Proton-Capture Rates on Carbon Isotopes and Their Impact on the Astrophysical C12/C13 Ratio

The C12/C13 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 C12(p,γ)N13 and C13(p,γ)N14 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.=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 C12(p,γ)N13 reaction and by 30% for C13(p,γ)N14. We provide the most precise value up to now of 3.6±0.4 in the 20-140 MK range for the lowest possible C12/C13 ratio that can be produced during H burning in giant stars.