uvbyHbeta CCD Photometry of Clusters. IV. Solving the Riddle of NGC 3680

Abstract

CCD photometry on the uvbyCaHbeta system is presented for the open cluster, NGC 3680. Restricting the data to probable cluster members using the CMD and the photometric indices alone defines a sample of 34 stars at the cluster turnoff that imply E(b-y)=0.042(0.002) or E(B-V) = 0.058(0.003), where the errors (s.e.m) refer to internal errors alone. With this reddening, [Fe/H] is derived from both m1 and hk using both b-y and Hbeta as the temperature indices. The agreement among the four approaches is excellent, leading to final value of [Fe/H] = -0.14 (0.03) for the cluster and removing the apparent discrepancy between the past uvby analyses and extensive results from the red giants for which the cause has been traced to an apparent zero-point offset in the original m1 indices. Using the homogenized and combined V, b-y data from a variety of studies transformed to B-V, the cluster CMD is compared to NGC 752, IC 4651, and the core-convective-overshoot isochrones of Girardi et al. (2002). By interpolation to the proper metallicity, it is found that the E(B-V), (m-M), and age for NGC 752, IC 4651, and NGC 3680 are (0.03, 8.30, 1.55 Gyr), (0.10, 10.20, 1.7 Gyr), and (0.06, 10.20, 1.85 Gyr), respectively. The revised age and metallicity sequence and the color distribution of the giants provide evidence for the suggestion that the giants defining the apparent clump in NGC 3680 are predominantly first-ascent giants, as indicated by their Li abundance, while the clump stars in NGC 752, 0.1 mag bluer in (B-V), are He-core-burning stars. When combined with the color distribution in IC 4651, it is suggested that over this modest age range where He-core flash becomes important, the distribution of so-called clump stars switches from being dominated by He-core burning stars to first-ascent giants in the bump phase.