Wfpc2 Observations of the Small Magellanic Cloud Intermediate-age Populous

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 ageNGC416/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, 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 of [Fe/H] = −1.44±0.12, −1.35±0.08, −1.24±0.11 dex, and reddenings of E(B−V )=0.08±0.03, 0.06±0.02, 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 & 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 MV ≈+4 mag is currently not possible or practical. Subject headings: galaxies: clusters: individual (Lindsay 1, Lindsay 113, NGC 416) — galaxies: individual (Small Magellanic Cloud) — galaxies: star clusters — stars: Population II