Sulfur abundances derived from optical emission line measurements and ionization correction factors (ICFs) in planetary nebulae are systematically lower than expected for the objects' metallicities. We have carefully considered a large range of explanations for this "sulfur anomaly," including: (1) correlations between the size of the sulfur deficit and numerous nebular and central star properties, (2) ICFs which undercorrect for unobserved ions, (3) effects of dielectronic recombination on the sulfur ionization balance, (4) sequestering of S into dust and/or molecules, and (5) excessive destruction of S or production of O by asymptotic giant branch stars. It appears that all but the second scenario can be ruled out. However, we find evidence that the sulfur deficit is generally reduced but not eliminated when S+3 abundances determined directly from IR measurements are used in place of the customary sulfur ICF. We tentatively conclude that the sulfur anomaly is caused by the inability of commonly used ICFs to properly correct for populations of ionization stages higher than S+2.

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Published in The Astrophysical Journal, v. 749, no. 1, 61, p. 1-15.

© 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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R.B.C.H. gratefully acknowledges support from NSF (AST-0806490). A.S. is grateful for a Big XII Fellowship that enabled her to visit the University of Oklahoma. A.I.K. is grateful for the support of the NCI National Facility at the ANU. G.J.F. acknowledges support by NSF (0908877), NASA (07-ATFP07-0124, 10-ATP10-0053, and 10-ADAP10-0073), and STScI (HST-AR-12125.01 and HST-GO-12309).