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 less than or similar to 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 Ne2 + is located in comparison with t(3) for the O2 + ion. We find that the differences (D) between Ne2 +/H+ ionic abundances calculated from IR-method and T-e-method (assuming t(3) in the Ne2 +/H+ derivation) are similar to the derivations in star-forming regions (SFs) and they are reduced by a mean factor of similar to 3 when is considered. We propose a semi-empirical Ionization Correction Factor (ICF) for the neon, based on [Neii]12.81, [Neiii]15.56, 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)]) greater than or similar to 8.80])an increase in Ne/O with O/H is found, which likely indicates secondary stellar production for the neon.