Integrated spectral energy distributions of binary star composite stellar populations

This paper presents the theoretically integrated spectral energy distributions (SEDs) of binary star composite stellar populations (bsCSPs) in early-type galaxies and how the bsCSP model can be used for the spectral studies of galaxies. All bsCSPs are built based on three adjustable inputs (metallicity, ages of old and young components). The effects of binary interactions and stellar population mixture are taken into account. The results show some ultraviolet (UV) upturn SEDs naturally for bsCSPs. The SEDs of bsCSPs are affected obviously by all of the three stellar population parameters, and the effects of three parameters are degenerate. This suggests that the effects of metallicity, and the ages of the old (major in stellar mass) and young (minor) components of stellar populations should be taken into account in the SED studies of early-type galaxies. The sensitivities of SEDs at different wavelengths to the inputs of a stellar population model are also investigated. It is shown that UV SEDs are sensitive to all of the three stellar population parameters, rather than to only stellar age. Special wavelength ranges according to some SED features that are relatively sensitive to the stellar metallicity, young-component age and old-component age of bsCSPs are found by this work. For example, the shapes of SEDs with the wavelength ranges of 51105250, 52505310, 53105350, 58305970 and 20 95023 550 angstrom are relatively sensitive to the stellar metallicity of bsCSPs. The shapes of SEDs within 965985, 10051055 and 12051245 angstrom are sensitive to the old-component age, while SED features within the wavelength ranges of 21852245, 24552505, 25052555, 27752825 and 28252875 angstrom are sensitive to the young-component age. The results suggest that some line indices within these special wavelength ranges are possibly better for stellar population studies compared to the others, and greater weights may be given to these special SED parts in the determination of the stellar population parameters of early-type galaxies from fitting SEDs via bsCSPs.