WIYN OPEN CLUSTER STUDY. LXXII. A uvbyCaHβ CCD ANALYSIS OF THE METAL-DEFICIENT OPEN CLUSTER NGC 2506

Abstract

Precision uvbyCaHβ photometry of the metal-deficient, old open cluster NGC 2506 is presented. The survey covers an area of $20^{\prime} \times 20^{\prime} $ and extends to $V\sim 18$ for b − y and Hβ and to $V\sim 17.0$ for c 1 and hk. For V brighter than 16.0, photometric scatter among the indices leads to the recovery of six known variables within the cluster core and five new variables in the outer $5^{\prime} $ of the survey field. Proper motions, radial velocities, and precise multicolor indices are used to isolate a highly probable sample of cluster members from the very rich color–magnitude diagram. From 257 highly probable members at the cluster turnoff, we derive a reddening estimate of $E(b-y)=0.042\pm 0.001$ ($E(B-V)=0.058\pm 0.001$), where the errors refer to the internal standard errors of the mean. [Fe/H] is derived from the A/F dwarf members using both m 1 and hk, leading to [Fe/H] = −0.296 ± 0.011 (sem) and −0.317 ± 0.004 (sem), respectively. The weighted average, heavily dominated by hk, is [Fe/H] = −0.316 ± 0.033. Based on red giant members, we place an upper limit of ±0.010 on the variation in the reddening across the face of the cluster. We also identify two dozen potential red giant cluster members outside the cluster core. Victoria–Regina isochrones on the Strömgren system produce an excellent match to the cluster for an apparent modulus of $(m-M)=12.75\pm 0.1$ and an age of 1.85 ± 0.05 Gyr.