Spectral population synthesis including massive binaries

We have constructed a new code to produce synthetic spectra of stellar populations that includes massive binaries. We have tested this code against the broad-band colours of unresolved young massive stellar clusters in nearby galaxies, the equivalent widths of the Red and Blue Wolf–Rayet bumps in star-forming Sloan Digital Sky Survey galaxies and the ultraviolet and optical spectra of the star-forming regions Tol-A and B in NGC 5398. In each case, we find a good agreement between our models and observations. We find that in general binary populations are bluer and have fewer red supergiants, and thus significantly less flux in the I band and at longer wavelengths, than single star populations. Also we find that Wolf–Rayet stars occur over a wider range of ages up to 107 yr in a stellar population including binaries, increasing the ultraviolet flux and Wolf–Rayet spectral features at later times. In addition, we find that nebula emission contributes significantly to these observed properties and must be considered when comparing stellar models with observations of unresolved stellar populations. We conclude that incorporation of massive stellar binaries can improve the agreement between observations and synthetic spectral synthesis codes, particularly for systems with young stellar populations.