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

TL;DR

This study uses Kepler data to analyze red giants in three open clusters, deriving their physical properties and examining the effects of evolution and metallicity on their asteroseismic characteristics and positions in the H-R diagram.

Contribution

It provides new insights into how mass and metallicity influence asteroseismic parameters and stellar positions, with implications for understanding stellar evolution in different cluster environments.

Findings

Mass significantly affects delta_nu vs. nu_max relation.

Metallicity has negligible impact on oscillation excitation/damping.

Field star masses are bracketed by cluster star masses.

Abstract

Context: Four open clusters are present in the Kepler field of view and timeseries of nearly a year in length are now available. These timeseries allow us to derive asteroseismic global oscillation parameters of red-giant stars in the three open clusters NGC 6791, NGC 6819 and NGC 6811. From these parameters and effective temperatures, we derive mass, radii and luminosities for the clusters as well as field red giants. Aims: We study the influence of evolution and metallicity on the observed red-giant populations. Methods: 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 delta_nu vs. nu_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.