Proton-Capture Rates on Carbon Isotopes and Their Impact on the Astrophysical 12C/13C Ratio

The 12C/13C ratio is a significant indicator of nucleosynthesis and mixing processes during hydrogen burning in stars. Its value mainly depends on the relative rates of the 12Cðp; γÞ13N and 13Cðp; γÞ14N reactions. Both reactions have been studied at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy down to the lowest energies to date (Ec:m: ¼ 60 keV) reaching for the first time the high energy tail of hydrogen burning in the shell of giant stars. Our cross sections, obtained with both prompt γ-ray detection and activation measurements, are the most precise to date with overall systematic uncertainties of 7%–8%. Compared with most of the literature, our results are systematically lower, by 25% for the 12Cðp; γÞ13N reaction and by 30% for 13Cðp; γÞ14N. We provide the most precise value up to now of 3.6 +/- 0.4 in the 20–140 MK range for the lowest possible 12C/13C ratio that can be produced during H burning in giant stars.