An Assessment of Broadband Optical Colours as Age Indicators for Star Clusters

We present an empirical assessment of the use of broadband optical colours as age indicators for unresolved extragalactic clusters and investigate stochastic sampling effects on integrated colours. We use the integrated properties of Galactic open clusters as models for unresolved extragalactic clusters. The population synthesis code Starburst99 (Leitherer et al. 1999) and four optical colours were used to estimate how well we can recover the ages of 62 well-studied Galactic open clusters with published ages. We provide a method for estimating the ages of unresolved clusters and for reliably determining the uncertainties in the age estimates. Our results support earlier conclusions based on comparisons to synthetic clusters, namely the (U-B) colour is critical to the estimation of the ages of star forming regions. We compare the observed optical colours with those obtained from Starburst99 using the published ages and get good agreement. The scatter in the (B-V)_observed-(B-V)_model is larger for lower luminosity clusters, perhaps due to stochastic effects.