Abstract
ABSTRACT 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.
Highlights
Stellar population and spectral synthesis (SPS) models provide the bridge between unresolved observations of galaxies or stellar clusters, and interpretation of their stellar content
In Stanway & Eldridge (2019) we explored the impact of varying our initial mass function, for a grid of variant models which were truncated at 1 Gyr to reduce the run-time, but otherwise followed the BPASS v2.2 prescription described by Stanway & Eldridge (2018)
In this paper we have explored the impact of empirical uncertainties on the parameter distributions in binary fraction as a function of initial primary mass, initial orbital period and initial mass ratio
Summary
Stellar population and spectral synthesis (SPS) models provide the bridge between unresolved observations of galaxies or stellar clusters, and interpretation of their stellar content. The Binary Population and Spectral Synthesis (BPASS, Eldridge, Stanway, Xiao, McClelland , Taylor, Ng, Greis & Bray 2017; Stanway & Eldridge 2018) project produces simple stellar population models from a custom grid of detailed stellar evolution tracks which incorporate the evolution of isolated single stars, and binary interactions resulting from a range of binary initial mass ratio and initial separations. The current BPASS data release model set (as of v2.2) incorporates 9 representative initial mass functions (IMFs), with or without including binary evolution tracks, each calculated at 13 metallicities - a total of 208 models which must be run independently, with the results output at 51 time steps.
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