Dating of H II Regions from Ionization Modeling

The emission-line spectrum of an H II region essentially reflects the shape of the EUV spectral energy distribution of the OB stars that ionize the gas. In principle one can use this spectrum to extract information about the age of the underlying association. In practice, a number of effects can lead to systematic errors in derived ages. The star-gas geometry is an important consideration, as it determines the ionization parameter; high-spatial-resolution imaging is thus essential to constrain the geometry. Dust grains absorb ionizing photons and decrease the Balmer equivalent widths and derived Lyman-continuum luminosity; differential extinction between stars and gas can also affect equivalent widths. Starburst synthesis models appear to over-predict the numbers of very hot Wolf-Rayet stars, especially for metal-rich starbursts; this can have deleterious effects on the modeling for such objects.