Metallicity-dependent isotopic abundances and the impact of helium rate uncertainties in massive stars

Abstract

All stellar evolution models for nucleosynthesis require an initial <italic>isotopic</italic> abundance set to use as a starting point, because nuclear reactions occur between isotopes. Generally, our knowledge of isotopic abundances of stars is fairly incomplete except for the Solar System. We develop a first model for a complete average isotopic decomposition as a function of metallicity. Our model is based on the underlying nuclear astrophysics processes, and is fitted to observational data, rather than traditional forward galactic chemical evolution modeling which integrates stellar yields beginning from big bang nucleosynthesis. We first decompose the isotopic solar abundance pattern into contributions from astrophysical sources. Each contribution is then assumed to scale as a function of metallicity. The resulting total isotopic abundances are summed into elemental abundances and fitted to available halo and disk stellar data to constrain the model's free parameter values. This procedure allows us to use available elemental observational data to reconstruct and constrain both the much needed complete isotopic evolution that is not accessible to current observations, and the underlying astrophysical processes. Our model finds a best fit for Type Ia supernovae contributing ∼0.7 to the solar Fe abundance, and Type Ia onset occurring at [Fe/H]∼−1.2, in agreement with typical values. The completed model can be used in future nucleosynthesis studies. We also perform a preliminary analysis to assess the impact of our isotopic scaling model on the resulting nucleosynthesis of massive stars, compared to a linear interpolation method. Using these two input methods we compute a limited grid of stellar models, and compare the final nucleosynthesis to observations. The compactness parameter was first used to assess which models would likely explode as successful supernovae, and contribute explosive nucleosynthesis yields. We find a better agreement to solar observations using the scaling model compared to the linear interpolation method, for the six s–only isotopes along the weak s–process path. As a second project, we study the sensitivity of presupernova evolution and supernova nucleosynthesis yields of massive stars to variations of the helium-burning reaction rates within the range of their uncertainties. The current solar abundances from Lodders (2010) are used for the initial stellar composition. We compute a grid of 12 initial stellar masses and 176 models per stellar mass to explore the effects of independently varying the <super>12</super>C(α,γ)<super>16</super>O</and 3α reaction rates, denoted R_α,12 and R_3α, respectively. The production factors of both the intermediate-mass elements (A=16–40) and the s–only isotopes along the weak s–process path (<super>70</super>Ge, <super>76</super>Se, <super>80</super>Kr, <super>82</super>Kr, <super>86</super>Sr, and <super>87</super>Sr) were found to be in reasonable agreement with predictions for variations of R_3α and R_α,12 of ±25%; the s–only isotopes, however, tend to favor higher values of R_3α than the intermediate-mass isotopes. The experimental uncertainty (one standard deviation) in R_3α(R_α,12) is approximately ±10%(±25%). The results show that a more accurate measurement of one of these rates would decrease the uncertainty in the other as inferred from the present calculations. We also observe sharp changes in production factors and standard deviations for small changes in the reaction rates, due to differences in the convection structure of the star. The compactness parameter was used to assess which models would likely explode as successful supernovae, and hence contribute explosive nucleosynthesis yields. We also provide the approximate remnant masses for each model and the carbon mass fractions at the end of core-helium burning as a key parameter for later evolution stages.