Colour indices of single stellar populations

Abstract

We present integrated colours for instantaneous burst single stellar populations (SSPs) of different metallicities and investigate their colour evolution. The principal difference between this study and previous work is the adoption of stellar evolutionary models that employ, amongst other things, recent opacities and a revised equation of state (EOS), and include evolutionary processes such as convective overshooting, thermal pulses and dredge-up. The models are used in the convenient form of analytic fitting functions. In addition, we use the BASEL model for the library of stellar spectra. This model provides an extensive low-resolution theoretical flux distribution and UBVRIJHKLM colours, which have been calibrated empirically or semi-empirically, for a wide range of stellar parameters. We calculate several sets of SSPs and explore the influence of relevant parameters on the integrated colours. The ages of the SSPs are from 0.5 to 19.5 Gyr and the metallicities range from 0.0001 to 0.03. We include a variety of prescriptions for the initial mass function (IMF) in our investigation, including different choices for the lower and upper mass cut-offs to the distribution, as well as the slope. We find that the age, the metallicity and the lower mass cut-off all play an important role in determining the integrated colours of the instantaneous burst SSPs. In particular, variations in the choice of IMF can lead to deviations of up to 0.47 mag and 2.25 mag in integrated B−V and V−K colours, respectively. Conversely, we find that altering the rate of mass loss in a stellar wind, or the choice of the upper mass cut-off to the IMF, does not have a significant effect on the results. We also investigate the discrepancies that exist between the results of various SSP studies. We compare the isochrones and the colour–temperature calibrations adopted in each of these studies, and estimate the deviations owing to the choice of stellar models, spectral library, and/or IMF. We find that the deviation arising from changes in the IMF is greater than that caused by changes to the evolutionary models or spectral calibrations utilized by each study. Therefore, the form of the IMF should be chosen with care when conducting SSP studies.