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Abundance patterns among very metal-poor stars in the halo of the galaxy

Grant number: 05/01023-8
Support type:Scholarships in Brazil - Master
Effective date (Start): September 01, 2005
Effective date (End): August 31, 2007
Field of knowledge:Physical Sciences and Mathematics - Astronomy - Stellar Astrophysics
Principal researcher:Silvia Cristina Fernandes Rossi
Grantee:Vinicius Moris Placco
Home Institution: Instituto de Astronomia, Geofísica e Ciências Atmosféricas (IAG). Universidade de São Paulo (USP). São Paulo , SP, Brazil


"Snapshot" spectra of high resolution for a sample of ~ 500 targets with [Fe/H] < -2.0 selected from HK and HEs surveys are being obtained with UVES/VLT, as part of the HERES survey (Hamburg/ESO R-process Enhanced Star). The resolving power (R=20000) and the S/N (~ 30/1) requirements are sufficient to allow confident identification of the presence (or absence) of the Eu II 4129A line in the target stars. Eu (Z=63) is thought to be almost exclusively associated with r-process in very MP stars (s-process enhancement of Eu does not become significant until [Fe/H] > -2.5, hence its presence immediately identifies a star that is likely to exhibit additional r-process elements (including Th, and in some cases, U), when higher quality spectroscopy is obtained. Futhermore, with those spectra we can obtain information on elemental abundances for ~ 20 species per star, including the ones formed by neutron-capture. This procedure enable us to select the most interesting ones (p.ex., those with probable r-process detection) and re-observe them using higher resolution and S/N. These data provide the best possible sample for studying, among them : (a) the dispersion in the observed patterns of the r-process elements in the range 40 < Z < 50 (elements that tend to fall below the scaled solar pattern , and probably exhibit star-to-star scatter even for objects of identical overall metalicitty, indicating the existence of a second r-process mechanism), and in the interval 56 < Z < 76 (elements that closely follow the scaled solar pattern of the r-process elements, suggesting that, at least for these elements, the astrophysical conditions leading to the r-process operatein a consistent manner throughtout the history of the Galaxy), and (b) the general pattern and dispersion of the elemental abundances for metal-poor stars.The objective of this study , based on automated abundance analysis of the large spectral database generated in the initial stage of this study, is to explore the elemental abundance patterns produced by the first generations of stars in the Galaxy. Using a sample of some 500 giants with [Fe/H] < -2.0, we intend to identify the general behavior of the abundance patterns as a function of metallicity, provide measures of their observed scatter, and specify the frequency of different classes of objects with similar abundance patterns.

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