Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. IV. Synthesis and the Sulfur Anomaly

Abstract

We have compiled a large sample of O, Ne, S, Cl, and Ar abundances that have been determined for 85 Galactic planetary nebulae in a consistent and homogeneous manner using spectra extending from 3600 to 9600 A. Sulfur abundances have been computed using the near-IR lines of [S III] λλ9069, 9532 along with [S III] temperatures. We find average values, expressed logarithmically with a standard deviation, of log(S/O) = -1.91 ± 0.24, log(Cl/O) = -3.52 ± 0.16, and log(Ar/O) = -2.29 ± 0.18, numbers consistent with previous studies of both planetary nebulae and H II regions. We also find a strong correlation between [O III] and [S III] temperatures among planetary nebulae. In analyzing abundances of Ne, S, Cl, and Ar with respect to O, we find a tight correlation for Ne-O, and loose correlations for Cl-O and Ar-O. All three trends appear to be colinear with observed correlations for H II regions. S and O also show a correlation, but there is a definite offset from the behavior exhibited by H II regions and stars. We suggest that this S anomaly is most easily explained by the existence of S+3, whose abundance must be inferred indirectly when only optical spectra are available, in amounts in excess of what is predicted by model-derived ionization correction factors in PNe. Finally for the disk PNe, abundances of O, Ne, S, Cl, and Ar all show gradients when plotted against Galactocentric distance. The slopes are statistically indistinguishable from one another, a result which is consistent with the notion that the cosmic abundances of these elements evolve in lockstep.