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Oscillating blue stragglers, γ Doradus stars and eclipsing binaries in the open cluster NGC 2506^*,**

¹ Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C., Denmark e-mail: toar@phys.au.dk
² Danish AsteroSeismology Centre, University of Aarhus, 8000 Aarhus C., Denmark
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, 3001 Heverlee, Belgium
⁴ LESIA, Observatoire de Paris-Meudon, UMR 8109, 92190 Meudon, France
⁵ GRAAG, Algiers Observatory, BP 63 Bouzareah, 16340 Algiers, Algeria

Received: 14 December 2006
Accepted: 25 January 2007

Abstract

Context.This is the first step in a project to combine studies of eclipsing binaries and oscillating stars to probe the interior of Blue Stragglers (BS). This may imply a way to discriminate observationally between different birth mechanisms of BS stars.

Aims.We study the open cluster NGC 2506 which contains oscillating BS stars and detached eclipsing binaries for which accurate parameters can be derived. This will tightly constrain the cluster isochrone and provide an absolute mass, radius and luminosity-scale for the cluster stars along with the cluster age, metallicity and distance. The present workfocuses on obtaining the light curves of the binaries anddetermine their orbital periods, on obtaining power spectra of the oscillating BS stars to select targets for follow-up studies, and on searching for γ Doradus type variables which are also expected to be present in the cluster.

Methods.With a two-colour, dual-site photometric campaign we obtained 3120 CCD-images of NGC 2506 spread over four months.We analysed theBI time-series of the oscillatingstars and used simulations to derive statistical uncertainties of the resulting frequencies, amplitudes and phases. A preliminary mode-identification was performed using frequency ratios for the oscillating BS stars, and amplitude ratios and phase differences for a population of newly detected γ Doradus stars.

Results.We quadrupled the number of known variables in NGC 2506 by discovering 3 newoscillating BS stars, 15 γ Doradus stars and four new eclipsing binaries. The orbital periods of 2 known, detached eclipsing binaries were derived. We discovered a BS star with both p-mode and g-mode variabilityand we confronted our γ Doradus observations with state-of-the-art seismic models, but found significant discrepancy between theory and observations.

Conclusions.NGC 2506 is an excellent target for asteroseismic tests of stellar models,as strong external constraints can be imposed on the models of a population of more than 20 oscillating stars of different types.