823 research outputs found
The tidal effects on the lithium abundance of binary systems with giant component
We analise the behavior of lithium abundance as a function of effective
temperature, projected rotational velocity, orbital period and eccentricity for
a sample of 68 binary systems with giant component and orbital period ranging
from about 10 to 6400 days. For these binary systems the Li abundances show a
gradual decrease with temperature, paralleling the well established result for
single giants. We have also observed a dependence of lithium content on
rotation. Binary systems with moderate to high rotation present also moderate
to high Li content. This study shows also that synchronized binary systems with
giant component seems to retain more of their original lithium than the
unsynchronized systems. For orbital periods lower than 100 to 250 days,
typically the period of synchronization for this kind of binary systems,
lithium depleted stars seems to be unusual. The suggestion is made that there
is an 'inhibited zone' in which synchronized binary systems with giant
component having lithium abundance lower than a threshold level should be
unusual.Comment: 6 pages, 3 Postscript figures, uses: aa.cls, psfig.st
Structured star formation in the Magellanic inter-Cloud region
We use a new contiguous imaging survey conducted using the Dark Energy Camera to investigate the distribution and properties of young stellar populations in the Magellanic inter-Cloud region. These young stars are strongly spatially clustered, forming a narrow chain of low-mass associations that trace the densest H I gas in the Magellanic Bridge and extend, in projection, from the SMC to the outer disc of the LMC. The associations in our survey footprint have ages ≲ 30 Myr, masses in the range ∼100-1200 M⊙ and very diffuse structures with half-light radii of up to ∼100 pc. The two most populous are strongly elliptical and aligned to ≈10°, with the axis joining the centres of the LMC and the SMC. These observations strongly suggest that the young inter-Cloud populations formed in situ, likely due to the compression of gas stripped during the most recent close LMC-SMC encounter. The associations lie at distances intermediate between the two Clouds, and we find no evidence for a substantial distance gradient across the imaged area. Finally, we identify a vast shell of young stars surrounding a central association, that is spatially coincident with a low column density bubble in the H I distribution. The properties of this structure are consistent with a scenario where stellar winds and supernova explosions from massive stars in the central cluster swept up the ambient gas into a shell, triggering a new burst of star formation. This is a prime location for studying stellar feedback in a relatively isolated environment
Surface superconductivity in multilayered rhombohedral graphene: Supercurrent
The supercurrent for the surface superconductivity of a flat-band
multilayered rhombohedral graphene is calculated. Despite the absence of
dispersion of the excitation spectrum, the supercurrent is finite. The critical
current is proportional to the zero-temperature superconducting gap, i.e., to
the superconducting critical temperature and to the size of the flat band in
the momentum space
Field-induced polarisation of Dirac valleys in bismuth
Electrons are offered a valley degree of freedom in presence of particular
lattice structures. Manipulating valley degeneracy is the subject matter of an
emerging field of investigation, mostly focused on charge transport in
graphene. In bulk bismuth, electrons are known to present a threefold valley
degeneracy and a Dirac dispersion in each valley. Here we show that because of
their huge in-plane mass anisotropy, a flow of Dirac electrons along the
trigonal axis is extremely sensitive to the orientation of in-plane magnetic
field. Thus, a rotatable magnetic field can be used as a valley valve to tune
the contribution of each valley to the total conductivity. According to our
measurements, charge conductivity by carriers of a single valley can exceed
four-fifth of the total conductivity in a wide range of temperature and
magnetic field. At high temperature and low magnetic field, the three valleys
are interchangeable and the three-fold symmetry of the underlying lattice is
respected. As the temperature lowers and/or the magnetic field increases, this
symmetry is spontaneously lost. The latter may be an experimental manifestation
of the recently proposed valley-nematic Fermi liquid state.Comment: 14 pages + 5 pages of supplementary information; a slightly modified
version will appear as an article in Nature physic
The Andromeda Project. I. Deep HST-WFPC2 V,I photometry of 16 fields toward the disk and the halo of the M31 galaxy. Probing the stellar content and metallicity distribution
HST-WFPC2 F555W and F814W photometry were obtained for 16 fields of the
luminous nearby spiral galaxy M31, sampling the stellar content of the disk and
the halo at different distances from the center, from ~ 20 to ~ 150 arcmin
(i.e. ~ 4.5 to 35 kpc), down to limiting V and I magnitudes of ~ 27. The
Color-Magnitude diagrams (CMD) show the presence of complex stellar
populations, including an intermediate age/young population and older
populations with a wide range of metallicity. Those fields superposed on the
disk of M31 generally show a blue plume of stars which we identify with main
sequence members. Accordingly, the star formation rate over the last 0.5 Gyr
appears to have varied dramatically with location in the disk. All the CMDs
show a prominent Red Giant Branch (RGB) with a descending tip in the V band,
characteristic of metallicity higher than 1/10 Solar. A red clump is detected
in all of the fields, and a weak blue horizontal branch is frequently present.
The metallicity distributions (MDs), obtained by comparison of the RGB stars
with globular cluster templates, are basically similar in all the sampled
fields: they all show a long, albeit scantly populated metal-poor tail and a
main component at [Fe/H] ~ -0.6. However, some differences also exist, e.g. in
some fields a very metal-rich ([Fe/H] >= -0.2) component is present. Whereas
the fraction of metal-poor stars seems to be approximately constant in all
fields, the fraction of very-metal-rich stars varies with position and seems to
be more prominent in those fields superposed on the disk and/or with the
presence of streams or substructures. This might indicate and possibly trace
interaction effects with some companion, e.g. M32.Comment: 23 pages (including 5 tables), 22 figures, submitted to A&
Large-scale physically accurate modelling of real proton exchange membrane fuel cell with deep learning
Proton exchange membrane fuel cells, consuming hydrogen and oxygen to generate clean electricity and water, suffer acute liquid water challenges. Accurate liquid water modelling is inherently challenging due to the multi-phase, multi-component, reactive dynamics within multi-scale, multi-layered porous media. In addition, currently inadequate imaging and modelling capabilities are limiting simulations to small areas (<1 mm2) or simplified architectures. Herein, an advancement in water modelling is achieved using X-ray micro-computed tomography, deep learned super-resolution, multi-label segmentation, and direct multi-phase simulation. The resulting image is the most resolved domain (16 mm2 with 700 nm voxel resolution) and the largest direct multi-phase flow simulation of a fuel cell. This generalisable approach unveils multi-scale water clustering and transport mechanisms over large dry and flooded areas in the gas diffusion layer and flow fields, paving the way for next generation proton exchange membrane fuel cells with optimised structures and wettabilities
Flat bands in topological media
Topological media are systems whose properties are protected by topology and
thus are robust to deformations of the system. In topological insulators and
superconductors the bulk-surface and bulk-vortex correspondence gives rise to
the gapless Weyl, Dirac or Majorana fermions on the surface of the system and
inside vortex cores. Here we show that in gapless topological media, the
bulk-surface and bulk-vortex correspondence is more effective: it produces
topologically protected gapless fermions without dispersion -- the flat band.
Fermion zero modes forming the flat band are localized on the surface of
topological media with protected nodal lines and in the vortex core in systems
with topologically protected Fermi points (Weyl points). Flat band has an
extremely singular density of states, and we show that this property may give
rise in particular to surface superconductivity which could exist even at room
temperature.Comment: 9 pages, 5 figures, version to appear in JETP Letter
A systematic review on health resilience to economic crises
Background The health effects of recent economic crises differ markedly by population group. The objective of this systematic review is to examine evidence from longitudinal studies on factors influencing resilience for any health outcome or health behaviour among the general population living in countries exposed to financial crises. Methods We systematically reviewed studies from six electronic databases (EMBASE, Global Health, MEDLINE, PsycINFO, Scopus, Web of Science) which used quantitative longitudinal study designs and included: (i) exposure to an economic crisis; (ii) changes in health outcomes/behaviours over time; (iii) statistical tests of associations of health risk and/or protective factors with health outcomes/behaviours. The quality of the selected studies was appraised using the Quality Assessment Tool for Quantitative Studies. PRISMA reporting guidelines were followed. Results From 14,584 retrieved records, 22 studies met the eligibility criteria. These studies were conducted across 10 countries in Asia, Europe and North America over the past two decades. Ten socio-demographic factors that increased or protected against health risk were identified: gender, age, education, marital status, household size, employment/occupation, income/ financial constraints, personal beliefs, health status, area of residence, and social relations. These studies addressed physical health, mortality, suicide and suicide attempts, mental health, and health behaviours. Women’s mental health appeared more susceptible to crises than men’s. Lower income levels were associated with greater increases in cardiovascular disease, mortality and worse mental health. Employment status was associated with changes in mental health. Associations with age, marital status, and education were less consistent, although higher education was associated with healthier behaviours. Conclusions Despite widespread rhetoric about the importance of resilience, there was a dearth of studies which operationalised resilience factors. Future conceptual and empirical research is needed to develop the epidemiology of resilience
Abundance analysis of barium and mild barium stars
High signal to noise, high resolution spectra were obtained for a sample of
normal, mild barium, and barium giants. Atmospheric parameters were determined
from the FeI and FeII lines. Abundances for Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr,
Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, Eu, and Gd, were
determined from equivalent widths and model atmospheres in a differential
analysis, with the red giant Eps Vir as the standard star. The different levels
of s-process overabundances of barium and mild barium stars were earlier
suggested to be related to the stellar metallicity. Contrary to this
suggestion, we found in this work no evidence of barium and mild barium to
having different range in metallicity. However, comparing the ratio of
abundances of heavy to light s-process elements, we found some evidence that
they do not share the same neutron exposure parameter. The exact mechanism
controlling this difference is still not clear. As a by-product of this
analysis we identify two normal red giants misclassified as mild barium stars.
The relevance of this finding is discussed. Concerning the suggested
nucleosynthetic effects possibly related to the s-process, for elements like
Cu, Mn, V and Sc, we found no evidence for an anomalous behavior in any of the
s-process enriched stars analyzed here. However, further work is still needed
since a clear [Cu/Fe] vs. [Ba/H] anticorrelation exists for other s-process
enriched objects.Comment: 34 pages, 18 figures, A&A accepted (revised version
Anisotropy in the Hubble constant as observed in the HST Extragalactic Distance Scale Key Project results
Based on general relativity, it can be argued that deviations from a uniform
Hubble flow should be thought of as variations in the Universe's expansion
velocity field, rather than being thought of as peculiar velocities with
respect to a uniformly expanding space. The aim of this paper is to use the
observed motions of galaxies to map out variations in the Universe's expansion,
and more importantly, to investigate whether real variations in the Hubble
expansion are detectable given the observational uncertainties. All-sky maps of
the observed variation in the expansion are produced using measurements
obtained along specific lines-of-sight and smearing them across the sky using a
Gaussian profile. A map is produced for the final results of the HST
Extragalactic Distance Scale Key Project for the Hubble constant, a comparison
map is produced from a set of essentially independent data, and Monte Carlo
techniques are used to analyse the statistical significance of the variation in
the maps. A statistically significant difference in expansion rate of 9
km/s/Mpc is found to occur across the sky. Comparing maps of the sky at
different distances appears to indicate two distinct sets of extrema with even
stronger statistically significant variations. Within our supercluster,
variations tend to occur near the supergalactic plane, and beyond our
supercluster, variations tend to occur away from the supergalactic plane.
Comparison with bulk flow studies shows some concordance, yet also suggests the
bulk flow studies may suffer confusion, failing to discern the influence of
multiple perturbations.Comment: 23 pages, 5 figures, to be published in New Astronom
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