F eb 1 99 5 . A ST R O - P H - 9 50 20 20 Derived Parameters for NGC 6791 from High - Metallicity Isochrones

We have computed 8, 10, and 12 Gyr isochrones and physically consistent models of zero-age red horizontal branch stars for stellar masses between 0.55 and 1.3 M , all at [Fe/H] = +0:15. Comparison to the NGC 6791 BVI photometry of Ka lu_ zny & Udalski (1992, Acta Astron., 42, 29) and Montgomery, Janes & Phelps (1994, AJ, 108, 585) yields an age of 10:0 0:5 Gyr at an apparent distance modulus 13:49 < (m M)V < 13:70. The color o sets required to t the isochrones, combined with the spectroscopic results of Friel & Janes (1993, A&A, 267,75), imply that the foreground reddening to NGC 6791 lies in the range 0:24 > E(B V ) > 0:19 with +0:27 < [Fe/H] < +0:44. These results are derived using a technique by which we predict color shifts and apply these to the isochrones to simulate progressively higher metallicities. The zero-age horizontal branch models suggest that the red horizontal branch stars of NGC 6791 have masses < 0:7M . The masses are similar to those found for M67 red horizontal branch stars by Tripicco, Dorman & Bell (1993, AJ, 106, 618) and for globular cluster red horizontal branch stars, even though the M67 progenitors are 0:2M more massive, while the progenitors of globular cluster horizontal branch stars are similarly less massive. This suggests the presence of a mechanism, not strongly dependent on metallicity, which reduces stellar envelopes on the zero-age horizontal branch to a given mass rather than by a given amount. Subject headings: clusters: open { clusters: individual (NGC 6791) { stars: evolution