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 1.3script M sign⊙, all at [Fe/H]=+0.15. Comparison to the NGC 6791 BVI photometry of Kałużny & Udalski [AcA, 42, 29 (1992)] and Montgomery et al. [AJ, 108, 585 (1994)] yields an age of 10.0±0.5 Gyr at an apparent distance modulus 13.49<(m-M)V<13.70. The color offsets required to fit the isochrones, combined with the spectroscopic results of Friel & Janes [A&A, 267, 75 (1993)], 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 ≲0.7script M sign⊙. The masses are similar to those found for M67 red horizontal branch stars by Tripicco et al. [AJ, 106, 618 (1993)] and for globular cluster red horizontal branch stars, even though the M67 progenitors are ∼0.2script M sign⊙ 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.