The Astrophysical Journal, 757:190 (13pp), 2012 October 1 doi:10.1088/0004-637X/757/2/190 C  2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A. ASTEROSEISMOLOGY OF THE OPEN CLUSTERS NGC 6791, NGC 6811, AND NGC 6819 FROM 19 MONTHS OF KEPLER PHOTOMETRY Enrico Corsaro 1 ,2 , Dennis Stello 3 , Daniel Huber 3 ,4 , Timothy R. Bedding 3 , Alfio Bonanno 2 , Karsten Brogaard 5 , Thomas Kallinger 6 , Othman Benomar 3 , Timothy R. White 3 , Benoit Mosser 7 , Sarbani Basu 8 , William J. Chaplin 9 ,10 , Jørgen Christensen-Dalsgaard 10 ,11 , Yvonne P. Elsworth 9 , Rafael A. Garc´ ıa 10 ,12 , Saskia Hekker 13 , Hans Kjeldsen 11 , Savita Mathur 9 ,14 , Søren Meibom 15 , Jennifer R. Hall 16 , Khadeejah A. Ibrahim 16 , and Todd C. Klaus 16 1 Department of Physics and Astronomy, Astrophysics Section, University of Catania, Via S. Sofia 78, I-95123 Catania, Italy 2 I.N.A.F.-Astrophysical Observatory of Catania, Via S. Sofia 78, I-95123 Catania, Italy 3 Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006, Australia 4 NASA-Ames Research Center, Moffett Field, CA 94035-0001, USA 5 Department of Physics and Astronomy, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada 6 Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium 7 LESIA-Observatoire de Paris, CNRS, Universit´ e Pierre et Marie Curie, Universit´ e Denis Diderot, F-92195 Meudon cedex, France 8 Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520-8101, USA 9 School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 10 Kavli Institute for Theoretical Physics, Kohn Hall, University of California, Santa Barbara, CA 93106, USA 11 Danish AsteroSeismology Centre (DASC), Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark 12 Laboratoire AIM, CEA/DSM CNRS-Universit´ e Paris Diderot IRFU/SAp, F-91191 Gif-sur-Yvette Cedex, France 13 Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands 14 High Altitude Observatory, NCAR, P.O. Box 3000, Boulder, CO 80307, USA 15 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 16 Orbital Sciences Corporation/NASA Ames Research Center, Moffett Field, CA 94035, USA Received 2012 March 4; accepted 2012 August 6; published 2012 September 17 ABSTRACT We studied solar-like oscillations in 115 red giants in the three open clusters, NGC 6791, NGC 6811, and NGC 6819, based on photometric data covering more than 19 months with NASA’s Kepler space telescope. We present the asteroseismic diagrams of the asymptotic parameters δν 02 , δν 01 , and ǫ , which show clear correlation with fundamental stellar parameters such as mass and radius. When the stellar populations from the clusters are compared, we see evidence for a difference in mass of the red giant branch stars and possibly a difference in structure of the red clump stars, from our measurements of the small separations δν 02 and δν 01 . Ensemble ´ echelle diagrams and upper limits to the linewidths of ℓ = 0 modes as a function of Δν of the clusters NGC 6791 and NGC 6819 are also shown, together with the correlation between the ℓ = 0 ridge width and the T eff of the stars. Lastly, we distinguish between red giant branch and red clump stars through the measurement of the period spacing of mixed dipole modes in 53 stars among all the three clusters to verify the stellar classification from the color–magnitude diagram. These seismic results also allow us to identify a number of special cases, including evolved blue stragglers and binaries, as well as stars in late He-core burning phases, which can be potentially interesting targets for detailed theoretical modeling. Key words: open clusters and associations: individual – stars: evolution – stars: oscillations – techniques: photometric Online-only material: color figures 1. INTRODUCTION Many recent studies of solar-like oscillations of red giant stars have been focused on large ensembles of stars, made possible by the flood of high-quality photometric data provided by the space missions CoRoT (e.g., De Ridder et al. 2009; Kallinger et al. 2010b; Mosser et al. 2011b) and Kepler (e.g., Borucki et al. 2010; Koch et al. 2010; Bedding et al. 2010). Particular attention has been given to the three open clusters NGC 6791, NGC 6811, and NGC 6819 in the Kepler field (Stello et al. 2010; Basu et al. 2011; Hekker et al. 2011; Stello et al. 2011a, 2011b; Miglio et al. 2012), due to the well-known advantage of cluster stars sharing common properties, which allows for more stringent investigations of stellar evolution theory. Among the highlights in recent results relevant for our study are the measurements of the small frequency separations δν 02 , δν 01 and of the dimensionless term ǫ , their correlation with the large frequency separation (see Bedding et al. 2010; Huber et al. 2010; Kallinger et al. 2012, for previous results on field red giant stars), and their dependence on stellar mass (Montalb´ an et al. 2010; Kallinger et al. 2012). Also, the results on the ensemble ´ echelle diagrams have allowed for the investigation of ensemble properties of the modes, including the measurement of the mean small spacing δν 03 (Bedding et al. 2010; Huber et al. 2010) and the linewidths of the dipole modes and their correlation to fundamental stellar properties (e.g., see Chaplin et al. 2009; Baudin et al. 2011; Appourchaux et al. 2012; Belkacem et al. 2012), which are important for the comprehension of the physics responsible for the excitation and damping of solar- like oscillations. Finally, the period spacing analysis for the investigation of the evolutionary stage of red giants (Bedding et al. 2011; Mosser et al. 2011a) now allows us to distinguish between He-core burning red giants and those only burning hydrogen in a shell. 1