Asteroseismology of the open clusters ngc6791, ngc6811, and ngc6819 from 19 months of kepler photometry

We studied solar-like oscillations in 115 red giants in the three open clusters, NGC6791, NGC6811, and NGC6819, based on photometric data covering more than 19 months with NASA's Kepler space telescope. We present the asteroseismic diagrams of the asymptotic parameters δν₀₂, δν₀₁, and ε, 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 δν₀₂ and δν₀₁. Ensemble échelle diagrams and upper limits to the linewidths of ℓ = 0 modes as a function of Δν of the clusters NGC6791 and NGC6819 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.