Stochasticity, a variable stellar upper-mass limit, binaries and
  star-formation rate indicators
  Using our Binary Population And Spectral Synthesis (BPASS) code we explore
the effects on star-formation rate indicators of stochastically sampling the
stellar initial mass function, adding a cluster mass dependent stellar
upper-mass limit and including binary stars. We create synthetic spectra of
young clusters and star-forming galaxies and compare these to observations of
H(alpha) emission from isolated clusters and the relation between H(alpha) and
FUV emission from nearby galaxies. We find that observations of clusters tend
to favour a purely stochastic sampling of the initial mass function for
clusters less than 100M(Sun), rather than the maximum stellar mass being
dependant on the total cluster mass. It is more difficult to determine whether
the same is true for more massive clusters. We also find that binary stars blur
some of the observational differences that occur when a cluster-mass dependent
stellar upper-mass limit is imposed when filling the IMF. The effect is
greatest when modelling the observed H(alpha) and FUV star-formation rate
ratios in galaxies. This is because mass transfer and merging of stars owing to
binary evolution creates more massive stars and stars that have greater mass
than the initial maximum imposed on the stellar population.
