WFPC2 Observations of the Small Magellanic Cloud Intermediate-Age Populous Cluster NGC 416

Abstract

We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 observations in F555W (~V) and F450W (~B) of the intermediate-age populous star cluster NGC 416 in the Small Magellanic Cloud (SMC). We use published photometry of two other SMC populous star clusters, Lindsay 1 and Lindsay 113, to investigate the age sequence of these three star clusters. We estimate that these clusters have age ratios of ageNGC 416/ageL1 ≈ 0.73±0.05 and ageL113/ageL1 ≈ 0.52±0.09, using an extrapolation of the d(B−V) method (which uses the color difference between the red horizontal branch and the red giant branch as an age indicator) of Sarajedini and coworkers. These age ratios provide absolute age estimates of 6.6±0.5 Gyr and 4.7±0.8 Gyr for NGC 416 and Lindsay 113, respectively, assuming that Lindsay 1 is 9 Gyr old. Metallicities of [Fe/H] = -1.44±0.12, -1.35±0.08, and -1.24±0.11 dex, and reddenings of E(B-V) = 0.08±0.03, 0.06±0.02, and 0.00±0.02 mag for NGC 416, Lindsay 1, and Lindsay 113, respectively, were determined using the simultaneous reddening and metallicity (SRM) method of Sarajedini and Layden. Accurate (relative) ages for the intermediate-age populous clusters in the SMC (e.g., via deep main-sequence photometry) would allow the d(B−V) method to be recalibrated with star clusters that are significantly younger than 7 Gyr. An extended d(B−V) method could prove to be a very useful age diagnostic for future studies of the intermediate-age metal-poor stellar populations in Local Group galaxies, where accurate main-sequence turnoff photometry at MV ≈ +4 mag is currently not possible or practical.