190 research outputs found
Main-Sequence and sub-giant stars in the Globular Cluster NGC6397: The complex evolution of the lithium abundance
Thanks to the high multiplex and efficiency of Giraffe at the VLT we have
been able for the first time to observe the Li I doublet in the Main Sequence
(MS) stars of a Globular Cluster. At the same time we observed Li in a sample
of Sub-Giant (SG) stars of the same B-V colour. Our final sample is composed of
84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same
temperature range we find that the equivalent widths of the Li I doublet in SG
stars are systematically larger than those in MS stars, suggesting a higher Li
content among SG stars. This is confirmed by our quantitative analysis. We
derived the effective temperatures, from H fitting, and NLTE Li
abundances of the stars in our the sample, using 3D and 1D models. We find that
SG stars have a mean Li abundance higher by 0.1dex than MS stars, using both 1D
and 3D models. We also detect a positive slope of Li abundance with effective
temperature. These results provide an unambiguous evidence that the Li
abundance changes with evolutionary status. The physical mechanisms responsible
for this behaviour are not yet clear, and none of the existing models seems to
describe accurately these observations. Based on these conclusions, we believe
that the cosmological lithium problem still remains an open question.Comment: Proceedings of the contributed talk presented at the IAU Symposium
26
Micron-sized single-crystal cathodes for sodium-ion batteries
Confining the particle-electrolyte interactions to the particle surface in electrode materials is vital to develop sustainable and safe batteries. Micron-sized single-crystal particles offer such opportunities. Owing to the reduced surface area and grain boundary-free core, particle-electrolyte interactions in micron-sized single-crystal particles will be confined to the particle surface. Here, we reveal the potential of such materials in sodium-ion batteries. We synthesized and investigated the chemical, electrochemical, and thermal properties of single-crystalline P2-type Na(0.7)Mn(0.9)Mg(0.1)O(2) as a cathode material for sodium-ion batteries. Single-crystalline Na(0.7)Mn(0.9)Mg(0.1)O(2) with a mean particle size of 8.1 μm exhibited high cycling and voltage stability. In addition, the exothermic heat released by the charged single-crystal Na(0.7)Mn(0.9)Mg(0.1)O(2) cathodes was four times lower than that of the corresponding polycrystalline Na(0.7)Mn(0.9)Mg(0.1)O(2). This significantly enhances the thermal stability of electrode materials and possibly mitigates thermal runaways in batteries. Surprisingly, single crystals of Na(0.7)Mn(0.9)Mg(0.1)O(2) were relatively stable in water and ambient atmosphere
An abundance analysis of a chemically peculiar B star – JL 87*,**
Aims. The aim of this study is to understand the nature and origin of a chemically peculiar star JL 87 by measuring its physical parameters and chemical abundances.
Methods. Physical parameters – effective temperature, surface gravity and helium abundance were measured from a moderate resolution optical spectrum using fully line-blanketed LTE model atmospheres. The effective temperature and extinction were verified by comparing FUSE, IUE spectrophotometry and optical/IR broadband photometry with theoretical flux distributions from LTE model atmospheres. The photospheric chemical abundances were measured from a high-resolution optical spectrum using LTE model atmospheres and spectral synthesis.
Results. On the basis of its physical parameters and chemical abundances, we confirm that JL 87 is a chemically peculiar subluminous B star. It is significantly cooler, has a lower surface gravity and is more helium-rich than previously believed. It is moderately enriched in carbon and nitrogen, but its overall metallicity is slightly subsolar.
Conclusions. The shallow-mixing model of a late core-flash on a white-dwarf cooling track currently provides the most consistent agreement with the observable properties of JL 87
Impact of granulation effects on the use of Balmer lines as temperature indicators
Balmer lines serve as important indicators of stellar effective temperatures
in late-type stellar spectra. One of their modelling uncertainties is the
influence of convective flows on their shape. We aim to characterize the
influence of convection on the wings of Balmer lines. We perform a differential
comparison of synthetic Balmer line profiles obtained from 3D hydrodynamical
model atmospheres and 1D hydrostatic standard ones. The model parameters are
appropriate for F,G,K dwarf and subgiant stars of metallicity ranging from
solar to 1/1000 solar. The shape of the Balmer lines predicted by 3D models can
never be exactly reproduced by a 1D model, irrespective of its effective
temperature. We introduce the concept of a 3D temperature correction, as the
effective temperature difference between a 3D model and a 1D model which
provides the closest match to the 3D profile. The temperature correction is
different for the different members of the Balmer series and depends on the
adopted mixing-length parameter in the 1D model. Among the investigated models,
the 3D correction ranges from -300K to +300K. Horizontal temperature
fluctuations tend to reduce the 3D correction. Accurate effective temperatures
cannot be derived from the wings of Balmer lines, unless the effects of
convection are properly accounted for. The 3D models offer a physically well
justified way of doing so. The use of 1D models treating convection with the
mixing-length theory do not appear to be suitable for this purpose. In
particular, there are indications that it is not possible to determine a single
value of the mixing-length parameter which will optimally reproduce the Balmer
lines for any choice of atmospheric parameters.Comment: 6 pages, 3 figures, accepted for publication in A&
Chemical abundances of distant extremely metal-poor unevolved stars
Aims: The purpose of our study is to determine the chemical composition of a
sample of 16 candidate Extremely Metal-Poor (EMP) dwarf stars, extracted from
the Sloan Digital Sky Survey (SDSS). There are two main purposes: in the first
place to verify the reliability of the metallicity estimates derived from the
SDSS spectra; in the second place to see if the abundance trends found for the
brighter nearer stars studied previously also hold for this sample of fainter,
more distant stars. Methods: We used the UVES at the VLT to obtain
high-resolution spectra of the programme stars. The abundances were determined
by an automatic analysis with the MyGIsFOS code, with the exception of lithium,
for which the abundances were determined from the measured equivalent widths of
the Li I resonance doublet. Results: All candidates are confirmed to be EMP
stars, with [Fe/H]<= -3.0. The chemical composition of the sample of stars is
similar to that of brighter and nearer samples. We measured the lithium
abundance for 12 stars and provide stringent upper limits for three other
stars, for a fourth star the upper limit is not significant, owing to the low
signal-to noise ratio of the spectrum. The "meltdown" of the Spite plateau is
confirmed, but some of the lowest metallicity stars of the sample lie on the
plateau. Conclusions: The concordance of the metallicities derived from
high-resolution spectra and those estimated from the SDSS spectra suggests that
the latter may be used to study the metallicity distribution of the halo. The
abundance pattern suggests that the halo was well mixed for all probed
metallicities and distances. The fact that at the lowest metallicities we find
stars on the Spite plateau suggests that the meltdown depends on at least
another parameter, besides metallicity. (abridged)Comment: A&A in pres
Universal geometric approach to uncertainty, entropy and information
It is shown that for any ensemble, whether classical or quantum, continuous
or discrete, there is only one measure of the "volume" of the ensemble that is
compatible with several basic geometric postulates. This volume measure is thus
a preferred and universal choice for characterising the inherent spread,
dispersion, localisation, etc, of the ensemble. Remarkably, this unique
"ensemble volume" is a simple function of the ensemble entropy, and hence
provides a new geometric characterisation of the latter quantity. Applications
include unified, volume-based derivations of the Holevo and Shannon bounds in
quantum and classical information theory; a precise geometric interpretation of
thermodynamic entropy for equilibrium ensembles; a geometric derivation of
semi-classical uncertainty relations; a new means for defining classical and
quantum localization for arbitrary evolution processes; a geometric
interpretation of relative entropy; and a new proposed definition for the
spot-size of an optical beam. Advantages of the ensemble volume over other
measures of localization (root-mean-square deviation, Renyi entropies, and
inverse participation ratio) are discussed.Comment: Latex, 38 pages + 2 figures; p(\alpha)->1/|T| in Eq. (72) [Eq. (A10)
of published version
The solar photospheric abundance of hafnium and thorium. Results from CO5BOLD 3D hydrodynamic model atmospheres
Context: The stable element hafnium (Hf) and the radioactive element thorium
(Th) were recently suggested as a suitable pair for radioactive dating of
stars. The applicability of this elemental pair needs to be established for
stellar spectroscopy. Aims: We aim at a spectroscopic determination of the
abundance of Hf and Th in the solar photosphere based on a \cobold 3D
hydrodynamical model atmosphere. We put this into a wider context by
investigating 3D abundance corrections for a set of G- and F-type dwarfs.
Method: High-resolution, high signal-to-noise solar spectra were compared to
line synthesis calculations performed on a solar CO5BOLD model. For the other
atmospheres, we compared synthetic spectra of CO5BOLD 3D and associated 1D
models. Results: For Hf we find a photospheric abundance A(Hf)=0.87+-0.04, in
good agreement with a previous analysis, based on 1D model atmospheres. The
weak Th ii 401.9 nm line constitutes the only Th abundance indicator available
in the solar spectrum. It lies in the red wing of an Ni-Fe blend exhibiting a
non-negligible convective asymmetry. Accounting for the asymmetry-related
additional absorption, we obtain A(Th)=0.09+-0.03, consistent with the
meteoritic abundance, and about 0.1 dex lower than obtained in previous
photospheric abundance determinations. Conclusions: Only for the second time,
to our knowledge, has am non-negligible effect of convective line asymmetries
on an abundance derivation been highlighted. Three-dimensional hydrodynamical
simulations should be employed to measure Th abundances in dwarfs if similar
blending is present, as in the solar case. In contrast, 3D effects on Hf
abundances are small in G- to mid F-type dwarfs and sub-giants, and 1D model
atmospheres can be conveniently used.Comment: A&A, in pres
Discovery of a variable lead-rich hot subdwarf: UVO 0825+15
UVO 0825+15 is a hot bright helium-rich subdwarf which lies in K2 Field 5 and in a sample of intermediate helium-rich subdwarfs observed the Subaru High Dispersion Spectrograph. The K2 light curve shows low-amplitude variations, whilst the Subaru spectrum shows Pb IV absorption lines, indicative of a very high lead overabundance. UVO 0825+15 also has a high proper motion with kinematics typical for a thick disc star. Analyses of ultraviolet and intermediate dispersion optical spectra rule out a short-period binary companion and provide fundamental atmospheric parameters of Teff=38 900±270 K, logg/cms−2=5.97±0.11, log nHe/nH = −0.57 ± 0.01, EB − V ≈ 0.03, and angular radius θ = 1.062 ± 0.006 × 10−11 radians (formal errors). The high-resolution spectrum shows that carbon is \u3e2 dex subsolar, iron is approximately solar, and all other elements heavier than argon are at least 2–4 dex overabundant, including germanium, yttrium and lead. Approximately 150 lines in the blue-optical spectrum remain unidentified. The chemical structure of the photosphere is presumed to be determined by radiatively dominated diffusion. The K2 light curve shows a dominant period around 10.8 h, with a variable amplitude, its first harmonic, and another period at 13.3 h. The preferred explanation is multiperiodic non-radial oscillation due to g modes with very high radial order, although this presents difficulties for pulsation theory. Alternative explanations fail for lack of radial-velocity evidence. UVO 0825+15 represents the fourth member of a group of hot subdwarfs having helium-enriched photospheres and 3–4 dex overabundances of trans-iron elements and is the first lead-rich subdwarf to show evidence of pulsations
Discovery of a variable lead-rich hot subdwarf: UVO 0825+15
UVO 0825+15 is a hot bright helium-rich subdwarf which lies in K2 Field 5 and in a sample of intermediate helium-rich subdwarfs observed the Subaru High Dispersion Spectrograph. The K2 light curve shows low-amplitude variations, whilst the Subaru spectrum shows Pb IV absorption lines, indicative of a very high lead overabundance. UVO 0825+15 also has a high proper motion with kinematics typical for a thick disc star. Analyses of ultraviolet and intermediate dispersion optical spectra rule out a short-period binary companion and provide fundamental atmospheric parameters of Teff=38 900±270 K, logg/cms−2=5.97±0.11, log nHe/nH = −0.57 ± 0.01, EB − V ≈ 0.03, and angular radius θ = 1.062 ± 0.006 × 10−11 radians (formal errors). The high-resolution spectrum shows that carbon is \u3e2 dex subsolar, iron is approximately solar, and all other elements heavier than argon are at least 2–4 dex overabundant, including germanium, yttrium and lead. Approximately 150 lines in the blue-optical spectrum remain unidentified. The chemical structure of the photosphere is presumed to be determined by radiatively dominated diffusion. The K2 light curve shows a dominant period around 10.8 h, with a variable amplitude, its first harmonic, and another period at 13.3 h. The preferred explanation is multiperiodic non-radial oscillation due to g modes with very high radial order, although this presents difficulties for pulsation theory. Alternative explanations fail for lack of radial-velocity evidence. UVO 0825+15 represents the fourth member of a group of hot subdwarfs having helium-enriched photospheres and 3–4 dex overabundances of trans-iron elements and is the first lead-rich subdwarf to show evidence of pulsations
The railroad switch effect of seasonally reversing currents on the Bay of Bengal high salinity core
The Southwest Monsoon Current (SMC) flows eastward from the Arabian Sea into the Bay of Bengal (BoB) during summer, advecting a core of high salinity water. This high salinity core has been linked with Arabian Sea High Salinity Water that is presumed to enter the BoB directly from the Arabian Sea via the SMC. Here we show that the high salinity core originates primarily from the western equatorial Indian Ocean, reaching the BoB via the Somali Current, the Equatorial Undercurrent and the SMC. Years with anomalously saline high salinity cores are linked with the East Africa Coastal Current and the Somali Current winter convergence, and an anomalously strong Equatorial Undercurrent. Seasonal reversals that occur at the Somali Current and SMC junctions act as 'railroad switches' diverting water masses to different basins in the northern Indian Ocean. Interannual fluctuations of the Equatorial Undercurrent are linked to wind stress and El Nino
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