Lederer, Claudia

Battino, Umberto

Murphy, Alex

Woods, Philip

Davinson, Thomas

Tattersall, Ashley

2025-03-11T11:25:50Z

2025-03-11T11:25:50Z

2025-03-11

To understand the origin of the solar system’s elemental abundances is a major challenge in astrophysics. Many generations of past stars and processes have contributed to the solar abundances. The abundances of the heavy elements beyond iron that we observe today in the solar system are mainly the result of two nucleosynthesis processes: the slow neutron-capture process (s-process) and the rapid neutron-capture process (r-process). Low-mass asymptotic giant branch (AGB) (2 < M /M⨀ < 3) and massive (M /M⨀ > 10) stars have been identified as the sites of the s-process. The s-process nucleosynthetic yields of low-mass AGB stars (initial mass M /M⨀ = 2, 3, and metallicity, Z = 0.01, 0.02 and 0.03) were calculated using stellar models with an updated mixing scheme. The ¹³C-pocket, a thin shell in the star where s-process material is produced, is formed through mixing induced by internal gravity waves (IGWs). This results in a pocket three times larger on average and hence larger s-process production compared to previous models. The full nucleosynthesis was calculated in post- processing using the NuGrid mppnp code. Isotopic and elemental abundances are compared to other stellar data sets available in the literature and to a wide range of observations, including carbon-stars, barium stars, and pre-solar grains. Good agreement was determined with few exceptions, for example ⁹⁶Mo, ¹³⁷Ba and ¹³⁸Ba. To judge the significance of (dis)agreements of our model results with observations, uncertainties of stellar yields and surface abundances due to uncertainties in nuclear reaction rates were determined, by adopting a Monte Carlo method. Rates for all neutron capture and neutron source reactions were simultaneously varied within their uncertainties in 10000 one zone nucleosynthesis calculations, resulting in an equal number of final isotopic abundances. The most important (key) nuclear reactions impacting on certain abundance changes were determined by calculation of Pearson correlation coefficients. Subsequently, these key nuclear reaction rates were varied within uncertainties in full multi zone post- processing calculations and results were used to estimate total uncertainties in stellar yields and surface abundances. Many abundance ratios can be constrained within 10% of the uncertainties from the nuclear physics input. Some are significantly higher than this, i.e. ⁸⁴Sr/⁸⁶Sr, ⁸⁸Sr/⁸⁶Sr, ⁹⁶Zr/⁹⁴Zr and ¹⁰⁰Mo/⁹⁶Mo. The higher uncertainties are due to the uncertain neutron capture cross sections of branching point nuclei, which have not been determined experimentally yet.

https://hdl.handle.net/1842/43191

http://dx.doi.org/10.7488/era/5732

en

The University of Edinburgh

asymptotic giant branch stars

AGB stars

slow neutron-capture process

rapid neutron-capture process

S-process nucleosynthesis

S-process nucleosynthesis of updated AGB star models and abundance uncertainties from nuclear reaction rates

Thesis or Dissertation

Doctoral

PhD Doctor of Philosophy