THE 12C + 12C REACTION AND THE IMPACT ON NUCLEOSYNTHESIS IN MASSIVE STARS

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 12C(12C, α)20Ne, 12C(12C, p)23Na, and 12C(12C, n)23Mg is poorly determined. Additionally, in C-burning conditions 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. Presented here is the impact of the 12C+12C 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 12C+12C rates, in particular an upper and lower rate limit of ∼50,000 higher and ∼20 lower than the standard rate at 5 × 108 K, five 25 M☉ stellar models are calculated. The enhanced s-process signature due to 13C(α, n)16O 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 eight compared with the standard case, efficiently producing the elusive Mo and Ru proton-rich isotopes. We also show that an s-process being driven by 13C(α, n)16O is a secondary process, even though the abundance of 13C 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.