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

Abstract

The 12 C/ 13 C 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 12 C(p, γ) 13 N and 13 C(p, γ) 14 N 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 12 C(p, γ) 13 N reaction and by 30 % for 13 C(p, γ) 14 N. We provide the most precise value up to now of 3.6 ± 0.4 in the 20 -140 MK range for the lowest possible 12 C/ 13 C ratio that can be produced during H burning in giant stars.Absorption lines in stellar spectra are commonly used to infer the atmospheric composition of a star. However, lines emitted by different isotopes of the same element can only be resolved for a limited number of cases.Among them, the carbon isotopic ratio 12 C/ 13 C ratio provides important insights about internal nucleosynthesis and its coupling with various mixing process, such as those induced by thermal convection, rotational instabilities, thermohaline circulation, magnetic buoyancy and turbulence due to the propagation of gravity waves [1][2][3][4][5][6][7][8][9]. Furthermore, the 12 C/ 13 C ratio can be directly and precisely measured in silicon carbide (SiC) grains that originated in giant stars and are recovered from mete-47 orites [10]. In the solar system, 12 C/ 13 C = 89 [11], while 48 in nearby molecular clouds an average 12 C/ 13 C ≃ 68 49 has been reported [12, 13]. This lower ratio likely re-50 flects the chemical evolution of the interstellar medium 51 that occurred since the formation of the solar system 52 4.6 Gyr ago [13]. The possible 13 C producers that con-53 tribute to lowering the isotopic ratio include massive 54 stars, Asymptotic Giant Branch (AGB) stars and clas-55 sical novae. Measurements of 12 C/ 13 C in the interstellar 56 medium located at different galactocentric distances [13] 57 confirm that this ratio is a tracer of the chemical evo-58 lution: it is lower toward the galactic center, where the 59 stellar density is higher and the chemical evolution faster, 60 and higher away from the center where the stellar density 61 is lower. 62 The carbon isotopic ratio in the atmospheres of evolved 63 stars shows variations during their evolution that are 64 the consequence of the combined action of internal nu-65 cleosynthesis and deep mixing processes. When a star, 66 after the central H exhaustion, leaves the main sequence 67 and becomes a red giant (RGB), a convective instabil-68 ity arises that starts from the surface and penetrates in-69 wards, down into the region whose composition has been 70 previously modified by H burning. A...