The Sulfur Abundance Anomaly in Planetary Nebulae

The alpha elements O, Ne, Mg, Si, S, Cl, and Ar are produced together in massive stars and their abundances are expected to evolve in lockstep. Both H II regions and planetary nebulae serve as interstellar probes of alpha element abundances. We have measured abundances for the alpha elements O, Ne, S, Cl, and Ar in over 130 Galactic Types I and II and halo PNe in a consistent manner from our own optical (3600–9600Å) spectra (Henry, Kwitter, & Balick 2004). Figure 1 is a plot of Ne vs. O for a combined dataset containing H II regions, blue compact galaxies, and our sample of planetary nebulae. All object types appear to be consistent with the expected linear relation between Ne and O. Similar plots of Cl and Ar vs. O show the same behavior. However, S vs. O is an exception.