NGC 6811: AN INTERMEDIATE-AGE CLUSTER IN THE KEPLER FIELD

NGC 6811 is one of the four open clusters located in the Kepler spacecraft field of view. We obtained UBVRI photometry of the cluster on six nights (four of them photometric) with the 1.08 m Hall and 1.83 m Perkins telescopes at Lowell Observatory. The mean photometric precision ranges from better than 0.01 mag in the V, B - V, V - R, and V - I indices among stars brighter than magnitude 15 to about 0.05 mag for U - B at magnitude 18. We followed a Bayesian statistical approach using the Markov Chain Monte Carlo algorithm to determine the cluster reddening, distance, age, and iron content from a maximum-likelihood fit to stellar isochrones. Using the Yale-Yonsei isochrones, we derived E(B - V) = 0.066 +/- 0.025, (m - M)(V) = 10.13 +/- 0.19, age = 1.06 +/- 0.19 Gyr, and Z = 0.012 +/- 0.004 ([Fe/H] = -0.18); with the Padova isochrones, we found E(B - V) = 0.081 +/- 0.015, (m - M)(V) = 10.31 +/- 0.11, age = 0.94 +/- 0.08 Gyr, and Z = 0.012 +/- 0.002 ([Fe/H] = -0.20). The uncertainties include possible errors in the photometric zero points of +/-0.01 mag, but do not include uncertainties in the models. Taking into account the differences between the two sets of models, we find the best estimate for the cluster parameters to be E(B - V) = 0.074 +/- 0.024, (m - M)(V) = 10.22 +/- 0.18, age = 1.00 +/- 0.17 Gyr, and Z = 0.012 +/- 0.004 ([Fe/H] = -0.19).