2003PASP..115...80K

This paper is the fourth in a series whose purpose is to study the interstellar abundances of sulfur, chlorine, and argon in the Galaxy using a sample of 86 planetary nebulae. Here we present new high-quality spectrophotometric observations of 20 Galactic planetary nebulae with spectral coverage from 3700 to 9600 Å. A major feature of our observations throughout the entire study has been the inclusion of the near-infrared lines of [S III] λλ9069, 9532, which allows us to calculate accurate S⁺² abundances and to either improve upon or convincingly confirm results of earlier sulfur abundance studies. For each of the 20 objects here, we calculate ratios of S/O, Cl/O, and Ar/O and find average values of S/O=(1.1±1.1)x10^–2, Cl/O=(4.2±5.3)x10^–4, and Ar/O=(5.7±4.3)x10^–3. For six objects, we are able to compare abundances of S⁺³ calculated directly from available [S IV] 10.5 µm measurements with those inferred indirectly from the values of the ionization correction factors for sulfur. In the final paper of the series, we will compile results from all 86 objects, search for and evaluate trends, and use chemical evolution models to interpret our results.