Evaluating the impact of binary parameter uncertainty on stellar population properties.

Binary stars have been shown to have a substantial impact on the integrated light of stellar populations, particularly at low metallicity and early ages – conditions prevalent in the distant Universe. But the fraction of stars in stellar multiples as a function of mass, their likely initial periods and distribution of mass ratios are all known empirically from observations only in the local Universe. Each has associated uncertainties. We explore the impact of these uncertainties in binary parameters on the properties of integrated stellar populations, considering which properties and time-scales are most susceptible to uncertainty introduced by binary fractions and whether observations of the integrated light might be sufficient to determine binary parameters. We conclude that the effects of uncertainty in the empirical binary parameter distributions are likely smaller than those introduced by metallicity and stellar population age uncertainties for observational data. We identify emission in the He  ii  1640 Å emission line and continuum colour in the ultraviolet–optical as potential indicators of a high-mass binary presence, although poorly constrained metallicity, dust extinction, and degeneracies in plausible star formation history are likely to swamp any measurable signal. [ABSTRACT FROM AUTHOR]