The Fe/Ni ratio in ionized nebulae: clues on dust depletion patterns

TL;DR

This study analyzes the Fe/Ni abundance ratio in various ionized nebulae and diffuse clouds, revealing a correlation between dust depletion levels and the Fe/Ni ratio, indicating nickel's higher affinity for dust grains.

Contribution

It provides a homogeneous analysis of Fe/Ni ratios in multiple nebulae and clouds, introducing an ICF for nickel based on extensive photoionization models, and compares these ratios across different environments.

Findings

Fe/Ni ratios correlate with dust depletion factors.

Nickel is more efficiently incorporated into dust grains than iron.

Results align with dust formation and growth theories.

Abstract

We perform a homogeneous analysis of the Fe/Ni abundance ratio in eight Galactic planetary nebulae (PNe) and three Galactic H II regions that include the Orion nebula, where we study four nebular zones and one shocked region. We use [Fe ii], [Fe iii], and [Ni iii] lines, and ionization correction factors (ICFs) that account for the unobserved ions. We derive an ICF for nickel from an extensive grid of photoionization models. We compare our results with those derived by other authors for 16 neutral clouds in the solar neighbourhood with available Fe/Ni ratios in the literature. We find an excellent agreement between the ionized nebulae and the diffuse clouds, with both types of regions showing a clear correlation between the Fe/Ni ratios and the iron and nickel depletion factors. The trend shows that the objects with a relatively low depletion have near solar Fe/Ni ratios whereas at higher depletions the Fe/Ni ratio increases with the depletion. Our results confirm that, compared to iron atoms, nickel ones are more efficiently stuck to the dust grains in ambients where dust formation or growth have been more efficient.