3,344 research outputs found
Inconsistencies in the application of harmonic analysis to pulsating stars
Using ultra-precise data from space instrumentation we found that the
underlying functions of stellar light curves from some AF pul- sating stars are
non-analytic, and consequently their Fourier expansion is not guaranteed. This
result demonstrates that periodograms do not provide a mathematically
consistent estimator of the frequency content for this kind of variable stars.
More importantly, this constitutes the first counterexample against the current
paradigm which considers that any physical process is described by a contin-
uous (band-limited) function that is infinitely differentiable.Comment: 9 pages, 8 figure
Electronic structure of Fe and magnetism in the double perovskites CaFeReO and BaFeReO
The Fe electronic structure and magnetism in (i) monoclinic CaFeReO
with a metal-insulator transition at K and (ii) quasi-cubic
half-metallic BaFeReO ceramic double perovskites are probed by soft
x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD).
These materials show distinct Fe XAS and XMCD spectra, which are
primarily associated with their different average Fe oxidation states (close to
Fe for CaFeReO and intermediate between Fe and Fe
for BaFeReO) despite being related by an isoelectronic
(Ca/Ba) substitution. For CaFeReO, the powder-averaged Fe
spin moment along the field direction ( T), as probed by the XMCD
experiment, is strongly reduced in comparison with the spontaneous Fe moment
previously obtained by neutron diffraction, consistent with a scenario where
the magnetic moments are constrained to remain within an easy plane. For
T, the unsaturated XMCD signal is reduced below consistent with a
magnetic transition to an easy-axis state that further reduces the
powder-averaged magnetization in the field direction. For BaFeReO, the
field-aligned Fe spins are larger than for CaFeReO ( T) and the
temperature dependence of the Fe magnetic moment is consistent with the
magnetic ordering transition at K. Our results illustrate the
dramatic influence of the specific spin-orbital configuration of Re
electrons on the Fe local magnetism of these Fe/Re double perovskites.Comment: 7 pages, 3 figure
Investigation of element-specific and bulk magnetism, electronic and crystal structures of La{0.70}Ca{0.30}Mn{1-x}Cr{x}O{3}
The magnetic interactions in La{0.70}Ca{0.30}Mn{1-x}Cr{x}O{3} (x = 0.15, 0.50
and 0.70) are investigated by x-ray absorption spectroscopy (XAS), x-ray
magnetic circular dichroism (XMCD), high-resolution x-ray powder diffraction,
and bulk magnetization measurements. XAS in the Mn and Cr L{2,3} edges support
stable single valent Cr{3+} ions and a varying Mn valence state with x, while
the O K edge XAS spectrum reveals local maxima in the O 2p density of states
close to the Fermi level due to mixing with Mn and Cr 3d states. A robust
antiferromagnetic state is found for x=0.70 below TN = 258 K. For x=0.15,
combined XMCD and bulk magnetization measurements indicate a fully polarized
ferrimagnetic state for the Mn and Cr spins below Tc=224 K. For x=0.50, a
reduced ferrimagnetic component dominated by Mn spins is present below Tc=154
K. No evidence of lattice anomalies due to cooperative charge and orbital
orderings is found by x-ray diffraction for all samples. The magnetic
properties of this system are rationalized in terms of a competition of
ferromagnetic Mn-Mn double exchange and antiferromagnetic Cr-Cr and Cr-Mn
superexchange interactions.Comment: 25 pages, 9 figure
Litter Windrows in the South-East Coast of the Bay of Biscay: An Ocean Process Enabling Effective Active Fishing for Litter
Large scale convergence regions of floating marine litter are commonly observed in semi-enclosed seas as the Bay of Biscay. However, clean-up activities on such accumulation regions are limited by the spread of the large-size floating litter on the sea surface. Data gathered by a small-scale fishing vessel devoted to active fishing for floating litter activities during the spring and summer of 2018 reveals that the linear streaks of high concentration of floating litter (so-called litter "windrows") are common accumulation structures in the south-east coast of the Bay of Biscay. The random search of litter windrows for their collection through surface tows of macro-nets was proved to be an effective action for floating litter mitigation. A total of 196 tows collected 16.2 tons of floating marine litter in 68 working days. Most of the litter windrows were around 1 km length and, on average, accumulated 77.75 kg of floating marine litter. Fishing, shipping and aquaculture sectors were the source of 35% of the 4,130 litter items analyzed (55% in weight of the sourced items), and plastic was the most common type of material (96% in terms of items). A better understanding of the phenomenon of the litter windrows, capable to guide clean-up efforts in space and time, would provide a considerable improvement in the efficiency of mitigation actions to reduce the marine litter pollution. The observations of litter windrows in the coastal area of the south-east of the Bay of Biscay demonstrate the key role of submesoscale processes in the distribution of FML. The present work provides a thorough description of floating litter windrows in nature, which it was non-existent to date. The results are the kind of proof necessary to boost the research addressed on the submesoscale aggregations of FML. Coupling litter windrows observations with remote-sensing technology and high-resolution modeling techniques offer great opportunities for the mitigation actions against marine litter
Impact of gaps in the asteroseismic characterization of pulsating stars. I. On the efficiency of pre-whitening
It is known that the observed distribution of frequencies in CoRoT and Kepler
{\delta} Scuti stars has no parallelism with any theoretical model.
Pre-whitening is a widespread technique in the analysis of time series with
gaps from pulsating stars located in the classical instability strip such as
{\delta} Scuti stars. However, some studies have pointed out that this
technique might introduce biases in the results of the frequency analysis. This
work aims at studying the biases that can result from pre-whitening in
asteroseismology. The results will depend on the intrinsic range and
distribution of frequencies of the stars. The periodic nature of the gaps in
CoRoT observations, just in the range of the pulsational frequency content of
the {\delta} Scuti stars, is shown to be crucial to determine their oscillation
frequencies, the first step to perform asteroseismolgy of these objects. Hence,
here we focus on the impact of pre-whitening on the asteroseismic
characterization of {\delta} Scuti stars. We select a sample of 15 {\delta}
Scuti stars observed by the CoRoT satellite, for which ultra-high quality
photometric data have been obtained by its seismic channel. In order to study
the impact on the asteroseismic characterization of {\delta} Scuti stars we
perform the pre-whitening procedure on three datasets: gapped data, linearly
interpolated data, and ARMA interpolated data. The different results obtained
show that at least in some cases pre-whitening is not an efficient procedure
for the deconvolution of the spectral window. therefore, in order to reduce the
effect of the spectral window to the minimum it is necessary to interpolate
with an algorithm that is aimed to preserve the original frequency content, and
not only to perform a pre-whitening of the data.Comment: 27 pages, 47 figures Tables and typos fixe
MIARMA: An information preserving method for filling gaps in time series. Application to CoRoT light curves
The method here presented intends to minimize the effect of the gaps in the
power spectra by gap-filling preserving the original information, that is, in
the case of asteroseismology, the stellar oscillation frequency content. We
make use of a forward-backward predictor based on autoregressive moving average
modelling (ARMA) in the time domain. The method MIARMA is particularly suitable
for replacing invalid data such as those present in the light curves of the
CoRoT satellite due to the pass through the South Atlantic Anomaly, and
eventually for the data gathered by the NASA planet hunter Kepler. We select a
sample of stars from the ultra-precise photometry collected by the
asteroseismic camera on board the CoRoT satellite: the {\delta} Scuti star HD
174966, showing periodic variations of the same order as the CoRoT
observational window, the Be star HD 51193, showing longer time variations, and
the solar-like HD 49933, with rapid time variations. We showed that in some
cases linear interpolations are less reliable to what was believed. In
particular: the power spectrum of HD 174966 is clearly aliased when this
interpolation is used for filling the gaps; the light curve of HD 51193
presents a much more aliased spectrum than expected for a low frequency
harmonic signal; and finally, although the linear interpolation does not affect
noticeably the power spectrum of the CoRoT light curve of the solar-like star
HD 49933, the ARMA interpolation showed rapid variations previously
unidentified that ARMA interprets as a signal. In any case, the ARMA
interpolation method provides a cleaner power spectrum, that is, less
contaminated by spurious frequencies. In conclusion, MIARMA appears to be a
suitable method for filling gaps in the light curves of pulsating stars
observed by CoRoT since the method preserves their frequency content, which is
a necessary condition for asteroseismic studies.Comment: 9 pages, 9 figures, submitted to A&
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