Emission-line diagnostics of nearby HII regions including interacting
  binary populations
  We present numerical models of the nebular emission from H II regions around
young stellar populations over a range of compositions and ages. The synthetic
stellar pop- ulations include both single stars and interacting binary stars.
We compare these models to the observed emission lines of 254 HII regions of 13
nearby spiral galax- ies and 21 dwarf galaxies drawn from archival data. The
models are created using the combination of the Binary Population and Spectral
Synthesis (bpass) code with the photoionization code cloudy to study the
differences caused by the inclusion of interacting binary stars in the stellar
population. We obtain agreement with the observed emission line ratios from the
nearby star-forming regions and discuss the effect of binary star evolution
pathways on the nebular ionization of H II regions. We find that at population
ages above 10 Myr, single-star models rapidly decrease in flux and ionization
strength, while binary-star model still produce strong flux and high O
III/H\b{eta} ratios. Our models can reproduce the metallicity of H II regions
from spiral galaxies but we find higher metallicities than previously estimated
for the H II regions from dwarf galaxies. Comparing the equivalent width of
H\b{eta} emission between mod- els and observations, we find that accounting
for ionizing photon leakage can affect age estimates for HII regions. When it
is included, the typical age derived for HII regions is 5Myr from single-star
models, and up to 10Myr with binary-star models. This is due to the existence
of binary-star evolution pathways which produce more hot WR and helium stars at
older ages. For future reference, we calculate new BPASS binary maximal
starburst lines as a function of metallicity, and for the total model
population, and present these in Appendix A.
