Chemical abundance analysis of 19 barium stars

We aim at deriving accurate atmospheric parameters and chemical abundances of 19 barium (Ba) stars, including both strong and mild Ba stars, based on the high signal-to-noise ratio and high resolution Echelle spectra obtained from the 2.16 m telescope at Xinglong station of National Astronomical Observatories, Chinese Academy of Sciences. The chemical abundances of the sample stars were obtained from an LTE, plane-parallel and line-blanketed atmospheric model by inputting the atmospheric parameters (effective temperatures, surface gravities, metallicity and microturbulent velocity) and equivalent widths of stellar absorption lines. These samples of Ba stars are giants indicated by atmospheric parameters, metallicities and kinematic analysis about UVW velocity. Chemical abundances of 17 elements were obtained for these Ba stars. Their light elements (O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn and Ni) are similar to the solar abundances. Our samples of Ba stars show obvious overabundances of neutron-capture (n-capture) process elements relative to the Sun. Their median abundances of [Ba/Fe], [La/Fe] and [Eu/Fe] are 0.54, 0.65 and 0.40, respectively. The YI and ZrI abundances are lower than Ba, La and Eu, but higher than the light elements for the strong Ba stars and similar to the iron-peak elements for the mild stars. There exists a positive correlation between Ba intensity and [Ba/Fe]. For the n-capture elements (Y, Zr, Ba, La), there is an anti-correlation between their [X/Fe] and [Fe/H]. We identify nine of our sample stars as strong Ba stars with [Ba/Fe]>0.6 where seven of them have Ba intensity Ba=2-5, one has Ba=1.5 and another one has Ba=1.0. The remaining ten stars are classified as mild Ba stars with 0.17<[Ba/Fe]<0.54

Наноструктурные эмиссионно-активные сплавы алюминия для сильноточных фотокатодов с лазерным возбуждением

В неравновесных условиях магнетронного распыления обнаружен эффект растворимости бария (до 2 ат.%) в ГЦК-решетке алюминия. Плотность наночастиц в пленке составляет ~10¹² см⁻². Первопринципными методами рассчитана электронная структура твердого раствора Al-Ba. Результаты вычислений ab initio предсказывают гибридизацию атомных орбиталей Al и Ba посредством перекрытия электронных s-, p-, d-состояний Al и d-состояний Ba. Дополнительные вычисления ab initio электронной структуры атомных группировок с использованием теории функционала электронной плотности доказывают принципиальную возможность существования в твердом растворе Al-Ba нанокластера Al₁₈Ba₉, структура которого близка к структуре Al₄Ba. Картины электронной микродифракции и данные оже-спектроскопии доказывают образование нанокластеров AlmBan в сплавах Al-Ba и Al-Ba,Li.У нерівноважних умовах магнетронного розпилення виявлено ефект розчинності барію (до 2 ат.%) в ГЦК-решітці алюмінію. Щільність наночастинок у плівці становить ~10¹² см⁻². Першопринципними методами розраховано електронну структуру твердого розчину Al-Ba. Результати обчислень ab initio передбачають гібридизацію атомних орбіталей Al та Ba через перекриття електронних s-, p-, d-станів Al та d-станів Ba. Додаткові обчислення ab initio електронної структури атомних групувань із використанням теорії функціонала електронної щільності доводять принципову можливість існування у твердому розчині Al-Ba нанокластера Al₁₈Ba₉, структура якого близька до структури Al₄Ba. Картини електронної мікродифракції та дані оже-спектроскопії доводять утворення нанокластерів AlmBan у сплавах Al-Ba, Al-Ba,Li.The effect of barium solubility (up to 2 at.%) in aluminum was revealed in Al-Ba films with structure close to that of Al₄Ba. The films were deposited in nonequilibrium conditions of magnetron sputtering. The density of nanoparticles in film is ~10¹² cm⁻². Ålectron structure of Al-Ba solid solution was calculated based on ab initio principle. Results of calculations predict the hybridization of atomic orbitals of Al and Ba by overlapping of s-, p-, d-states of Al with d-states of Ba. Calculations of the electronic structure of atomic groups on the base of the density functional theory prove the possibility of the existence of Al₁₈Ba₉ nanoclusters inside the Al-Ba solid solution. Investigations of crystal structure of Al-Ba-Li film alloy using electron diffraction and Auger-spectroscopy prove the formation of AlmBan nanoclusters in Al-Ba and Al-Ba-Li alloy films

Abundances in giant stars of the globular cluster NGC 6752

Recent theoretical yields and chemical evolution models demonstrate that intermediate-mass AGB stars cannot reproduce the observed abundance distributions of O, Na, Mg, and Al. As a further observational test of this finding, we present elemental abundance ratios [X/Fe] for 20 elements in 38 bright giants of the globular cluster NGC 6752. Our mean abundance ratios [X/Fe] are in good agreement with previous studies of this cluster and are also consistent with other globular clusters and field stars at the same metallicity. The mean abundance ratios [Ba/Eu] and [La/Eu] exhibit values, in agreement with field stars at the same metallicity, that lie approximately midway between the pure r-process and the solar (s-process + r-process) mix, indicating that AGB stars have played a role in the chemical evolution of the proto-cluster gas. For the first time, we find possible evidence for an abundance variation for elements heavier than Al in this cluster. We find a correlation between [Si/Fe] and [Al/Fe] which is consistent with the abundance anomalies being synthesized via proton captures at high temperatures. Leakage from the Mg-Al chain into 28Si may explain the Si excess in stars with the highest [Al/Fe]. We identify correlations between [Y/Fe] and [Al/Fe], [Zr/Fe] and [Al/Fe], and [Ba/Fe] and [Al/Fe] suggesting that Y, Zr, and Ba abundances may increase by about 0.1 dex as Al increases by about 1.3 dex. While the correlations are statistically significant, the amplitudes of the variations are small. If the small variations in Y, Zr, and Ba are indeed real, then the synthesis of the Al anomalies must have taken place within an unknown class of stars that also ran the s-process. [Abridged]Comment: Accepted for publication in A&

Predicting ground-state configurations and electronic properties of the thermoelectric clathrates Ba8_{8}Alx_{x}Si46−x_{46-x} and Sr8_{8}Alx_{x}Si46−x_{46-x}

The structural and electronic properties of the clathrate compounds Ba$_{8}$Al$_{x}$Si$_{46-x}$ and Sr$_{8}$Al$_{x}$Si$_{46-x}$ are studied from first principles, considering an Al content $x$ between 6 and 16. Due to the large number of possible substitutional configurations we make use of a special iterative cluster-expansion approach, to predict ground states and quasi-degenerate structures in a highly efficient way. These are found from a simulated annealing technique where millions of configurations are sampled. For both compounds, we find a linear increase of the lattice constant with the number of Al substituents, confirming experimental observations for Ba$_{8}$Al$_{x}$Si$_{46-x}$. Also the calculated bond distances between high-symmetry sites agree well with experiment for the full compositional range. For $x$ being below 16, all configurations are metallic for both materials. At the charge-balanced composition ($x=16$), the substitutional ordering leads to a metal-semiconductor transition, and the ground states of Ba$_{8}$Al$_{16}$Si$_{30}$ and Sr$_{8}$Al$_{16}$Si$_{30}$ exhibit indirect Kohn-Sham band gaps of 0.36 and 0.30 eV, respectively, while configurations higher in energy are metals. The finding of semiconducting behavior is a promising result in view of exploiting these materials in thermoelectric applications.Comment: 9 figure

The chemical composition of nearby young associations: s-process element abundances in AB Doradus, Carina-Near, and Ursa Major

Recently, several studies have shown that young, open clusters are characterised by a considerable over-abundance in their barium content. In particular, D'Orazi et al. (2009) reported that in some younger clusters [Ba/Fe] can reach values as high as ~0.6 dex. The work also identified the presence of an anti-correlation between [Ba/Fe] and cluster age. For clusters in the age range ~4.5 Gyr-500 Myr, this is best explained by assuming a higher contribution from low-mass asymptotic giant branch stars to the Galactic chemical enrichment. The purpose of this work is to investigate the ubiquity of the barium over-abundance in young stellar clusters. We analysed high-resolution spectroscopic data, focusing on the s-process elemental abundance for three nearby young associations, i.e. AB Doradus, Carina-Near, and Ursa Major. The clusters have been chosen such that their age spread would complement the D'Orazi et al. (2009) study. We find that while the s-process elements Y, Zr, La, and Ce exhibit solar ratios in all three associations, Ba is over-abundant by ~0.2 dex. Current theoretical models can not reproduce this abundance pattern, thus we investigate whether this unusually large Ba content might be related to chromospheric effects. Although no correlation between [Ba/Fe] and several activity indicators seems to be present, we conclude that different effects could be at work which may (directly or indirectly) be related to the presence of hot stellar chromospheres.Comment: Accepted for publication in MNRA

The NLTE Barium Abundance in Dwarf Stars in the Metallicity Range of -1 < [Fe/H] < +0.3

We present the results of determination of the barium abundance considering the non-LTE (NLTE) effects in 172 dwarf stars in the metallicity range of -1< [Fe/H] <+0.3, assigned to different Galactic substructures by kinematic criteria. We used a model of the Ba atom with 31 levels of Ba I and 101 levels of Ba II. The atmosphere models for the investigated stars were computed using the ATLAS9 code modified by new opacity distribution functions. The NLTE profiles of the unblended Ba II (4554 A, 5853 A, 6496 A) were computed and then compared to those observed. The line 6141 A was also used, but with an allowance for its correlation with the iron line. The average barium abundances in the thin and thick discs are 0.01 +/- 0.08 and -0.03 +/- 0.07, respectively. The comparison to the calculations of the Galactic chemical evolution by Serminato et al. (2009) was conducted. The trend obtained for the Ba abundance versus [Fe/H] suggests a complex barium production process in the thin and thick discs

Superconductivity of MI(MII0.5, Si0.5)2 (MI=Sr and Ba, MII=Al and Ga), ternary silicides with the AlB2-type structure

Ternary silicides MI(MII0.5, Si0.5)2 (MI=Sr and Ba, MII=Al and Ga) were prepared by Ar arc melting. Powder X-ray diffraction indicates that they have the AlB2-type structure, in which Si and MII atoms are arranged in honeycomb layers and MI atoms are intercalated between them. Electrical resistivity and dc magnetization measurements revealed that Sr(Al0.5, Si0.5)2 is superconductive, with a critical temperature for superconductivity (TC) of 4.2 K, while Ba(Al0.5, Si0.5)2 is not at temperatures ranging above 2.0 K. Sr(Ga0.5, Si0.5)2 and Ba(Ga0.5, Si0.5)2 are also superconductors, with TCs of 5.1 and 3.3 K, respectively.Comment: 5 pages, 3 tables, 4 figure

Comparison of charge modulations in La1.875_{1.875}Ba0.125_{0.125}CuO4_4 and YBa2_2Cu3_3O6.6_{6.6}

A charge modulation has recently been reported in (Y,Nd)Ba$_2$Cu$_3$O$_{6+x}$ [Ghiringhelli {\em et al.} Science 337, 821 (2013)]. Here we report Cu $L_3$ edge soft x-ray scattering studies comparing the lattice modulation associated with the charge modulation in YBa$_2$Cu$_3$O$_{6.6}$ with that associated with the well known charge and spin stripe order in La$_{1.875}$Ba$_{0.125}$CuO$_4$. We find that the correlation length in the CuO$_2$ plane is isotropic in both cases, and is $259 \pm 9$ \AA for La$_{1.875}$Ba$_{0.125}$CuO$_4$ and $55 \pm 15$ \AA for YBa$_2$Cu$_3$O$_{6.6}$. Assuming weak inter-planar correlations of the charge ordering in both compounds, we conclude that the order parameters of the lattice modulations in La$_{1.875}$Ba$_{0.125}$CuO$_4$ and YBa$_2$Cu$_3$O$_{6.6}$ are of the same order of magnitude.Comment: 3 pages, 2 figure