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 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.
Highlights
After leaving the asymptotic giant branch (AGB), a post-AGB star may evolve to high temperatures (> 30, 000 K) on the timescale required to ionise the surrounding shell of ejected material and become visible as a planetary nebula (PN)
As planetary nebulae (PNe) are composed of envelope material from a progenitor AGB star, measurements of PN chemical abundances provide a way to test the predictions of AGB nucleosynthesis models (e.g., Marigo et al 2003; Karakas et al 2009; Pottasch & Bernard-Salas 2010; Karakas & Lugaro 2010)
The main finding of this study is that variation in the uncertainties that affect nucleosynthesis of AGB stars has little impact on the abundance of sulphur in AGB models
Summary
After leaving the asymptotic giant branch (AGB), a post-AGB star may evolve to high temperatures (> 30, 000 K) on the timescale required to ionise the surrounding shell of ejected material and become visible as a planetary nebula (PN). As PNe are composed of envelope material from a progenitor AGB star, measurements of PN chemical abundances provide a way to test the predictions of AGB nucleosynthesis models (e.g., Marigo et al 2003; Karakas et al 2009; Pottasch & Bernard-Salas 2010; Karakas & Lugaro 2010). Infrared observations of PNe (e.g., Bernard-Salas et al 2008) have directly measured S+3 abundances using the [S IV] emission line at 10.5 μm. This was done without the need for an ICF and these observations show that the sulphur anomaly still exists and is in need of explanation
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