A BV Color-Magnitude Diagram for the Galactic Globular Cluster NGC 1851

A B and V band color-magnitude diagram is presented for the Galactic globular cluster NGC 1851. The cluster reddening is E(B-V) = 0.02 and the abundance [Fe/H] = -1.29 +/- 0.07 is confirmed. There is no evidence for abundance variations in excess of +/- 0.1 dex. A sample of stars complete to 1.5 mag fainter than the horizontal branch is discussed extensively. The bimodal distribution of stars on the horizontal branch is interpreted as being due to a larger than usual mass dispersion parameter rather than a consequence of post-ZAHB evolution. The RGB clump is clearly identified and its position agrees with the horizontal-branch stars being little-evolved from the ZAHB. Faint stars in the periphery of the cluster are used to prepare a CMD which extends from the subgiant branch to well below the main sequence turn-off, which occurs at V = 19.6 +/- 0.1, thus Delta-V_TO-HB = 3.45 +/- 0.1. The main sequence-red giant branch color difference method, when calibrated from the Revised Yale Isochrones, gives an age of 14 +/- 1 Gyr, identical to that found for M5 using the same technique. (SECTION: Stellar Clusters and Associations)