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.

arXiv comment10 pages (LaTeX+aaspp4.sty), 1 table (in LaTeX file) and 2 figures (PostScript format). The PostScript version of the paper, table, and full-resolution figures is available at http://www.noao.edu/noao/staff/mighell/ngc416 To appear in the Astrophysical Journal Letters

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• A Search for Old Star Clusters in the Large Magellanic Cloud
• WFPC2 Observations of Star Clusters in the Magellanic Clouds. II. The Oldest Star Clusters in the Small Magellanic Cloud
• WFPC2 Observations of the Carina Dwarf Spheroidal Galaxy
• WFPC2 Observations of Star Clusters in the Magellanic Clouds: I. The LMC Globular Cluster Hodge 11

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• A Comparison of Optical and Near-Infrared Colours of Magellanic Cloud Star Clusters with Predictions of Simple Stellar Population Models
• The Distances of the Magellanic Clouds
• A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters
• Integrated-light VRI imaging photometry of globular clusters in the Magellanic clouds
• The Faint Cepheids of the Small Magellanic Cloud: an evolutionary selection effect?
• High dispersion spectroscopy of two A supergiant systems in the Small Magellanic Cloud with novel properties
• Basic parameters of three star clusters in the Small Magellanic Cloud: Kron 11, Kron 63 and NGC 121