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Investigating ageing methods of Large Magellanic Cloud star clusters using integrated colours


E-mail: (RSA); (MMH)


We present an investigation of 84 star clusters in the Large Magellanic Cloud (LMC) galaxy using different broad-band photometric, age-estimation methods. Because of its intermediate distance, the LMC is uniquely positioned to compare its clusters that have previously been aged using both resolved photometry [colour–magnitude diagrams (CMDs)] of the constituent stars and unresolved methods (integrated, broad-band colour photometry). In our comparison between the published CMD ages to three similar, but different methods based on UBV integrated colours, we find poor matches. We attribute this primarily to two things. First, the UBV integrated broad-band ageing methods require matching a cluster with an expected model prediction of the cluster colours as a function of age. The biggest problem we find is that the stellar clusters in our sample do not typically lie on the model line based on their known age and extinction. That is to say, real cluster colours often do not match the model colours and can be found some distance from expected model values. The second issue, which has been previously documented in numerous studies, is the strong degeneracy between age and extinction in the UBV plane. Certainly, providing more photometric bands will reduce degeneracy between age and reddening. Better yet, if extinction can be independently determined, we show that ages from methods based on integrated colours will more closely match those obtained from CMD ages. However, the underlying issue remains. Simple stellar population models often do not accurately represent the colours of real stellar clusters due to the incomplete and stochastic sampling of the stellar mass function in low- and moderate-mass stellar clusters.