The Iron abundance in Galactic Planetary Nebulae

TL;DR

This study measures iron abundances in 33 Galactic planetary nebulae, revealing significant dust depletion of iron and providing insights into dust formation and destruction processes in different astrophysical environments.

Contribution

It presents the first systematic analysis of iron depletion in a sizable sample of planetary nebulae using [Fe III] lines and ionization correction factors, highlighting the extent of dust condensation.

Findings

Over 90% of iron is condensed onto dust grains.

Depletion factors vary by about two orders of magnitude.

No difference in iron depletion between C-rich and O-rich PNe.

Abstract

We constrain the iron abundance in a sample of 33 low-ionization Galactic planetary nebulae (PNe) using [Fe III] lines and correcting for the contribution of higher ionization states with ionization correction factors (ICFs) that take into account uncertainties in the atomic data. We find very low iron abundances in all the objects, suggesting that more than 90% of their iron atoms are condensed onto dust grains. This number is based on the solar iron abundance and implies a lower limit on the dust-to-gas mass ratio, due solely to iron, of M_dust/M_gas>1.3x10^{-3} for our sample. The depletion factors of different PNe cover about two orders of magnitude, probably reflecting differences in the formation, growth, or destruction of their dust grains. However, we do not find any systematic difference between the gaseous iron abundances calculated for C-rich and O-rich PNe, suggesting similar iron depletion efficiencies in both environments. The iron abundances of our sample PNe are similar to those derived following the same procedure for a group of 10 Galactic H II regions. These high depletion factors argue for high depletion efficiencies of refractory elements onto dust grains both in molecular clouds and AGB stars, and low dust destruction efficiencies both in interstellar and circumstellar ionized gas.