Discrepancies between the [OIII] and [SIII] Temperatures in HII Regions

TL;DR

This study investigates the discrepancy between [OIII] and [SIII] temperatures in HII regions, exploring various models including metallicity inhomogeneities, shock heating, and electron energy distributions to explain the observations.

Contribution

It introduces and evaluates multiple models such as metallicity inhomogeneities, shock waves, and kappa electron distributions to explain temperature discrepancies in HII regions.

Findings

Metallicity inhomogeneities alone cannot explain the temperature differences.

Shock heating with velocities < 60 km/s can reproduce observed [OIII] temperature excess.

Kappa-distributions of electron energies successfully match the observed temperature discrepancies.

Abstract

An analysis of the OIII and SIII temperatures measurements compiled by Perez-Montero et al. of emission line objects consisting of HII galaxies, giant extragalactic HII regions, Galactic HII regions and HII regions from the Magellanic Clouds, reveals that the OIII temperatures are higher than the corresponding values from SIII in most objects with gas metallicities in excess of 0.2 solar. We explore the possibility of temperature inhomogeneities. We also explored metallicity inhomogeneities by combining two models of widely different metallicity. We calculate models that consider a non-Mawell-Boltzmann distributions for the electron energies (kappa parametrization). We also consider shock heating within the photoionized nebula. Varying the various input parameters in the pure photoionization case does not reproduce the observed nebular temperatures and neither does having local temperature inhomogeneities. We find that (1) metallicity inhomogeneities of the nebular gas, (2) shock waves of velocities < 60 km/s propagating in a photoionized plasma, and (3) an electron energy distribution given by a kappa-distribution, are successful in reproducing the observed excess in the [OIII] temperatures. Shock models, however, would require proper 3D hydrodynamical simulations to become a fully developed alternative while models with metallicity inhomogeneities appear to fail in metal poor nebulae, since they result in T_O++(rec)> T_OIII.