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 article, 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.

Non-rotating stellar models considering five different initial masses, 15, 20, 25, 32 and 60 M$_{\odot}$, 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 $\simeq$ 1100 isotopes was used to track the s-process nucleosynthesis.

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.

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 12C + 12C rate using the current branching ratio for $\alpha$- and p-exit channels.

image image
Kippenhahn diagrams, 15 & 20M

The 12C +12C reaction and the impact on nucleosynthesis in massive stars (2012)
Despite much effort in the past decades, the C-burning reaction rate is uncertain by several orders of magnitude, and the relative strength between the different channels $^{12}$C($^{12}$C,$\alpha$)$^{20}$Ne, $^{12}$C($^{12}$C,p)$^{23}$Na and $^{12}$C($^{12}$C,n)$^{23}$Mg is poorly determined. Additionally, in C-burning conditions a high $^{12}$C+$^{12}$C rate may lead to lower central C-burning temperatures and to $^{13}$C($\alpha$,n)$^{16}$O emerging as a more dominant neutron source than $^{22}$Ne($\alpha$,n)$^{25}$Mg, increasing significantly the $s$-process production. This is due to the chain $^{12}$C(p,$\gamma$)$^{13}$N followed by $^{13}$N($\beta$$^+$)$^{13}$C, where the photodisintegration reverse channel $^{13}$N($\gamma$,p)$^{12}$C is strongly decreasing with decreasing temperature.

In this article, we explore the impact of the $^{12}$C+$^{12}$C reaction uncertainties on the $s$-process and on explosive $p$-process nucleosynthesis in massive stars, including also fast rotating massive stars at low metallicity. Using various $^{12}$C+$^{12}$C rates, in particular an upper and lower rate limit of $\sim$ 50000 higher and $\sim$ 20 lower than the standard rate at 5$\times$10$^8$ K, five 25 M$_{\odot}$ stellar models are calculated. The enhanced $s$-process signature due to $^{13}$C($\alpha$,n)$^{16}$O activation is considered, taking into account the impact of the uncertainty of all three C-burning reaction branches. Consequently, we show that the $p$-process abundances have an average production factor increased up to about a factor of 8 compared to the standard case, efficiently producing the elusive Mo and Ru proton-rich isotopes. We also show that an $s$-process being driven by $^{13}$C($\alpha$,n)$^{16}$O is a secondary process, even though the abundance of $^{13}$C does not depend on the initial metal content. Finally, implications for the Sr-peak elements inventory in the Solar System and at low metallicity are discussed.

Maxwellian-averaged cross-sections

Fluxes in and out of 23Na
Fluxes in and out of 22Ne