Asteroseismic inferences on red giants in open clusters NGC 6791, NGC 6819, and NGC 6811 using Kepler

¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
e-mail: S.Hekker@uva.nl
² School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
³ Department of Astronomy, Yale University, PO Box 208101, New Haven CT 06520-8101, USA
⁴ Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006, Australia
⁵ Department of Physics and Astronomy, University of British Colombia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
⁶ Department of Physics and Astronomy, Building 1520, Aarhus University, 8000 Aarhus C, Denmark
⁷ High Altitude Observatory, NCAR, PO Box 3000, Boulder, CO 80307, USA
⁸ Laboratoire AIM, CEA/DSM-CNRS, Université Paris 7 Diderot, IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette, France
⁹ LESIA, UMR8109, Université Pierre et Marie Curie, Université Denis Diderot, Observatoire de Paris, 92195 Meudon Cedex, France
¹⁰ Konkoly Observatory of the Hungarian Academy of Sciences, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
¹¹ Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ Bay Area Environmental Research Institute/Nasa Ames Research Center, Moffett Field, CA 94035, USA
¹⁴ Orbital Sciences Corporation/Nasa Ames Research Center, Moffett Field, CA 94035, USA
¹⁵ SETI Institute/Nasa Ames Research Center, Moffett Field, CA 94035, USA

Received: 13 December 2010
Accepted: 19 April 2011

Abstract

Context. Four open clusters are present in the Kepler field of view and timeseries of nearly a year in length are now available. These timeseries allow us to derive asteroseismic global oscillation parameters of red-giant stars in the three open clusters NGC 6791, NGC 6819 and NGC 6811. From these parameters and effective temperatures, we derive masses, radii and luminosities for the clusters as well as field red giants.

Aims. We study the influence of evolution and metallicity on the observed red-giant populations.

Methods. The global oscillation parameters are derived using different published methods and the effective temperatures are derived from 2MASS colours. The observational results are compared with BaSTI evolution models.

Results. We find that the mass has significant influence on the asteroseismic quantities Δν vs. ν_max relation, while the influence of metallicity is negligible, under the assumption that the metallicity does not affect the excitation/damping of the oscillations. The positions of the stars in the H-R diagram depend on both mass and metallicity. Furthermore, the stellar masses derived for the field stars are bracketed by those of the cluster stars.

Conclusions. Both the mass and metallicity contribute to the observed difference in locations in the H-R diagram of the old metal-rich cluster NGC 6791 and the middle-aged solar-metallicity cluster NGC 6819. For the young cluster NGC 6811, the explanation of the position of the stars in the H-R diagram challenges the assumption of solar metallicity, and this open cluster might have significantly lower metallicity [Fe/H] in the range  − 0.3 to  − 0.7 dex. Also, nearly all the observed field stars seem to be older than NGC 6811 and younger than NGC 6791.