of magnitude, and we find that the highest depletion factors are found in C-rich objects with SiC or the 30 $\mu$m feature in their infrared spectra, whereas some of the O-rich objects with silicates show the lowest depletion factors.'
author:
- 'Gloria Delgado-Inglada and Mónica Rodríguez'
title: 'C/O abundance ratios, iron depletions, and infrared dust features in Galactic Planetary Nebulae'
---

Introduction {#sec:intro}
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Planetary nebulae (PNe) are suitable sites to study the evolution of dust grains since their progenitors, asymptotic giant branch (AGB) stars, are among the most efficient sources of circumstellar dust in the Galaxy [@Whittet_03]. These stars lose large amounts of material and create circumstellar envelopes with both low temperatures (below 1500 K in the regions of dust formation) and high densities ($\sim$ 10$^{13}$ cm$^{-3}$), where dust grains are efficiently formed.

In @DelgadoInglada_09, we studied iron depletions into dust grains in a sample of 33 Galactic disk PNe. Iron is an important contributor to the mass of dust grains [@Sofia_94], and its gaseous abundance can be used as a proxy for the amount of depletion of other elements. We found low iron gaseous abundances in all the objects, suggesting that more than 90% of their iron atoms 