15,996 research outputs found
Spinning Relativistic Particle in an External Electromagnetic Field
The Hamiltonian formulation of the motion of a spinning relativistic particle
in an external electromagnetic field is considered. The approach is based on
the introduction of new coordinates and their conjugated momenta to describe
the spin degrees of freedom together with an appropriate set of constraints in
the Dirac formulation. For particles with gyromagnetic ratio , the
equations of motion do not predict any deviation from the standard Lorentz
force, while for an additional force, which corresponds to the
magnetic dipole force, is obtained.Comment: Latex file, 11 page
Constraints on the braneworld from compact stars
According to the braneworld idea, ordinary matter is confined on a
3-dimensional space (brane) that is embedded in a higher-dimensional space-time
where gravity propagates. In this work, after reviewing the limits coming from
general relativity, finiteness of pressure and causality on the brane, we
derive observational constraints on the braneworld parameters from the
existence of stable compact stars. The analysis is carried out by solving
numerically the brane-modified Tolman-Oppenheimer-Volkoff equations, using
different representative equations of state to describe matter in the star
interior. The cases of normal dense matter, pure quark matter and hybrid matter
are considered.Comment: 13 pages, 11 figures, 2 tables; new EoS considered, references and
comments adde
Detection of the ISW effect and corresponding dark energy constraints made with directional spherical wavelets
Using a directional spherical wavelet analysis we detect the integrated
Sachs-Wolfe (ISW) effect, indicated by a positive correlation between the
first-year Wilkinson Microwave Anisotropy Probe (WMAP) and NRAO VLA Sky Survey
(NVSS) data. Detections are made using both a directional extension of the
spherical Mexican hat wavelet and the spherical butterfly wavelet. We examine
the possibility of foreground contamination and systematics in the WMAP data
and conclude that these factors are not responsible for the signal that we
detect. The wavelet analysis inherently enables us to localise on the sky those
regions that contribute most strongly to the correlation. On removing these
localised regions the correlation that we detect is reduced in significance, as
expected, but it is not eliminated, suggesting that these regions are not the
sole source of correlation between the data. This finding is consistent with
predictions made using the ISW effect, where one would expect weak correlations
over the entire sky. In a flat universe the detection of the ISW effect
provides direct and independent evidence for dark energy. We use our detection
to constrain dark energy parameters by deriving a theoretical prediction for
the directional wavelet covariance statistic for a given cosmological model.
Comparing these predictions with the data we place constraints on the
equation-of-state parameter and the vacuum energy density .
We also consider the case of a pure cosmological constant, i.e. . For
this case we rule out a zero cosmological constant at greater than the 99.9%
significance level. All parameter estimates that we obtain are consistent with
the standand cosmological concordance model values.Comment: 16 pages, 13 figures; replaced to match version accepted by MNRA
Focused directed evolution of aryl-alcohol oxidase in Saccharomyces cerevisiae by using chimeric signal peptides
Aryl-alcohol oxidase (AAO) is an extracellular flavoprotein that supplies ligninolytic peroxidases with H2O2 during natural wood decay. With a broad substrate specificity and highly stereoselective reaction mechanism, AAO is an attractive candidate for studies into organic synthesis and synthetic biology, and yet the lack of suitable heterologous expression systems has precluded its engineering by directed evolution. In this study, the native signal sequence of AAO from Pleurotus eryngii was replaced by those of the mating a-factor and the K1 killer toxin, as well as different chimeras of both prepro-leaders in order to drive secretion in Saccharomyces cerevisiae. The secretion of these AAO constructs increased in the following order: preproa-AAO> preaproK-AAO>preKproa-AAO>preproK-AAO. The chimeric preaproK-AAO was subjected to focused-directed evolution with the aid of a dual screening assay based on the Fenton reaction. Random mutagenesis and DNA recombination was concentrated on two protein segments (Meta1]-Val109 and Phe392-Gln566), and an array of improved variants was identified, among which the FX7 mutant (harboring the H91N mutation) showed a dramatic 96-fold improvement in total activity with secretion levels of 2 mg/liter. Analysis of the N-terminal sequence of the FX7 variant confirmed the correct processing of the preaproK hybrid peptide by the KEX2 protease. FX7 showed higher stability in terms of pH and temperature, whereas the pH activity profiles and the kinetic parameters were maintained. The Asn91 lies in the flavin attachment loop motif, and it is a highly conserved residue in all members of the GMC superfamily, except for P. eryngii and P. pulmonarius AAO. The in vitro involution of the enzyme by restoring the consensus ancestor Asn91 promoted AAO expression and stability
The Limits on Cosmological Anisotropies and Inhomogeneities from COBE Data
Assuming that the cosmological principle holds, Maartens, Ellis and Stoeger
(MES) recently constructed a detailed scheme linking anisotropies in the cosmic
background radiation (CMB) with anisotropies and inhomogeneities in the large
scale structure of the universe and showed how to place limits on those
anisotropies and inhomogeneities simply by using CMB quadrupole and octupole
limits. First we indicate and discuss the connection between the covariant
multipole moments of the temperature anisotropy used in the MES scheme and the
quadrupole and octupole results from COBE. Then we introduce those results into
the MES limit equations to obtain definite quantitative limits on the complete
set of cosmological measures of anisotropy and inhomogeneity.
We find that all the anisotropy measures are less than 10^{-4} in the case of
those not affected by the expansion rate H, and less than 10^{-6} Mpc^{-1} in
the case of those which are. These results quantitatively demonstrate that the
observable universe is indeed close to Friedmann-Lemaitre-Robertson-Walker
(FLRW) on the largest scales, and can be adequately modelled by an almost-FLRW
model -- that is, the anisotropies and inhomogeneities characterizing the
observable universe on the largest scales are not too large to be considered
perturbations to FLRW.Comment: Original paper with corrections. ApJ 476 435 (1997) erratum to appear
ApJ Sept 199
Proper motions of the HH1 jet
We describe a new method for determining proper motions of extended objects,
and a pipeline developed for the application of this method. We then apply this
method to an analysis of four epochs of [S~II] HST images of the HH~1 jet
(covering a period of ~yr).
We determine the proper motions of the knots along the jet, and make a
reconstruction of the past ejection velocity time-variability (assuming
ballistic knot motions). This reconstruction shows an "acceleration" of the
ejection velocities of the jet knots, with higher velocities at more recent
times. This acceleration will result in an eventual merging of the knots in
~yr and at a distance of from the outflow source, close to
the present-day position of HH~1.Comment: 12 pages, 8 figure
Filtering techniques for the detection of Sunyaev-Zel'dovich clusters in multifrequency CMB maps
The problem of detecting Sunyaev-Zel'dovich (SZ) clusters in multifrequency
CMB observations is investigated using a number of filtering techniques. A
multifilter approach is introduced, which optimizes the detection of SZ
clusters on microwave maps. An alternative method is also investigated, in
which maps at different frequencies are combined in an optimal manner so that
existing filtering techniques can be applied to the single combined map. The SZ
profiles are approximated by the circularly-symmetric template , with and , where the core radius and the overall amplitude of the effect
are not fixed a priori, but are determined from the data. The background
emission is modelled by a homogeneous and isotropic random field, characterized
by a cross-power spectrum with . The
filtering methods are illustrated by application to simulated Planck
observations of a patch of sky in 10 frequency
channels. Our simulations suggest that the Planck instrument should detect
SZ clusters in 2/3 of the sky. Moreover, we find the catalogue
to be complete for fluxes mJy at 300 GHz.Comment: 12 pages, 7 figures; Corrected figures. Submitted to MNRA
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