Proton-Capture Rates on Carbon Isotopes and Their Impact on the Astrophysical C12/C13 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 12C(p, gamma )13N and 13C(p, gamma )14N 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 gamma-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 12C(p, gamma )13N reaction and by 30% for 13C(p, gamma)14N. 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.