An assessment of broad-band optical colours as age indicators for star clusters

• Published in: Monthly notices of the Royal Astronomical Society Vol. 389; no. 3; pp. 1470 - 1478
• Main Authors: Hancock, M., Smith, B. J., Giroux, M. L., Struck, C.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 21-09-2008; Blackwell Science; Oxford University Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; galaxies: star clusters; galaxies: stellar content; methods: data analysis; open clusters and associations: general; Spectrum analysis; Stars & galaxies; methods: data analysis; open clusters and associations: general; galaxies: stellar content; galaxies: star clusters; Data analysis; Uncertainty; Galaxies; Stellar content; Stellar formation region; Color; Luminosity; Open clusters; Star clusters; Galactic cluster; Cluster model; Galaxy clusters; Age
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2008.13705.x

We present an empirical assessment of the use of broad-band 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 (OCs) as models for unresolved extragalactic clusters. The population synthesis code Starburst99 (SB99) and four optical colours were used to estimate how well we can recover the ages of 62 well-studied Galactic OCs 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 SB99 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.