Chemical abundances in Seyfert galaxies -- VII. Direct abundance
  determination of neon based on optical and infrared emission lines
  For the first time, neon abundance has been derived in the narrow line region
from a sample of Seyfert~2 nuclei. In view of this, we compiled from the
literature fluxes of optical and infrared (IR) narrow emission lines for 35
Seyfert 2 nuclei in the local universe ($z < 0.06$). The relative intensities
of emission lines were used to derive the ionic and total neon and oxygen
abundances through electron temperature estimations ($T_{e}$-method). For the
neon, abundance estimates were obtained by using both $T_{e}$-method and
IR-method. Based on photoionization model results, we found a lower electron
temperature [$t_{e}([Ne III])$] for the gas phase where the Ne$^{2+}$ is
located in comparison with $t_{3}$ for the O$^{2+}$ ion. We find that the
differences (D) between Ne$^{2+}$/H$^{+}$ ionic abundances calculated from
IR-method and $T_{e}-$method (assuming $t_{3}$ in the Ne$^{2+}$/H$^{+}$
derivation) are similar to the derivations in star-forming regions (SFs) and
they are reduced by a mean factor of $\sim3$ when $t_{e}([Ne III])$ is
considered. We propose a semi-empirical Ionization Correction Factor (ICF) for
the neon, based on [Ne II]12.81$\mu$m, [\ion{Ne}{iii}]15.56$\mu$m and oxygen
ionic abundance ratios. We find that the average Ne/H abundance for the Seyfert
2s sample is nearly 2 times higher than similar estimate for SFs. Finally, for
the very high metallicity regime (i.e. [$12+log(O/H) > 8.80$]) an increase in
Ne/O with O/H is found, which likely indicates secondary stellar production for
the neon.
