Chemical abundances in Seyfert galaxies VI. Empirical abundance
  calibration
  We derived a bi-dimensional calibration between the emission line ratios
R23=([O II]3726+3729+[O II]4959+5007)/Hb, P=[([O II]4959+5007)/Hb]/R23 and the
oxygen abundance relative to hydrogen (O/H) in the gas phase of Seyferts 1 and
2 nuclei. In view of this, emission-line intensity ratios for a sample of
objects taken from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7)
measured by the MPA/JHU group and direct estimates of O/H based on Te-method,
adapted for AGNs, are considered. We find no variation of R23 observed along
the radii of AGNs which shows that this line ratio is a good oxygen abundance
(O/H) indicator for the class of objects considered in this work. The derived
O/H = f(R23, P) relation produces O/H values similar to estimations via
Te-method in a wide range of metallicities [8.0 < 12+log(O/H) < 9.2].
Conversely to star-forming regions in the high metallicity regime, R23 shows a
positive correlation trend with O/H in AGNs. This indicates that the hardness
of ionizing radiation is not affected by the metallicities in these objects or
Narrow Line Regions (NLRs) are not significantly modified by changes in the
Spectral Energy Distribution due to metallicity variations.
