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[en] The ρ Puppis stars are mid-F-type stars that show peculiar chemical-abundance patterns similar to those of the Am stars. Typically they exhibit overabundances of iron-peak elements such as Fe and Ni and s- and r-process elements such as Sr and Eu, and underabundances of certain other elements including He, Ca, and Sc. It has been proposed that these stars are evolving Am stars passing through the short-lived phase that occurs between the re-establishment of convection and the consequent erasure of their chemical peculiarities. In this paper we suggest a second hypothesis: these stars may have acquired their peculiar abundance patterns in a fashion similar to the Barium stars, i.e., they may have gained the s-process element enhancements via mass transfer from a once asymptotic giant-branch companion star, now turned white dwarf. This study will detail our efforts to investigate the ρ Puppis stars with regard to these two hypotheses, with a view to understanding these stars and their significance in terms of stellar evolution. We have performed a spectral classification survey, and a detailed chemical-abundance analysis of selected ρ Puppis stars. This paper gives an overview of the ρ Puppis stars and describes the spectral classification survey which has increased the number of ρ Puppis stars currently known from 6 to 49. A future paper will describe the chemical-abundance analysis which should provide insight into the nature of these stars.
[en] The atmospheric parameters of the components of the 16 Cygni binary system, in which the secondary has a gas giant planet detected, are measured accurately using high-quality observational data. Abundances relative to solar are obtained for 25 elements with a mean error of σ([X/H]) = 0.023 dex. The fact that 16 Cyg A has about four times more lithium than 16 Cyg B is normal considering the slightly different masses of the stars. The abundance patterns of 16 Cyg A and B, relative to iron, are typical of that observed in most of the so-called solar twin stars, with the exception of the heavy elements (Z > 30), which can, however, be explained by Galactic chemical evolution. Differential (A-B) abundances are measured with even higher precision (σ(Δ[X/H]) = 0.018 dex, on average). We find that 16 Cyg A is more metal-rich than 16 Cyg B by Δ[M/H] = +0.041 ± 0.007 dex. On an element-to-element basis, no correlation between the A-B abundance differences and dust condensation temperature (TC) is detected. Based on these results, we conclude that if the process of planet formation around 16 Cyg B is responsible for the observed abundance pattern, the formation of gas giants produces a constant downward shift in the photospheric abundance of metals, without a TC correlation. The latter would be produced by the formation of terrestrial planets instead, as suggested by other recent works on precise elemental abundances. Nevertheless, a scenario consistent with these observations requires the convective envelopes of ≅ 1 Msun stars to reach their present-day sizes about three times quicker than predicted by standard stellar evolution models.
[en] We report Zn abundances for 18 very metal-poor stars studied in our previous work, covering the metallicity range -3.2< [Fe/H] <-2.5. The [Zn/Fe] values of most stars show an increasing trend with decreasing [Fe/H] in this metallicity range, confirming the results found by previous studies. However, the extremely metal-poor star BS 16920-017([Fe/H] =-3.2) exhibits a significantly high [Zn/Fe] ratio ([Zn/Fe] = +1.0). Comparison of the chemical abundances of this object with HD 4306, which has similar atmospheric parameters to BS 16920-017, clearly demonstrates a deficiency of α elements and neutron-capture elements in this star, along with enhancements of Mn and Ni, as well as Zn. The association with a hypernova explosion that has been proposed to explain the high Zn abundance ratios found in extremely metal-poor stars is a possible explanation, although further studies are required to fully interpret the abundance pattern of this object.
[en] The behaviour of the OI triplet at λ 7773 in a sample of 74 Ap stars is analysed and compared with the results derived for a set of 50 normal stars. These abundance determinations are made in the NLTE frame by introducing a correction to the LTE model atmosphere. Among the Ap stars, the oxygen abundance varies greatly from one group to another and shows a clear separation between the different classes of peculiarities. An underabundance of up to a factor 400 is found for the (Sr-Cr-Eu) stars
[en] Results of a detailed abundance analysis of the solar twins 16 Cyg A and 16 Cyg B based on high-resolution, high signal-to-noise ratio echelle spectroscopy are presented. 16 Cyg B is known to host a giant planet while no planets have yet been detected around 16 Cyg A. Stellar parameters are derived directly from our high-quality spectra, and the stars are found to be physically similar, with ΔTeff = +43 K, Δlog g = -0.02 dex, and Δξ = +0.10 km s-1 (in the sense of A - B), consistent with previous findings. Abundances of 15 elements are derived and are found to be indistinguishable between the two stars. The abundances of each element differ by ≤0.026 dex, and the mean difference is +0.003 ± 0.015 (σ) dex. Aside from Li, which has been previously shown to be depleted by a factor of at least 4.5 in 16 Cyg B relative to 16 Cyg A, the two stars appear to be chemically identical. The abundances of each star demonstrate a positive correlation with the condensation temperature of the elements (Tc); the slopes of the trends are also indistinguishable. In accordance with recent suggestions, the positive slopes of the [m/H]-Tc relations may imply that terrestrial planets have not formed around either 16 Cyg A or 16 Cyg B. The physical characteristics of the 16 Cyg system are discussed in terms of planet formation models, and plausible mechanisms that can account for the lack of detected planets around 16 Cyg A, the disparate Li abundances of 16 Cyg A and B, and the eccentricity of the planet 16 Cyg B b are suggested.
[en] Stellar streams are the kinematically coherent groups of stars, scattered all over the sky, and are not gravitationally bound. They appear as over-densities in the velocity space. Open clusters are groups of coeval, chemically homogeneous stars, gravitationally bound with each other. The striking similarity between stellar streams and open clusters is that, both are kinematically coherent groups. It has been long thought that stellar streams formed as a result of open cluster disruption, the dispersed members having retained the parental cluster kinematics, and were the intermediate phase between open clusters and field stars in the Galaxy. While many of the proposed stellar streams in the literature share the motion of well known open clusters, some streams do exist without any defining clusters. Here, we have tried to probe the origin of a few stellar streams in the disc of the Galaxy, by chemically tagging them, performing the detailed abundance analysis. We discuss 3 streams that share well known cluster kinematics (Hyades, Pleiades, Sirius) and 3 streams that do not have any known association with clusters (Arcturus, Hercules and AF06) in the context of various origin scenarios. The streams being the result dynamical perturbation in the Galaxy and the debris of accreted satellite will also be discussed. The significant result of this study is that none of the streams show chemical homogeneity, and the abundance pattern imply member stars may not have originated solely from their corresponding defining clusters. (author)
[en] Standard (or mildly inhomogeneous) Big Bang nucleosynthesis theory is well confirmed by abundance measurements of light elements up to 7Li and the resulting upper limit to the number of neutrino families confirmed in accelerator experiments. Extreme inhomogeneous models with a closure density in form of baryons seem to be ruled out and there is no evidence for a cosmic 'floor' to 9Be or heavier elements predicted in some versions of those models. Galaxies show a correlation between luminous mass and abundance of carbon and heavier elements, usually attributed to escape of hot gas from shallow potential wells. Uncertainties include the role of dark matter and biparametric behaviour of ellipticals. Spirals have radial gradients which may arise from a variety of causes. In our own Galaxy one can distinguish three stellar populations - disk, halo and bulge - characterised by differing metallicity distribution functions. Differential abundance effects are found among different elements in stars as a function of metallicity and presumably age, notably in the ratio of oxygen and α-particle elements to iron. These may eventually be exploitable to set a time scale for the formation of the halo, bulge and disk. (orig.)
[en] The IUE satellite has supplied several thousands of high resolution stellar spectra which has only partially been interpreted in terms of synthetic spectra of relevant wavelength intervals. In many cases the comparison between observed and computed spectra gives poor results. To establish the real needs to interpret correctly a high resolution IUE spectrum we perform a rough estimation of what would be the abundance or the f minimal values that correspond to isolated lines in the limit of detection given by instrumental resolution. In this way, we stress the need to develop the determination or calculation of atomic data concerning very weak spectral lines. Thus, with more complete and reliable atomic data, systematic synthetic spectra calculations could lead to better understand the physical conditions of stellar atmospheres