Enrico Corsaro, Dennis Stello, Daniel Huber, Timothy R. Bedding, Alfio Bonanno, Karsten Brogaard, Thomas Kallinger, Othman Benomar, Timothy R. White, Benoit Mosser, Sarbani Basu, William J. Chaplin, Jørgen Christensen-Dalsgaard, Yvonne P. Elsworth, Rafael A. García, Saskia Hekker, Hans Kjeldsen, Savita Mathur, Søren Meibom, Jennifer R. Hall, Khadeejah A. Ibrahim, Todd C. Klaus
We studied 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 \Delta\nu, \delta\nu_01, \delta\nu_02 and \epsilon. 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 the small separations \delta\nu_01 and \delta\nu_02. Ensemble \'echelle diagrams and lower limits to the linewidth of l = 0 modes as a function of Dnu of the clusters NGC 6791 and NGC 6819 are also shown, together with a correlation between the l = 0 ridge width and T_eff. Lastly, we distinguish between red giant branch and red clump stars through the measurement of the period spacing of mixed 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 potentially interesting stars including evolved blue stragglers and binaries, as well as stars in late He-core burning phases.
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http://arxiv.org/abs/1205.4023
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