Chemical abundances of Seyfert 2 AGNs IV. Composite models calculated by
  photoionization + shocks
  We build detailed composite models of photoionization and shock ionization
based on the SUMA code to reproduce emission lines emitted from the Narrow Line
Regions (NLR) of Seyfert 2 nuclei. The aim of this work is to investigate
diagram AGN positions according to shock parameters, shock effects on the gas
temperature and ionization structures and derive a semi-empirical abundance
calibration based on emission-line ratios little sensitive to the shock
presence. The models were used to reproduce optical (3000 < A < 7000) emission
line intensities of 244 local (z < 0.4) Seyfert 2s, whose observational data
were selected from Sloan Digital Sky Survey DR7. Our models suggest that shocks
in Seyfert 2 nuclei have velocities in the range of 50-300 km/s and imply a
narrower metallicity range (0.6 < (Z/Z) < 1.6) than those derived using pure
photoionization models. Our results indicate that shock velocity in AGNs can
not be estimated using standard optical line ratio diagrams, based on
integrated spectra. Our models predict a different temperature structure and
O+/O and O2+/O fractional abundances throughout the NLR clouds than those
derived from pure photoionization models, mainly in shock-dominated objects.
This suggests that, in order to minimize the shock effects, the combination of
emission-lines emitted by ions with similar intermediate ionization potential
could be good metallicity indicators. Finally, we derive two calibrations
between the N/O abundance ratio and the N2O2=log([N II]6584/[O II]3727) and
N2=log([N II]6584/H{\alpha}) indexes which agree with that derived from pure
photoionization models.
