manner with the column
density of either CO isotopologue, owing to varying levels of competition
between isotopic charge exchange and selective photodissociation in the
fractionation of CO. Finally, rotational temperatures of H_2 show that all
sight lines with detected amounts of ^13CO pass through gas that is on average
colder by 20 K than the gas without ^13CO. This colder gas is also sampled by
CN and C_2 molecules, the latter indicating gas kinetic temperatures of only 28
K, enough to facilitate an efficient charge exchange reaction that lowers the
value of ^12CO/^13CO.Comment: 1-column emulateapj, 23 pages, 9 figure
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