[ITAL]BV[/ITAL] Photometry for the ∼2.5 [CLC]Gyr[/CLC] Open Cluster NGC 6819: More Evidence for Convective Core Overshooting on the Main Sequence

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Abstract
New BV photometry is presented for the populous, intermediate-age open cluster NGC 6819. From ~4 mag below the turnoff to ~2 mag above the red giant clump, the cluster's color-magnitude diagram is well defined. Of particular note is the fact that the bright end of the main-sequence distribution exhibits a pronounced curvature to the red. This morphology can be matched by theoretical stellar models only if a significant amount of convective core overshooting is assumed. Indeed, canonical, nonovershooting calculations fail by a wide margin to explain the observations. Comparisons with up-to-date evolutionary tracks, isochrones, and zero-age horizontal-branch loci for [Fe/H] = 0.0 and -0.10 indicate that the [Fe/H] ≈ -0.05 cluster has (m - M)V = 12.35, E(B - V) = 0.16, and an age near 2.4 Gyr. The existence of several clump stars that appear to be appreciably more massive than the turnoff stars is noted: we suggest that they are the descendants of blue stragglers, given that NGC 6819 has a confirmed blue straggler population.