The Te[N II]-Te[O III] temperature relation in H II regions and the reliability of strong-line methods

TL;DR

This study investigates the Te[O III]-Te[N II] temperature relation in H II regions, introduces new relations accounting for ionization, and assesses the reliability of strong-line methods for metallicity estimation, highlighting their limitations.

Contribution

It provides new temperature relations considering ionization effects and evaluates the accuracy of common strong-line methods against direct measurements.

Findings

New temperature relations improve abundance calculations.

Strong-line methods can differ by ~0.2 dex or more from direct methods.

Ionization degree affects the reliability of metallicity estimates.

Abstract

We use a sample of 154 observations of 124 H II regions that have measurements of both Te[O III] and Te[N II], compiled from the literature, to explore the behaviour of the Te[O III]-Te[N II] temperature relation. We confirm that the relation depends on the degree of ionization and present a new set of relations for two different ranges of this parameter. We study the effects introduced by our temperature relations and four other available relations in the calculation of oxygen and nitrogen abundances. We find that our relations improve slightly on the results obtained with the previous ones. We also use a sample of 26 deep, high-resolution spectra to estimate the contribution of blending to the intensity of the temperature-sensitive line [O III] $\lambda4363$, and we derive a relation to correct Te[O III] for this effect. With our sample of 154 spectra, we analyse the reliability of the R, S, O3N2, N2, ONS, and C strong-line methods by comparing the metallicity obtained with these methods with the one implied by the direct method. We find that the strong-line methods introduce differences that reach ~ 0.2 dex or more, and that these differences depend on O/H, N/O, and the degree of ionization.