Spectral 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 broadband colours of unresolved young massive stellar clusters in nearby
galaxies, the equivalent widths of the Red and Blue Wolf-Rayet bumps in
star-forming SDSS galaxies and the UV and optical spectra of the star forming
regions Tol-A and B in NGC5398. 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 10^7 years in a
stellar population including binaries, increasing the UV 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.
