How binary interactions affect stellar population synthesis

The single star-stellar population (ss-SSP) models are usually used for stellar population studies. However, there are more than 50% stars that are in binaries and evolve differently from single stars. This suggests when we model the stellar populations of galaxies and star clusters, the effects of binary interactions should be considered. Although a few works tried to model stellar populations via binaries, there is no quantitative study about the effects of binary interactions on stellar population synthesis. Via a rapid stellar population synthesis (RPS) model, we study how binary interactions affect the results of stellar population synthesis and the results of stellar population studies. Our results show that binary interactions make the colours of stellar populations less luminous and bluer, age-sensitive Lick index Hβ larger, and the metallicity-sensitive index such as Mgb, Fe5270 and Fe5335 smaller than those of ss-SSPs. The results also show that the results (luminosity-weighted stellar ages and metallicities) of relative studies obtained via ss-SSPs and binary star-stellar populations (bs-SSPs) are similar, especially when we use Hβ and [MgFe] indices to measure stellar age and metallicity. Thus binary interactions do not change the most results we obtained via ss-SSPs before. The bs-SSP fitted luminosityweighted stellar ages of populations can be calculated from ss-SSP fitted results via, e.g., tbs = 14.01Zss + (1.46 − 17.61Zss)tss (for the results fitted by Hβ and [MgFe] indices; tbs, Zss and tss are the bs-SSP fitted age, ss-SSP fitted metallicity and ss-SSP fitted age, respectively). This can help us to get some absolute values of stellar-population parameters and is useful for absolute studies. As a whole, our results suggest that ss-SSPs can be used for most stellar population studies, except some special ones. Subject headings: galaxies: stellar content— galaxies: elliptical and lenticular, cD. Graduate University of the Chinese Academy of Sciences