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.