The effect of 12C +12C rate uncertainties on the evolution and nucleosynthesis of massive stars

Abstract

Over the last 40 years, the 12 C + 12 C fusion reaction has been the subject of considerable experimental efforts to constrain uncertainties at temperatures relevant for stellar nucleosynthesis. Recent studies have indicated that the reaction rate may be higher than that currently used in stellar models. In order to investigate the effect of an enhanced carbon-burning rate on massive star structure and nucleosynthesis, new stellar evolution models and their yields are presented exploring the impact of three different 12 C + 12 C reaction rates. Non-rotating stellar models considering five different initial masses, 15, 20, 25, 32 and 60 M , at solar metallicity, were generated using the Geneva Stellar Evolution Code (GENEC) and were later post-processed with the NuGrid Multi-zone Post-Processing Network tool (MPPNP). A dynamic nuclear reaction network of ∼1100 isotopes was used to track the s-process nucleosynthesis. An enhanced 12 C + 12 C reaction rate causes core carbon burning to be ignited more promptly and at lower temperature. This reduces the neutrino losses, which increases the core carbonburning lifetime. An increased carbon-burning rate also increases the upper initial mass limit for which a star exhibits a convective carbon core (rather than a radiative one). Carbon-shell burning is also affected, with fewer convective-shell episodes and convection zones that tend to be larger in mass. Consequently, the chance of an overlap between the ashes of carbon-core burning and the following carbon shell convection zones is increased, which can cause a portion of the ashes of carbon-core burning to be included in the carbon shell. Therefore, during the supernova explosion, the ejecta will be enriched by s-process nuclides synthesized from the carbon-core s-process. The yields were used to estimate the weak s-process component in order to compare with the Solar system abundance distribution. The enhanced rate models were found to produce a significant proportion of Kr, Sr, Y, Zr, Mo, Ru, Pd and Cd in the weak component, which is primarily the signature of the carbon-core s-process. Consequently, it is shown that the production of isotopes in the Kr-Sr region can be used to constrain the 12 C + 12 C rate using the current branching ratio for αand p-exit channels.

The Compactness of Presupernova Stellar Cores

“…The effects of varying the three uncertain reaction rates on massive stellar evolution, and nucleosynthesis in particular, have been previously explored by Weaver & Woosley (1993), Tur et al (2007), and West et al (2013) for 3α, and 12 C(α, γ ) 16 O and by Pignatari et al (2013);Bennett et al (2012) for 12 C+ 12 C, but none of these works explicitly focused on how these rates affect the structure of presupernova stars. We will not attempt a survey of all possibilities at the present time, but focus here on just the one rate for 12 C(α, γ ) 16 O. Compactness parameter as a function of main sequence mass for non-rotating, solar metallicity stars using variable rates for the 12 C(α, γ ) 16 O reaction rate.…”

Code dependencies of pre-supernova evolution and nucleosynthesis in massive stars: evolution to the end of core helium burning

“…This nuclear burning step is then followed by a mixing step that solves the diffusion equation using diffusion coefficients from the stellar evolution calculations. For more details of the NuGrid post-processing tool MPPNP and the reaction rate compilations that we use, we refer the reader to Bennett et al (2012) and Pignatari et al (2013b). We used the same key energy-producing reaction rates as are used in the MESA and GENEC stellar evolution calculations to ensure consistency where possible.…”

“…The uncertainties in these reaction rates, which are important during helium-and carbonburning, were shown by Tur et al to induce large changes in the remnant (proto-neutron star) masses, which propagate to the final (explosive) nucleosynthesis yields of massive stars. Uncertainties in the 12 C+ 12 C reaction rate also propagate through into uncertainties in weak s-process element production, primarily via the impact that enhancing or reducing the rate has on the stellar structure during carbon burning (Gasques et al 2007;Bennett et al 2012;Pignatari et al 2013b).…”

Stellar structure, evolution and nucleosynthesis

“…Progress in one field can be made with the help of the other. One may constrain nuclear physics uncertainties and determine their impact by using stellar evolution models as virtual nuclear physics laboratories (see e. g. [86,40]). At the same time, nucleosynthesis signatures can help constrain stellar evolution models (see e. g. [72]).…”