Chlorine and Sulfur in Nearby Planetary Nebulae and H II Regions

TL;DR

This study measures chlorine abundances in nearby planetary nebulae and H II regions, revealing similar Cl/H ratios and highlighting the importance of ionization correction factors for sulfur abundance estimates.

Contribution

It provides a homogeneous method for deriving element abundances and compares chlorine levels across different nebulae types, emphasizing the impact of dust depletion and ionization corrections.

Findings

Cl/H ratios are similar in PNe and H II regions.

Oxygen underabundance in H II regions is due to dust depletion.

Sulfur abundance estimates can be significantly biased by ionization correction factors.

Abstract

We derive the chlorine abundances in a sample of nearby planetary nebulae (PNe) and H II regions that have some of the best available spectra. We use a nearly homogeneous procedure to derive the abundance in each object and find that the Cl/H abundance ratio shows similar values in H II regions and PNe. This supports our previous interpretation that the underabundance we found for oxygen in the H II regions is due to the depletion of their oxygen atoms into organic refractory dust components. For other elements, the bias introduced by ionization correction factors in their derived abundances can be very important, as we illustrate here for sulfur using photoionization models. Even for low-ionization PNe, the derived sulfur abundances can be lower than the real ones by up to 0.3 dex, and the differences found with the abundances derived for H II regions that have similar S/H can reach 0.4 dex.