30,498 research outputs found
Gravitational waves in general relativity: XIV. Bondi expansions and the ``polyhomogeneity'' of \Scri
The structure of polyhomogeneous space-times (i.e., space-times with metrics
which admit an expansion in terms of ) constructed by a
Bondi--Sachs type method is analysed. The occurrence of some log terms in an
asymptotic expansion of the metric is related to the non--vanishing of the Weyl
tensor at Scri. Various quantities of interest, including the Bondi mass loss
formula, the peeling--off of the Riemann tensor and the Newman--Penrose
constants of motion are re-examined in this context.Comment: LaTeX, 28pp, CMA-MR14-9
Hot subdwarf stars in close-up view. I. Rotational properties of subdwarf B stars in close binary systems and nature of their unseen companions
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO)The origin of hot subdwarf B stars (sdBs) is still unclear. About half of the known sdBs are in close binary systems for which common envelope ejection is the most likely formation channel. Little is known about this dynamic phase of binary evolution. Since most of the known sdB systems are single-lined spectroscopic binaries, it is difficult to derive masses and unravel the companions' nature, which is the aim of this paper. Due to the tidal influence of the companion in close binary systems, the rotation of the primary becomes synchronised to its orbital motion. In this case it is possible to constrain the mass of the companion, if the primary mass, its projected rotational velocity as well as its surface gravity are known. For the first time we measured the projected rotational velocities of a large sdB binary sample from high resolution spectra. We analysed a sample of 51 sdB stars in close binaries, 40 of which have known orbital parameters comprising half of all such systems known today. Synchronisation in sdB binaries is discussed both from the theoretical and the observational point of view. The masses and the nature of the unseen companions could be constrained in 31 cases. We found orbital synchronisation most likely to be established in binaries with orbital periods shorter than . Only in five cases it was impossible to decide whether the sdB's companion is a white dwarf or an M dwarf. The companions to seven sdBs could be clearly identified as late M stars. One binary may have a brown dwarf companion. The unseen companions of nine sdBs are white dwarfs with typical masses. The mass of one white dwarf companion is very low. In eight cases (including the well known system KPD1930+2752) the companion mass exceeds , four of which even exceed the Chandrasekhar limit indicating that they may be neutron stars. Even stellar mass black holes are possible for the most massive companions. The distribution of the inclinations of the systems with low mass companions appears to be consistent with expectations, whereas a lack of high inclinations becomes obvious for the massive systems. We show that the formation of such systems can be explained with common envelope evolution and present an appropriate formation channel including two phases of unstable mass transfer and one supernova explosion. The sample also contains a candidate post-RGB star, which rotates fast despite its long orbital period. The post-RGB stars are expected to spin-up caused by their ongoing contraction. The age of the sdB is another important factor. If the EHB star is too young, the synchronisation process might not be finished yet. Estimating the ages of the target stars from their positions on the EHB band, we found PG 2345+318, which is known not to be synchronised, to lie near the zero-age extreme horizontal branch as are the massive candidates PG 1232-136, PG 1432+159 and PG 1101+249. These star may possibly be too young to have reached synchronisation. The derived large fraction of putative massive sdB binary systems in low inclination orbits is inconsistent with theoretical predictions. Even if we dismiss three candidates because they may be too young and assume that the other sdB primaries are of low mass, PG 1743+477 and, in particular, HE 0532-4503 remain as candidates whose companions may have masses close to or above the Chandrasekhar limit. X-ray observations and accurate photometry are suggested to clarify their nature. As high inclination systems must also exist, an appropriate survey has already been launched to find such binaries.Peer reviewe
Evaluation of ASTER GDEM ver2 using GPS measurements and SRTM ver4.1 in China
The freely available ASTER GDEM ver2 was released by NASA and METI on October 17, 2011. As one of the most complete high resolution digital topographic data sets of the world to date, the ASTER GDEM covers land surfaces between 83°N and 83°S at a spatial resolution of 1 arc-second and will be a useful product for many applications, such as relief analysis, hydrological studies and radar interferometry. The stated improvements in the second version of ASTER GDEM benefit from finer horizontal resolution, offset adjustment and water body detection in addition to new observed ASTER scenes. This study investigates the absolute vertical accuracy of the ASTER GDEM ver2 at five study sites in China using ground control points (GCPs) from high accuracy GPS benchmarks, and also using a DEM-to-DEM comparison with the Consultative Group for International Agriculture Research Consortium for Spatial Information (CGIAR-CSI) SRTM DEM (Version 4.1). And then, the results are separated into GlobCover land cover classes to derive the spatial pattern of error. It is demonstrated that the RMSE (19m) and mean (-13m) values of ASTER GDEM ver2 against GPS-GCPs in the five study areas is lower than its first version ASTER GDEM ver1 (26m and -21m) as a result of the adjustment of the elevation offsets in the new version. It should be noted that the five study areas in this study are representative in terms of terrain types and land covers in China, and even for most of mid-latitude zones. It is believed that the ASTER GDEM offers a major alternative in accessibility to high quality elevation data
Chemical analysis and aqueous solution properties of Charged Amphiphilic Block Copolymers PBA-b-PAA synthesized by MADIX
We have linked the structural and dynamic properties in aqueous solution of
amphiphilic charged diblock copolymers poly(butyl acrylate)-b-poly(acrylic
acid), PBA-b-PAA, synthesized by controlled radical polymerization, with the
physico-chemical characteristics of the samples. Despite product imperfections,
the samples self-assemble in melt and aqueous solutions as predicted by
monodisperse microphase separation theory. However, the PBA core are abnormally
large; the swelling of PBA cores is not due to AA (the Flory parameter
chiPBA/PAA, determined at 0.25, means strong segregation), but to h-PBA
homopolymers (content determined by Liquid Chromatography at the Point of
Exclusion and Adsorption Transition LC-PEAT). Beside the dominant population of
micelles detected by scattering experiments, capillary electrophoresis CE
analysis permitted detection of two other populations, one of h-PAA, and the
other of free PBA-b-PAA chains, that have very short PBA blocks and never
self-assemble. Despite the presence of these free unimers, the self-assembly in
solution was found out of equilibrium: the aggregation state is history
dependant and no unimer exchange between micelles occurs over months
(time-evolution SANS). The high PBA/water interfacial tension, measured at 20
mN/m, prohibits unimer exchange between micelles. PBA-b-PAA solution systems
are neither at thermal equilibrium nor completely frozen systems: internal
fractionation of individual aggregates can occur.Comment: 32 pages, 16 figures and 4 tables submitted to Journal of Interface
and Colloidal Scienc
Very Long Baseline Array observations of the Intraday Variable source J1128+592
Short time-scale flux density variations of flat spectrum radio sources are
often explained by the scattering of radio waves in the turbulent, ionized
Interstellar Matter of the Milky Way. One of the most convincing observational
arguments in favor of this is the annual modulation of the variability
time-scale caused by the Earth orbital motion around the Sun. J1128+592 is an
IDV source with a possible annual modulation in its variability time-scale. We
observed the source in 6 epochs with the VLBA at 5, 8 and 15 GHz in total
intensity and polarization. The VLBA observations revealed an east-west
oriented core-jet structure. Its position angle agrees with the angle of
anisotropy derived from the annual modulation model. No significant long-term
structural changes were observed with VLBI on mas-scales, however, compared to
archival data, the VLBI core size is expanded. This expansion offers a possible
explanation to the observed decrease of the strength of IDV. VLBI polarimetry
revealed significant changes in the electric vector position angle and Rotation
Measure of the core and jet. Part of the observed RM variability could be
attributed to a scattering screen (37 pc distance), which covers the source
(core and jet) and which may be responsible for the IDV. Superposition of
polarized sub-components below the angular resolution limit may affect the
observed RM as well.Comment: accepted for A&A (11 pages, 11 figures
Non-Gaussian statistics and extreme waves in a nonlinear optical cavity
A unidirectional optical oscillator is built by using a liquid crystal
light-valve that couples a pump beam with the modes of a nearly spherical
cavity. For sufficiently high pump intensity, the cavity field presents a
complex spatio-temporal dynamics, accompanied by the emission of extreme waves
and large deviations from the Gaussian statistics. We identify a mechanism of
spatial symmetry breaking, due to a hypercycle-type amplification through the
nonlocal coupling of the cavity field
Core-Collapse Simulations of Rotating Stars
We present the results from a series of two-dimensional core-collapse
simulations using a rotating progenitor star. We find that the convection in
these simulations is less vigorous because a) rotation weakens the core bounce
which seeds the neutrino-driven convection and b) the angular momentum profile
in the rotating core stabilizes against convection. The limited convection
leads to explosions which occur later and are weaker than the explosions
produced from the collapse of non-rotating cores. However, because the
convection is constrained to the polar regions, when the explosion occurs, it
is stronger along the polar axis. This asymmetric explosion can explain the
polarization measurements of core-collapse supernovae. These asymmetries also
provide a natural mechanism to mix the products of nucleosynthesis out into the
helium and hydrogen layers of the star. We also discuss the role the collapse
of these rotating stars play on the generation of magnetic fields and neutron
star kicks. Given a range of progenitor rotation periods, we predict a range of
supernova energies for the same progenitor mass. The critical mass for black
hole formation also depends upon the rotation speed of the progenitor.Comment: 16 pages text + 13 figures, submitted to Ap
Gamma-Ray Lines from Asymmetric Supernovae
We present 3-dimensional SPH simulations of supernova explosions from 100
seconds to 1 year after core-bounce. By extending our modelling efforts to a
3-dimensional hydrodynamics treatment, we are able to investigate the effects
of explosion asymmetries on mixing and gamma-ray line emergence in supernovae.
A series of initial explosion conditions are implemented, including jet-like
and equatorial asymmetries of varying degree. For comparison, symmetric
explosion models are also calculated. A series of time slices from the
explosion evolution are further analyzed using a 3-dimensional Monte Carlo
gamma-ray transport code. The emergent hard X- and gamma-ray spectra are
calculated as a function of both viewing angle and time, including trends in
the gamma-ray line profiles. We find significant differences in the velocity
distribution of radioactive nickel between the symmetric and asymmetric
explosion models. The effects of this spatial distribution change are reflected
in the overall high energy spectrum, as well as in the individual gamma-ray
line profiles.Comment: 32 pages, 14 figures, LAUR-02-6114, http://qso.lanl.gov/~clf
"Clumping Asymmetry" section revise
Mass movement susceptibility mapping using satellite optical imagery compared with InSAR monitoring: Zigui County, Three Gorges region, China
Mass movements on steep slopes are a major hazard to
communities and infrastructure in the Three Gorges
region, China. Developing susceptibility maps of mass
movements is therefore very important in both current
and future land use planning. This study employed
satellite optical imagery and an ASTER GDEM (15 m)
to derive various parameters (namely geology; slope
gradient; proximity to drainage networks and proximity
to lineaments) in order to create a GIS-based map of
mass movement susceptibility. This map was then
evaluated using highly accurate deformation signals
processed using the Persistent Scatterer (PS) InSAR
technique. Areas of high susceptibility correspond well
to points of high subsidence, which provides a strong
support of our susceptibility map
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