Abundance determinations in HII regions: model fitting versus Te-method

TL;DR

This paper compares oxygen abundance measurements in HII regions obtained via the Te-method and model fitting, advocating for the Te-method's accuracy and proposing the use of interstellar oxygen abundance as a benchmark.

Contribution

It demonstrates the validity of the Te-method for high-metallicity HII regions by comparing it with interstellar measurements, and recommends calibrations based on Te-method derived abundances.

Findings

Strong agreement between Te-method and interstellar oxygen abundances at solar galactocentric distance.

Two-zone Te model is a realistic representation of HII region temperature structure.

Te-method provides accurate oxygen abundances in high-metallicity HII regions.

Abstract

The discrepancy between the oxygen abundances in high-metallicity HII regions determined through the Te-method (and/or through the corresponding "strong lines - oxygen abundance" calibration) and that determined through the model fitting (and/or through the corresponding "strong lines - oxygen abundance" calibration) is discussed. It is suggested to use the interstellar oxygen abundance in the solar vicinity, derived with very high precision from the high-resolution observations of the weak interstellar absorption lines towards the stars, as a "Rosetta stone" to verify the validity of the oxygen abundances derived in HII regions with the Te-method at high abundances. The agreement between the value of the oxygen abundance at the solar galactocentric distance traced by the abundances derived in HII regions through the Te-method and that derived from the interstellar absorption lines towards the stars is strong evidence in favor of that i) the two-zone model for Te seems to be a realistic interpretation of the temperature structure within HII regions, and ii) the classic Te-method provides accurate oxygen abundances in HII regions. It has been concluded that the "strong lines - oxygen abundance" calibrations must be based on the HII regions with the oxygen abundances derived with the Te-method but not on the existing grids of the models for HII regions.