The Curious Conundrum Regarding Sulfur Abundances In Planetary Nebulae

TL;DR

This paper investigates why sulfur abundances in planetary nebulae appear systematically lower than expected, exploring multiple hypotheses and finding that ionization correction factors likely cause the discrepancy.

Contribution

The study identifies that the sulfur anomaly is mainly due to inadequate ionization correction factors for higher sulfur ionization stages.

Findings

Using IR measurements reduces the sulfur deficit.

Most proposed explanations for the anomaly are ruled out.

ICFs are likely responsible for the sulfur abundance discrepancy.

Abstract

Sulfur abundances derived from optical emission line measurements and ionization correction factors in planetary nebulae are systematically lower than expected for the objects' metallicities. We have carefully considered a large range of explanations for this "sulfur anomaly", including: (1) correlations between the size of the sulfur deficit and numerous nebular and central star properties; (2) ionization correction factors which under-correct for unobserved ions; (3) effects of dielectronic recombination on the sulfur ionization balance; (4) sequestering of S into dust and/or molecules; and (5) excessive destruction of S or production of O by AGB stars. It appears that all but the second scenario can be ruled out. However, we find evidence that the sulfur deficit is generally reduced but not eliminated when S^+3 abundances determined directly from IR measurements are used in place of the customary sulfur ionization correction factor. We tentatively conclude that the sulfur anomaly is caused by the inability of commonly used ICFs to properly correct for populations of ionization stages higher than S^+2.