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Astron. Astrophys. 361, 92-100 (2000)


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High resolution abundance analysis of 16 giants and subgiants in the metal-poor globular cluster NGC 6397 *

B.V. Castilho 1, L. Pasquini 2, D.M. Allen 1, B. Barbuy 1 and P. Molaro 3

1 Universidade de São Paulo, CP 3386, São Paulo 01060-970, Brazil
2 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
3 Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131 Trieste, Italy

Received 7 April 2000 / Accepted 20 July 2000

Abstract

High-resolution échelle spectra of five giants and eleven subgiants of the metal-poor globular cluster NGC 6397 were obtained at the ESO 3.5m (NTT) and 3.6m telescopes.

A detailed analysis of the 16 sample stars was carried out, providing atmospheric parameters ([FORMULA], log g, [Fe/H], [FORMULA]) and element abundances.

The metallicity of the cluster is [Fe/H] = -2.0 with a r.m.s. of [FORMULA]0.05, irrespective of the position in the Colour - Magnitude Diagram, with no variations as a function of effective temperatures and gravities. The dispersion in metal abundance, obtained over a range of 5 magnitudes and 1400 K, is negligible and well within the measurement uncertainties.

[FORMULA]-elements are mildly overabundant while the s-elements are deficient relative to iron by about 0.2 dex. [O/Fe] is also mildly overabundant for the two stars where the line is detected.

The Li abundances in the present sample and those of turnoff stars by Pasquini and Molaro (1996) are complementary in terms of evolutionary stage. The Li abundances decrease off the main sequence along the red giant branch, confirming that extra depletion must occur for stars cooler than [FORMULA]5000 K and V brighter than [FORMULA]14.

Key words: stars: abundances – stars: Hertzsprung – Russell (HR) and C-M diagrams – stars: Population II – Galaxy: globular clusters: individual: NGC 6397 – Galaxy: halo

* Observations collected at the European Southern Observatory - ESO, Chile

Send offprint requests to: B.V. Castilho

© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000

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