Poster abstract details

The nature of the dust grains formed in the stellar envelopes of asymptotic giant branch (AGB) stars is controlled by the gas chemistry, in particular by the value of the C/O abundance ratio. We expect silicates and oxides to form in O-rich environments (C/O<1) and carbonaceous material such as amorphous carbon or SiC grains in C-rich environments (C/O >1). Since planetary nebulae (PNe) are the descendants of AGB stars, the value of C/O in the ionized gas is likely to be related to the type of grains present in the nebulae, which can be inferred from infrared spectroscopy. However, the calculation of C/O in PNe is not an easy task, requiring the combination of ultraviolet and optical lines and uncertain corrections for the unobserved ions. We present here a homogeneous analysis of the C/O abundance ratio in a sample of Galactic PNe with available optical and ultraviolet spectra of good quality. Some of these PNe also have infrared spectra from Spitzer, which we use to identify dust features such as SiC, PAHs, the 30 $\mu$m feature, and silicates. We compare the values of C/O obtained with different methods, including the ones that involve recombination lines, and we explore the relation between the C/O abundance ratios and the infrared dust features observed in the nebulae.