431 research outputs found
Perturbative calculation of quasi-normal modes of Schwarzschild black holes
We discuss a systematic method of analytically calculating the asymptotic
form of quasi-normal frequencies of a four-dimensional Schwarzschild black hole
by expanding around the zeroth-order approximation to the wave equation
proposed by Motl and Neitzke. We obtain an explicit expression for the
first-order correction and arbitrary spin. Our results are in agreement with
the results from WKB and numerical analyses in the case of gravitational waves.Comment: 11 pages; references added and a sign error corrected; to appear in
CQ
Symmetry of massive Rarita-Schwinger fields
We derive the general lagrangian and propagator for a vector-spinor field in
-dimensions and show that the physical observables are invariant under the
so-called point transformation symmetry. Until now the symmetry has not been
exploited in any non-trival way, presumably because it is not an invariance of
the classical action nor is it a gauge symmetry. Nevertheless, we develop a
technique for exploring the consequences of the symmetry leading to a conserved
vector current and charge. The current and charge are identically zero in the
free field case and only contribute in a background such as a electromagnetic
or gravitational field. The current can couple spin-3/2 fields to vector and
scalar fields and may have important consequences in intermediate energy hadron
physics as well as linearized supergravity. The consistency problem which
plagues higher spin field theories is then discussed and and some ideas
regarding the possiblity of solutions are presented.Comment: 26 pages, 1 figure; revised using referee comments, Journal ref.
adde
Modification of the twist angle in chiral nematic polymer films by photoisomerization of the chiral dopant
A method for the production of polarization sensitive recordings in liquid crystalline polymers is presented. The system is based on local modification of the twist angle of chiral nematic polymer films. The twist angle of the polymer film is varied by modifying the chemical structure of the chiral dopant. Here a photoisomerizable menthone derivative is used that has a fivefold difference in helical twisting power between the E and the Z isomer. The twist angle of the film can be varied between –90° and 0° by introducing a nonphotosensitive chiral dopant with opposite twisting sense. Complex pictures with gray scales can be recorded in the films with black and white contrasts higher than 20
Approximating RR Lyrae light curves using cubic polynomials
In this paper, we use cubic polynomials to approximate RR Lyrae light curves
and apply the method to HST data of RR Lyraes in the halo of M31. We compare
our method to the standard method of Fourier decomposition and find that the
method of cubic polynomials eliminates virtually all ringing effects and does
so with significantly fewer parameters than the Fourier technique. Further, for
RRc stars the parameters in the fit are all physical. Our study also reveals a
number of additional periodicites in this data not found previously: we find 23
RRc stars, 29 RRab stars and 3 multiperiodic stars.Comment: 6 pages, MNRAS accepte
CORS Baade-Wesselink method in the Walraven photometric system: the period-radius and the period-luminosity relation of classical Cepheids
We present a new derivation of the CORS Baade-Wesselink method in the
Walraven photometric system. We solved the complete Baade-Wesselink equation by
calibrating the surface brightness function with a recent grid of atmosphere
models. The new approach was adopted to estimate the mean radii of a sample of
Galactic Cepheids for which are available precise light curves in the Walraven
bands. Current radii agree, within the errors, quite well with Cepheid radii
based on recent optical and near-infrared interferometric measurements. We also
tested the impact of the projection factor on the Period-Radius relation using
two different values (p=1.36, p=1.27) that bracket the estimates available in
the literature. We found that the agreement of our Period-Radius relation with
similar empirical and theoretical Period-Radius relations in the recent
literature, improves by changing the projection factor from p=1.36 to p=1.27.
Our Period-Radius relation is log(R)=(0.75\pm 0.03)log(P)+(1.10 \pm 0.03), with
a rms=0.03 dex. Thanks to accurate estimates of the effective temperature of
the selected Cepheids, we also derived the Period-Luminosity relation in the V
band and we found Mv=(-2.78 \pm 0.11)log(P)+(-1.42 \pm 0.11) with rms=0.13 mag,
for p=1.27. It agrees quite well with recent results in the literature, while
the relation for p=1.36 deviates by more than 2sigma. We conclude that, even
taking into account the intrinsic dispersion of the obtained Period-Luminosity
relations, that is roughly of the same order of magnitude as the effect of the
projection factor, the results of this paper seem to favour the value p = 1.27.Comment: 17 pages, 14 figures, accepted for publication in MNRA
Pretargeting kit for imaging or therapy comprising a trans - cyclooctene dienophile and a diene
Variability of Active Galactic Nuclei from the Optical to X-ray Regions
Some progress in understanding AGN variability is reviewed. Reprocessing of
X-ray radiation to produce significant amounts of longer-wavelength continua
seems to be ruled out. In some objects where there has been correlated X-ray
and optical variability, the amplitude of the optical variability has exceeded
the amplitude of X-ray variability. We suggest that accelerated particles
striking material could be linking X-ray and optical variability (as in
activity in the solar chromosphere). Beaming effects could be significant in
all types of AGN. The diversity in optical/X-ray relationships at different
times in the same object, and between different objects, might be explained by
changes in geometry and directions of motion relative to our line of sight.
Linear shot-noise models of the variability are ruled out; instead there must
be large-scale organization of variability. Variability occurs on
light-crossing timescales rather than viscous timescales and this probably
rules out the standard Shakura-Sunyaev accretion disk. Radio-loud and
radio-quiet AGNs have similar continuum shapes and similar variability
properties. This suggests similar continuum origins and variability mechanisms.
Despite their extreme X-ray variability, narrow-line Seyfert 1s (NLS1s) do not
show extreme optical variability.Comment: Invited talk given at Euro Asian Astronomical Society meeting in
Moscow, June 2002. 20 pages, 4 figures. References update
Carbon-rich RR Lyr type stars
We have derived CNO abundances in 12 RR Lyrae stars. Four stars show [C/Fe]
near 0.0 and two stars show [C/Fe] = 0.52 and 0.65. Red giant branch stars,
which are known to be the predecessors of RR Lyrae stars, generally show a
deficiency of carbon due to proton captures during their evolution from the
main sequence up the giant branch. We suggest that the enhancement of carbon is
due to production during the helium flash combined with mixing to the surface
by vigorous convection induced by the flash itself.Comment: accepted for publication in ApJ
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