THE¹²C +¹²C REACTION AND THE IMPACT ON NUCLEOSYNTHESIS IN MASSIVE STARS

Abstract

Despite much effort in the past decades, the C-burning reaction rate is\nuncertain by several orders of magnitude, and the relative strength between the\ndifferent channels 12C(12C,alpha)20Ne, 12C(12C,p)23Na and 12C(12C,n)23Mg is\npoorly determined. Additionally, in C-burning conditions a high 12C+12C rate\nmay lead to lower central C-burning temperatures and to 13C(alpha,n)16O\nemerging as a more dominant neutron source than 22Ne(alpha,n)25Mg, increasing\nsignificantly the s-process production. This is due to the rapid decrease of\nthe 13N(gamma,p)12C with decreasing temperature, causing the 13C production via\n13N(beta+)13C. Presented here is the impact of the 12C+12C reaction\nuncertainties on the s-process and on explosive p-process nucleosynthesis in\nmassive stars, including also fast rotating massive stars at low metallicity.\nUsing various 12C+12C rates, in particular an upper and lower rate limit of ~\n50000 higher and ~ 20 lower than the standard rate at 5*10^8 K, five 25 Msun\nstellar models are calculated. The enhanced s-process signature due to\n13C(alpha,n)16O activation is considered, taking into account the impact of the\nuncertainty of all three C-burning reaction branches. Consequently, we show\nthat the p-process abundances have an average production factor increased up to\nabout a factor of 8 compared to the standard case, efficiently producing the\nelusive Mo and Ru proton-rich isotopes. We also show that an s-process being\ndriven by 13C(alpha,n)16O is a secondary process, even though the abundance of\n13C does not depend on the initial metal content. Finally, implications for the\nSr-peak elements inventory in the Solar System and at low metallicity are\ndiscussed.\n