The Curious Conundrum Regarding Sulfur Abundances In Planetary Nebulae

R.B.C. Henry (1), Angela Speck (2), Amanda I. Karakas (3), Gary J. Ferland (4), Mason Maguire (5) ((1) University of Oklahoma, Norman, OK; (2) University of Missouri, Columbia, MO, USA; (3) Mount Stromlo Observatory,  Australia; (4)  University of Kentucky, Lexington, KY USA; (5) University of Oklahoma, Norman, OK, USA).

Sulfur abundances derived from optical emission line measurements and ionization correction factors 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) ionization correction factors which under-correct 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 AGB 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 ionization correction factor. 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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2 Responses to The Curious Conundrum Regarding Sulfur Abundances In Planetary Nebulae

  1. grazyna stasinska says:

    Hello Dick,
    Available ionization correction factors (except those of Kingsburgh and Barlow) are based on grids of simple ab initio photoionization models and I would not put much faith in them because quite a lot of questions are not addressed (density structure, aperture effects, density “boundedness”, spectral energy distribution of the ionizing radiation field) in addition to atomic data. In the case of high excitation PNe, I do not think S abundances obtained in such a way are reliable. What about considering only low-excitation PNe in sulfur abundance studies?
    As for abundances obtained using IR data, there is the problem of aperture correction (except for the case of compact sources) which is rarely treated in a satisfactory way because it actually requires a tailored photoionization model taking into account the different observing apertures.

  2. Dick Henry says:

    Hi Grazyna-

    Thanks for your comments on our paper. I certainly can agree with you regarding the shortcomings of the photoionization model grids for developing ICFs. As we pointed out at the end of our paper, recent models which assume density gradients appear to offer a solution in the case of the sulfur problem. Regarding the Kingsburgh & Barlow ICFs, as far as I have been able to determine through literature searching and asking around, their formulae are based upon nine photoionization models the details of which have never been published. Your point about aperture correction is certainly worth considering and quite possibly explains why the use of S+++ abundances derived from IR lines (10.5 microns) does not reduce the offset more completely between expected and measured S abundances in PNe.


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