On the electron temperatures in high-metallicity HII regions

The electron temperatures of high-metallicity [12 + log(O/H) > 8.2] HII regions have been studied. The empirical flux-flux relations which express the nebular-to-auroral [OIII] line ratio Q_3,O (as well as the nebular-to-auroral [OII] line ratio Q_2,O, and the nebular-to-auroral [NII] line ratio Q_2,N) in terms of the nebular R₃ and R₂ line fluxes in spectra of high-metallicity HII regions are derived, and the electron temperatures t_3,O, t_2,O and t_2,N in a number of extragalactic HII regions are also determined. Furthermore, the t₂- t₃ diagram is discussed. It is found that there is a one-to-one correspondence between t₂ and t₃ electron temperatures for HII regions with a weak nebular R₃ lines (logR₃ <~ 0.5). The derived t_2,N-t_3,O relation for these HII regions is similar to commonly used t₂-t₃ relations. The HII regions with a strong nebular R₃ line flux (logR₃ >~ 0.5) do not follow this relation. A discrepancy between t_2,N and t_2,O temperatures is found, being the t_2,N temperatures systematically lower than the t_2,O ones. The differences are small at low electron temperatures, and increase with increasing electron temperatures up to 10 per cent at t = 1. The uncertainties in the atomic data may be the cause of this discrepancy.