1,067 research outputs found
Magnetic Doppler imaging of alpha^2 Canum Venaticorum in all four Stokes parameters. Unveiling the hidden complexity of stellar magnetic fields
Strong organized magnetic fields have been studied in the upper main sequence
chemically peculiar stars for more than half a century. However, only recently
have observational methods and numerical techniques become sufficiently mature
to allow us to record and interpret high-resolution four Stokes parameter
spectra, leading to the first assumption-free magnetic field models of these
stars. Here we present a detailed magnetic Doppler imaging analysis of the
spectropolarimetric observations of the prototypical magnetic Ap star alpha^2
CVn. The surface abundance distributions of Fe and Cr and a full vector map of
the stellar magnetic field are reconstructed in a self-consistent inversion
using our state-of-the-art magnetic Doppler imaging code Invers10. We succeeded
in reproducing most of the details of the available spectropolarimetric
observations of alpha^2 CVn with a magnetic map which combines a global
dipolar-like field topology with localized spots of higher field intensity. We
demonstrate that these small-scale magnetic structures are inevitably required
to fit the linear polarization spectra; however, their presence cannot be
inferred from the Stokes I and V observations alone. Our magnetic Doppler
imaging analysis of alpha^2 CVn and previous results for 53 Cam support the
view that the upper main sequence stars can harbour fairly complex surface
magnetic fields which resemble oblique dipoles only at the largest spatial
scales. Spectra in all four Stokes parameters are absolutely essential to
unveil and meaningfully characterize this field complexity in Ap stars. We
therefore suggest that understanding magnetism of stars in other parts of the
H-R diagram is similarly incomplete without investigation of their linear
polarization spectra.Comment: 16 pages, 12 figures; Accepted for publication by Astronomy &
Astrophysic
Dinuclear LnIII Complexes with 9âAnthracenecarboxylate Showing FieldâInduced SMM and Visible/NIR Luminescence
The reaction of several Ln(NO3)3·6H2O salts with 9-anthracenecarboxylic acid (9-HAC) and 2,2âČ-bipyridine (bpy) in a mixture of CH3OH/H2O has allowed the isolation of the dinuclear compounds 1-6 with formula [Ln2(ÎŒ2-9-AC)4(9-AC)2(bpy)2] [LnIII = Nd (1), Eu (2), Gd (3), Tb (4), Er (5), and Yb (6)]. The molar magnetic susceptibility measurements of 1-6 in the 2-300 K temperature range indicate weak antiferromagnetic ex-. change for the isotropic GdIII compound 3. Compounds 1, 5, and 6 exhibit field-induced single-molecule magnet (SMM) behavior. The luminescence properties of compounds 1-6 in the solid state have been studied at different temperatures and show sensitization of the 4f-4f emission bands in the NIR range for compounds 1, 5, and 6
First observation of two hyperfine transitions in antiprotonic He-3
We report on the first experimental results for microwave spectroscopy of the
hyperfine structure of antiprotonic He-3. Due to the helium nuclear spin,
antiprotonic He-3 has a more complex hyperfine structure than antiprotonic He-4
which has already been studied before. Thus a comparison between theoretical
calculations and the experimental results will provide a more stringent test of
the three-body quantum electrodynamics (QED) theory. Two out of four
super-super-hyperfine (SSHF) transition lines of the (n,L)=(36,34) state were
observed. The measured frequencies of the individual transitions are
11.12559(14) GHz and 11.15839(18) GHz, less than 1 MHz higher than the current
theoretical values, but still within their estimated errors. Although the
experimental uncertainty for the difference of these frequencies is still very
large as compared to that of theory, its measured value agrees with theoretical
calculations. This difference is crucial to be determined because it is
proportional to the magnetic moment of the antiproton.Comment: 8 pages, 6 figures, just published (online so far) in Physics Letters
Formation of the compact jets in the black hole GX 339-4
Galactic black hole binaries produce powerful outflows with emit over almost
the entire electromagnetic spectrum. Here, we report the first detection with
the Herschel observatory of a variable far-infrared source associated with the
compact jets of the black hole transient GX 339-4 during the decay of its
recent 2010-2011 outburst, after the transition to the hard state. We also
outline the results of very sensitive radio observations conducted with the
Australia Telescope Compact Array, along with a series of near-infrared,
optical (OIR) and X-ray observations, allowing for the first time the
re-ignition of the compact jets to be observed over a wide range of
wavelengths. The compact jets first turn on at radio frequencies with an
optically thin spectrum that later evolves to optically thick synchrotron
emission. An OIR reflare is observed about ten days after the onset of radio
and hard X-ray emission, likely reflecting the necessary time to build up
enough density, as well as to have acceleration (e.g. through shocks) along an
extended region in the jets. The Herschel measurements are consistent with an
extrapolation of the radio inverted power-law spectrum, but they highlight a
more complex radio to OIR spectral energy distribution for the jets.Comment: Accepted for publication in MNRAS Letter, 6 pages, 3 Figures + 1
online Tabl
On discretization in time in simulations of particulate flows
We propose a time discretization scheme for a class of ordinary differential
equations arising in simulations of fluid/particle flows. The scheme is
intended to work robustly in the lubrication regime when the distance between
two particles immersed in the fluid or between a particle and the wall tends to
zero. The idea consists in introducing a small threshold for the particle-wall
distance below which the real trajectory of the particle is replaced by an
approximated one where the distance is kept equal to the threshold value. The
error of this approximation is estimated both theoretically and by numerical
experiments. Our time marching scheme can be easily incorporated into a full
simulation method where the velocity of the fluid is obtained by a numerical
solution to Stokes or Navier-Stokes equations. We also provide a derivation of
the asymptotic expansion for the lubrication force (used in our numerical
experiments) acting on a disk immersed in a Newtonian fluid and approaching the
wall. The method of this derivation is new and can be easily adapted to other
cases
Predicting consumer biomass, size-structure, production, catch potential, responses to fishing and associated uncertainties in the world's marine ecosystems
Existing estimates of fish and consumer biomass in the worldâs oceans are disparate. This creates uncertainty about the roles of fish and other consumers in biogeochemical cycles and ecosystem processes, the extent of human and environmental impacts and fishery potential. We develop and use a size-based macroecological model to assess the effects of parameter uncertainty on predicted consumer biomass, production and distribution. Resulting uncertainty is large (e.g. median global biomass 4.9 billion tonnes for consumers weighing 1 g to 1000 kg; 50% uncertainty intervals of 2 to 10.4 billion tonnes; 90% uncertainty intervals of 0.3 to 26.1 billion tonnes) and driven primarily by uncertainty in trophic transfer efficiency and its relationship with predator-prey body mass ratios. Even the upper uncertainty intervals for global predictions of consumer biomass demonstrate the remarkable scarcity of marine consumers, with less than one part in 30 million by volume of the global oceans comprising tissue of macroscopic animals. Thus the apparently high densities of marine life seen in surface and coastal waters and frequently visited abundance hotspots will likely give many in society a false impression of the abundance of marine animals. Unexploited baseline biomass predictions from the simple macroecological model were used to calibrate a more complex size- and trait-based model to estimate fisheries yield and impacts. Yields are highly dependent on baseline biomass and fisheries selectivity. Predicted global sustainable fisheries yield increases â4 fold when smaller individuals (< 20 cm from species of maximum mass < 1kg) are targeted in all oceans, but the predicted yields would rarely be accessible in practice and this fishing strategy leads to the collapse of larger species if fishing mortality rates on different size classes cannot be decoupled. Our analyses show that models with minimal parameter demands that are based on a few established ecological principles can support equitable analysis and comparison of diverse ecosystems. The analyses provide insights into the effects of parameter uncertainty on global biomass and production estimates, which have yet to be achieved with complex models, and will therefore help to highlight priorities for future research and data collection. However, the focus on simple model structures and global processes means that non-phytoplankton primary production and several groups, structures and processes of ecological and conservation interest are not represented. Consequently, our simple models become increasingly less useful than more complex alternatives when addressing questions about food web structure and function, biodiversity, resilience and human impacts at smaller scales and for areas closer to coasts
A jump-growth model for predator-prey dynamics: derivation and application to marine ecosystems
This paper investigates the dynamics of biomass in a marine ecosystem. A
stochastic process is defined in which organisms undergo jumps in body size as
they catch and eat smaller organisms. Using a systematic expansion of the
master equation, we derive a deterministic equation for the macroscopic
dynamics, which we call the deterministic jump-growth equation, and a linear
Fokker-Planck equation for the stochastic fluctuations. The McKendrick--von
Foerster equation, used in previous studies, is shown to be a first-order
approximation, appropriate in equilibrium systems where predators are much
larger than their prey. The model has a power-law steady state consistent with
the approximate constancy of mass density in logarithmic intervals of body mass
often observed in marine ecosystems. The behaviours of the stochastic process,
the deterministic jump-growth equation and the McKendrick--von Foerster
equation are compared using numerical methods. The numerical analysis shows two
classes of attractors: steady states and travelling waves.Comment: 27 pages, 4 figures. Final version as published. Only minor change
H2O line mapping at high spatial and spectral resolution - Herschel observations of the VLA1623 outflow
Apart from being an important coolant, H2O is known to be a tracer of
high-velocity molecular gas. Recent models predict relatively high abundances
behind interstellar shockwaves. The dynamical and physical conditions of the
H2O emitting gas, however, are not fully understood yet. We aim to determine
the abundance and distribution of H2O, its kinematics and the physical
conditions of the gas responsible for the H2O emission. The observed line
profile shapes help us understand the dynamics in molecular outflows. We mapped
the VLA1623 outflow, in the ground-state transitions of o-H2O, with the HIFI
and PACS instruments. We also present observations of higher energy transitions
of o-H2O and p-H2O obtained with HIFI and PACS towards selected outflow
positions. From comparison with non-LTE radiative transfer calculations, we
estimate the physical parameters of the water emitting regions. The observed
water emission line profiles vary over the mapped area. Spectral features and
components, tracing gas in different excitation conditions, allow us to
constrain the density and temperature of the gas. The H2O emission originates
in a region where temperatures are comparable to that of the warm H2 gas
(T\gtrsim200K). Thus, the H2O emission traces a gas component significantly
warmer than the gas responsible for the low-J CO emission. The H2O column
densities at the CO peak positions are low, i.e. N(H2O) \simeq (0.03-10)x10e14
cm-2. The H2O abundance with respect to H2 in the extended outflow is estimated
at X(H2O)<1x10e-6, significantly lower than what would be expected from most
recent shock models. The H2O emission traces a gas component moving at
relatively high velocity compared to the low-J CO emitting gas. However, other
dynamical quantities such as the momentum rate, energy and mechanical
luminosity are estimated to be the same, independent of the molecular tracer
used, CO or H2O.Comment: 14 pages, 13 figures, 4 table
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