Is the sulphur anomaly in planetary nebulae caused by the s-process?

Abstract

Motivated by unexplained observations of low sulphur abundances in planetary nebulae (PNe) and the PG1159 class of post-asymptotic giant branch (AGB) stars, we investigate the possibility that sulphur may be destroyed by nucleosynthetic processes in low-to-intermediate mass stars during stellar evolution. We use a 3M⊙, Z = 0.01 evolutionary sequence to examine the consequences of high and low reaction rate estimates of neutron captures on to sulphur and neighbouring elements. In addition, we have also tested high and low rates for the neutron producing reactions 13C(α,n)16O and 22Ne(α,n)25Mg. We vary the mass width of a partially mixed zone (PMZ), which is responsible for the formation of a 13C pocket and is the site of the 13C(α,n)16O neutron source. We test PMZ masses from zero up to an extreme upper limit of the entire He-intershell mass at 10-2M⊙. We find that the alternative reaction rates and variations to the PMZ have almost no effect on surface sulphur abundances and do not reproduce the anomaly. To understand the effect of initial mass on our conclusions, 1.8 and 6M⊙ evolutionary sequences are also tested with similar results for sulphur abundances. We are able to set a constraint on the size of the PMZ, as PMZ sizes that are greater than half of the He-intershell mass (in the 3M⊙ model) are excluded by comparison with neon abundances in PNe. We compare the 1.8M⊙ model's intershell abundances with observations of PG1159-035, whose surface abundances are thought to reflect the intershell composition of a progenitor AGB star. We find general agreement between the patterns of F, Ne, Si, P and Fe abundances and a very large discrepancy for sulphur where our model predicts abundances that are 30-40 times higher than those observed in the star. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.