On the oxygen abundance determination in HII regions.

High-metallicity regions

Main Astronomical Observatory of National Academy of Sciences of Ukraine, Goloseevo, 03680 Kiev-127, Ukraine

Received: 26 October 2000
Accepted: 22 December 2000

Abstract

This is our second paper devoted to the problem of line intensity -oxygen abundance calibration starting from the idea of McGaugh ([CITE]) that the strong oxygen lines ( and ) contain the necessary information to determine accurate abundances in HII regions. In the previous study (Pilyugin 2000) the corresponding relations were obtained for the low-metallicity HII regions (, the lower branch of the O/H -R₂₃ diagram). The high-metallicity HII regions (, the upper branch of the O/H -R₂₃ diagram) are considered in the present study. A relation of the type ) between oxygen abundance and the value of abundance index R₂₃, introduced by Pagel et al. ([CITE]), and the excitation parameter P (which is defined here as the contribution of the radiation in lines to the "total" oxygen radiation) has been derived empirically using the available oxygen abundances determined via measurement of a temperature-sensitive line ratio [OIII]4959,5007/[OIII]4363 (T_e-method). By comparing oxygen abundances in high-metallicity HII regions derived with the T_e-method and those derived with the suggested relations (P-method), it was found that the precision of oxygen abundance determination with the P-method is around 0.1 dex (the mean difference for the 38 HII regions considered is ~0.08 dex) and is comparable to that of the T_e-method. A relation of the type ) between electron temperature and the values of abundance index R₂₃ and the excitation parameter P was derived empirically using the available electron temperatures determined via measurement of temperature-sensitive line ratios. The maximum value of differences between electron temperatures determined via measurement of temperature-sensitive line ratios and those derived with the suggested relation is around 1000 K for HII regions considered here, the mean value of differences for 38 HII regions is ~500 K, which is the same order of magnitude as the uncertainties of electron temperature determinations in high-metallicity HII regions via measured temperature-sensitive line ratios.