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