The age, extinction, and distance of the old, metal-rich open cluster NGC 6791

An extensive grid of metal-rich isochrones utilizing the latest available input physics has been calculated for comparison with the old, metal-rich open cluster NGC 6791. The isochrones have been simultaneously fitted to BV and VI color-magnitude diagrams, with the same composition, reddening, and distance modulus required for both colors. Our best-fitting isochrone assumes [Fe/H] = +0.4, scaled solar abundance ratios, and ΔY/ΔZ = 2 (Y = 0.31), yielding an excellent fit to the data at all points along the major sequences. The resulting age is 8 Gyr, with E(B-V) = 0.10 and (m-M)_V = 13.42. The derived cluster parameters are fairly robust to variations in the isochrone [Fe/H] and helium abundances. All the acceptable fits indicate that 0.08 ≤ E(B-V) ≤ 0.13 and 13.30 ≤ (m - M)_V ≤ 13.45 and that NGC 6791 has an age of 8.0 ± 0.5 Gyr. The fits also suggest that ΔY/ΔZ lies between 1 and 3. A metallicity as low as solar is clearly ruled out, as is ΔY/ΔZ = 0. Comparison with previous isochrone studies indicates that the derived reddening is primarily due to our use of the most recent color transformations, whereas the age depends on both the colors and the input physics. Our isochrones provide an excellent fit to the Hyades zero-age main sequence as determined by Hipparcos, providing evidence that our derived reddening and distance modulus are reliable.