On the oxygen abundance determination in HII regions. High-metallicity regions

Abstract

This is our second paper devoted to the problem of line intensity - oxygen abundance calibration starting from the idea of McGaugh (1991) that the strong oxygen lines ([OII]λλ3727, 3729 and [OIII]λλ4959, 5007) 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 (12 + log O/H ≤ 7.95, the lower branch of the O/H - R23 diagram). The high-metallicity HII regions (12 + log O/H ≥ 8.2, the upper branch of the O/H - R23 diagram) are considered in the present study. A relation of the type O/H = f(P, R23) between oxygen abundance and the value of abundance index R23, introduced by Pagel et al. (1979), and the excitation parameter P (which is defined here as the contribution of the radiation in [OIII]λλ4959, 5007 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 (Te-method). By comparing oxygen abundances in high-metallicity HII regions derived with the Te-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 Te-method. A relation of the type Te = f(P, R23) 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.