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

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 ∆ vs v(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.