Abundance anomalies in hot horizontal branch stars of the Galactic globular cluster NGC 2808

¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: pace@pd.astro.it
² Dpt. Cassiopée, UMR 6202, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France

Received: 25 November 2005
Accepted: 27 February 2006

Abstract

Aims.We present metallicity measurements of 25 stars in the blue horizontal branch of the Galactic globular cluster NGC 2808.

Methods. Our measurements are based on moderate–resolution spectra taken with the multi–object fiber facility FLAMES–UVES, mounted on Kueyen at the Very Large Telescope.

Results. We confirm that stars hotter than a threshold temperature have super–solar abundance, while the cooler ones respect the nominal metallicity of the cluster, i.e. . The threshold temperature is estimated to be about 12 000 K, corresponding to the so called u–jump, and coincides with the sudden departure of the cluster horizontal branch from the models. The metallicity increases with temperature for star hotter than the jump, confirming the hypothesis that the process responsible for this abrupt metallic enhancement is the levitation due to the strong radiation field in absence of a significative convective envelope. A metallicity dependence of the abundance enhancement is also suggested, with more metal poor clusters having a higher increase in metal content.

Conclusions.The slope in the temperature vs. abundance diagram is higher than the errors involved, and the metal content of the cluster plays possibly a role in determining the amplitude of the jump (more metal poor clusters show more enhancement after the jump), although other parameters, such as clusters' characteristics and even the atomic species involved, may also someway contribute.