The impact of strong recombination on temperature determination in planetary nebulae

TL;DR

This study investigates how recombination affects temperature diagnostics in planetary nebulae, revealing significant recombination contributions that challenge traditional assumptions and impact abundance measurements.

Contribution

The paper provides a new fit for the [O III] 4363 recombination coefficient and quantifies its contribution in specific nebulae, highlighting the importance of recombination in temperature diagnostics.

Findings

Recombination contribution can reach up to 70% in some nebulae.

The spatial distribution of recombination flux peaks in central regions.

Recombination significantly affects temperature diagnostics in high-abundance discrepancy nebulae.

Abstract

The long-standing difference in chemical abundances determined from optical recombination lines and collisionally excited lines raises questions about our understanding of atomic physics, as well as the assumptions made when determining physical conditions and chemical abundances in astrophysical nebulae. Here, we study the recombination contribution of [O III] 4363 and the validity of the line ratio [O III] 4363/4959 as a temperature diagnostic in planetary nebulae with a high abundance discrepancy. We derive a fit for the recombination coefficient of [O III] 4363 that takes into account the radiative and dielectronic recombinations, for electron temperatures from 200 to 30,000 K. We estimate the recombination contribution of [O III] 4363 for the planetary nebulae Abell 46 and NGC 6778 by subtracting the collisional contribution from the total observed flux. We find that the spatial distribution for the estimated recombination contribution in [O III] 4363 follows that of the O II 4649 recombination line, both peaking in the central regions of the nebula, especially in the case of Abell 46 which has a much higher abundance discrepancy. The estimated recombination contribution reaches up to 70% and 40% of the total [O III] 4363 observed flux, for Abell 46 and NGC 6778, respectively.