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 galaxy. 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 age_NGC416/age_L1 ~= 0.73 +- 0.05 and age_L113/age_L1 ~= 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, Lee, & Lee [ApJ, 450, 712 (1995)]. 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 and reddenings for NGC 416, Lindsay 1, and Lindsay 113, respectively, were determined using the simultaneous reddening and metallicity (SRM) method of Sarajedini & Layden [AJ, 113, 264, (1997)]. Accurate (relative) ages for the intermediate-age populous clusters in the Small Magellanic Cloud (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 M_V ~= +4 mag is currently not practical.