  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
3 Msun, Z=0.01 evolutionary sequence to examine the consequences of high and
low reaction rate estimates of neutron captures onto sulphur and neighbouring
elements. In addition, we have tested high and low rates for the neutron
producing reactions C13(alpha,n)O16 and Ne22(alpha,n)Mg25. We vary the mass
width of a partially mixed zone (PMZ), which is responsible for the formation
of a C13 pocket and is the site of the C13(alpha,n)O16 neutron source. We test
PMZ masses from zero up to an extreme upper limit of the entire He-intershell
mass at 10^-2 Msun. 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 Msun and 6 Msun 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 3 Msun model) are excluded by comparison with neon
abundances in planetary nebulae. We compare the 1.8 Msun 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.

Solar and Stellar Astrophysics