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 3 M , 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 also tested high and low rates for the neutron producing reactions 13 C(α,n) 16 O and 22 Ne(α,n) 25 Mg. We vary the mass width of a partially mixed zone (PMZ), which is responsible for the formation of a 13 C pocket and is the site of the 13 C(α,n) 16 O 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 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 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.

The far-ultraviolet spectra of “cool” PG 1159 stars

“…In particular, the abundances found in PG 1707+427 and PG 1424+535 (10 and 5×10 −5 ) are consistent with the range of abundances in models with different mass and reaction rates presented by Shingles & Karakas (2013) (2−8 × 10 −5 ), although their lowest mass model (1.8 M ) falls short by factors of 2.5 and 5 to correctly predict the observed high F abundances.…”

“…For phosphorus, we find 5.5 solar for PG 1424+535, matching the P overproduction of four times solar predicted by the 1.8 M model of Shingles & Karakas (2013). For PG 1707+427, 1.7 solar was determined, matching the 1.9 solar overproduction of P in a 3.0 M model.…”

“…Nevertheless, the various C and O abundances are consistent with models that include overshoot into the C-O core (Werner & Herwig 2006). (Such high values are not exhibited in the Shingles & Karakas (2013) models because they do not include that strong overshoot.) As to nitrogen, the presence of ample N in one of our stars was already discovered by Dreizler & Heber (1998).…”

“…In addition, one possible solution of the so-called sulphur anomaly in planetary nebulae, which describes an unexplained S depletion (Henry et al 2004), could originate from sulfur transformation into chlorine by neutron captures and subsequent beta decay in the central star's preceding AGB phase. Consequently, Shingles & Karakas (2013) have examined to what extent particular uncertainties of nucleosynthesis modeling (nuclear reaction rates, mass width of partially mixed zone) can affect the resulting intershell abundances of stars with a variety of initial masses. However, no explanation for the sulphur anomaly could be found.…”

Cross-Section Measurements of theKr86(γ,n)Reaction to Probe thes-Process Branching atRaut1, Tonchev2, Rusev3 et al. 2013Phys. Rev. Lett.593330View full textAdd to dashboardCite

“…We considered stellar models of 1.25 M ⊙ and 1.8 M ⊙ with a metallicity of Z=0.01 [28], 3 M ⊙ with Z=0.02 [27], and 3 M ⊙ with Z=0.01 [30]. In all of these models dredge-up carries enough carbon to the envelope to ensure a higher carbon abundance relative to oxygen, which is a necessary condition for the formation of carbon-rich dust such as SiC.…”