How binary interactions affect spectral stellar population synthesis

Single-star stellar population (ssSSP) models are usually used for stellar population studies. However, more than 50% of stars are in binaries and evolve differently from single stars. This suggests that the effects of binary interactions should be considered when modeling the stellar populations of galaxies and star clusters. Via a rapid spectral stellar population synthesis (RPS) model, we give detailed studies of the effects of binary interactions on the Lick indices and colours of stellar populations, and on the determination of the stellar ages and metallicities of populations. The results show that binary interactions make stellar populations less luminous, bluer, with larger age-sensitive Lick index (Hbeta) and less metallicity-sensitive indices (e.g., Mgb, Fe5270 and Fe5335) compared to ssSSPs. It also shows that when ssSSP models are used to determine the ages and metallicities of stellar populations, lower ages or metallicities will be obtained, taking two line indices (Hbeta and [MgFe]) and two colours (e.g., u-R and R-K), respectively. Some relations for linking the stellar-population parameters obtained by ssSSPs to those obtained by binary-star stellar populations (bsSSPs) are presented in the work. This can help us to get some absolute values for stellar-population parameters and is useful for absolute studies. However, it is found that the relative luminosity-weighted stellar ages and metallicities obtained via ssSSPs and bsSSPs are similar. This suggests that ssSSPs can be used for most stellar population studies, except in some special cases.