On the electron temperatures in high-metallicity HII regions

TL;DR

This study investigates electron temperatures in high-metallicity HII regions, deriving empirical relations between line ratios and temperatures, revealing a consistent t2-t3 relation for weak R3 lines and discrepancies for strong R3 lines.

Contribution

The paper introduces new empirical ff relations linking nebular and auroral line ratios to electron temperatures in high-metallicity HII regions, enhancing temperature diagnostics.

Findings

One-to-one t2-t3 temperature correspondence for weak R3 lines.

Discrepancy with strong R3 lines where t2,N < t2,O.

Uncertainties in atomic data may cause temperature differences.

Abstract

The electron temperatures of high-metallicity (12+log(O/H) > 8.2) HII regions have been studied. The empirical ff 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 R_3 and R_2 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 t_2 - t_3 diagram is discussed. It is found that there is a one-to-one correspondence between t_2 and t_3 electron temperatures for HII regions with a weak nebular R_3 lines (logR_ < 0.5). The derived t_2,N - t_3,O relation for these HII regions is similar to commonly used t_2 - t_3 relations. The HII regions with a strong nebular R_3 line flux (logR_3 > 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 t_2,O ones. The differences are small at low electron temperatures and increases with increasing electron temperatures up to 10% at t=1. The uncertainties in the atomic data may be the cause of this discrepancy.