Is the Sulphur Anomaly in Planetary Nebulae Caused by the s-Process?

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 pos-sibility that sulphur may be destroyed by nucleosynthetic processes in low-to-intermediate mass stars during stellar evolution. We use a 3 M, Z = 0.01 evolutionary sequence to exam-ine the consequences of high and low reaction rate estimates of neutron captures onto 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 10−2 M. We find that the alternative reaction rates and variations to the partially mixed zone 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 M and 6 M evolutionary sequences are also tested with similar results for sulphur abun-dances. 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 3 M model) are excluded by comparison with neon abundances in planetary nebulae. We compare the 1.8 M 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 is observed in the star