How binary interactions affect spectral stellar population synthesis

Single-star stellar population (ssSSP) models are usually used for spectral 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. Our results show that binary interactions make stellar populations less luminous, bluer, with larger age-sensitive Lick index (Hβ) and smaller metallicitysensitive 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, smaller ages or metallicities will be obtained, when using two line indices (Hβ 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 spectral stellar population studies, except in some special cases. Subject headings: galaxies: stellar content— galaxies: elliptical and lenticular, cD. Graduate University of the Chinese Academy of Sciences