Sounding open clusters: asteroseismic constraints from Kepler on the properties of NGC 6791 and NGC 6819

Abstract

We present initial results on some of the properties of open clusters NGC 6791 and NGC 6819 derived from asteroseismic data obtained by NASA’s Kepler mission. In addition to estimating the mass, radius, and log g of stars on the red giant branch (RGB) of these clusters, we estimate the distance to the clusters and their ages. Our model-independent estimate of the distance modulus of NGC 6791 is (m − M)[subscript 0] = 13.11 [plus or minus] 0.06. We find (m − M)[subscript 0] = 11.85 [plus or minus] 0.05 for NGC 6819. The average mass of stars on the RGB of NGC 6791 is 1.20 [plus or minus] 0.01 M [subscript circled dot], while that of NGC 6819 is 1.68 [plus or minus] 0.03 M[subscript circled dot] . It should be noted that we do not have data that cover the entire RGB and the actual mass will be somewhat lower. We have determined model-dependent estimates of ages of these clusters. We find ages between 6.8 and 8.6 Gyr for NGC 6791, however, most sets of models give ages around 7 Gyr. We obtain ages between 2 and 2.4 Gyr for NGC 6819.