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Looking into red giants in NGC 6819 using asteroseismology and vice versa
Rasmus Handberg (Stellar Astrophysics Centre, Aarhus University), K. F. Brogaard (Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Denmark), A. Miglio (School of Physics and Astronomy, University of Birmingham, United Kingdom), Y. Elsworth (School of Physics and Astronomy, University of Birmingham, United Kingdom), et al.
We present an extensive peakbagging effort on Kepler light curves of 51 red giant stars in the open star cluster NGC 6819, a unique sample, because of the length of observations and their common distance, metallicity, and age. By employing sophisticated pre-processing of the time series and Markov Chain Monte Carlo (MCMC) techniques we extracted individual frequencies, heights and linewidths for hundreds of oscillation modes in the sample of stars. We show that the “average” asteroseismic parameter δν02, derived from these, can be used to distinguish the stellar evolutionary state between the red giant branch (RGB) stars and red clump (RC) stars, without having to measure the often difficult dipole modes. The masses and radii of the giants are estimated using asteroseismic scaling relations, both empirically corrected to obtain self-consistency as well as agreement with independent measures of distance and age, and, alternatively, using updated theoretical corrections. Remarkable agreement is found, allowing the evolutionary state of the giants to be determined exclusively from the empirical correction to the scaling relations. Stars that are outliers relative to the ensemble reveal over-massive members that likely evolved via mass-transfer in a blue straggler phase. We also suggest that KIC 4937011, a low-mass Li-rich giant previously studied in the literature, is a cluster member in the RC phase that experienced very high mass-loss during its evolution. Such over- and under-massive stars need to be considered when studying field giants, since the true age of such stars cannot be known and there is currently no way to distinguish them from normal stars.