Effects of 12C +12C


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Contact: F.X.Timmes
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The effect of 12C +12C rate uncertainties on the evolution and nucleosynthesis of massive stars - 2012
In this paper, we explore recent suggestions that the 12C +12C 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 showing the impact of three different 12C +12C reaction rates.

An enhanced 12C +12C rate causes core carbon burning to be ignited more promptly and at lower temperature. This reduces the neutrino losses, which increases the core carbon- burning 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.

Maxwellian-averaged cross-sections
Kippenhahn diagrams, 15 & 20M
Carbon-core burning lifetimes

The 12C +12C reaction and the impact on nucleosynthesis in massive stars - 2012
In this paper, we explore the impacts of the uncertain C-burning reaction and the relative strengths between the different channels 12C(12C,α)20Ne, 12C(12C,p)23Na, 12C(12C,n)23Mg. A high 12C +12C rate may lead to lower central C-burning temperatures and to 13C(α,n)16O emerging as a more dominant neutron source than 22Ne(α,n)25Mg, increasing significantly the s-process production. This is due to the chain 12C(p,γ)13N followed by 13N(β+)13C, where the photodisintegration reverse channel 13N(γ,p)12C is strongly decreasing with increasing temperature. Here we show the impact of the 12C +12C reaction uncertainties on the s-process and on explosive p-process nucleosynthesis in massive stars.

Maxwellian-averaged cross-sections
Fluxes in and out of 23Na
Fluxes in and out of 22Ne