R-matrix analysis of the 14 N(p,γ ) 15 O astrophysical S -factor C. Angulo a , A.E. Champagne b and H.-P. Trautvetter c a Centre de Recherches du Cyclotron, Chemin du cyclotron 2, Louvain-la-Neuve, Belgium b The University of North Carolina, Chapel Hill, and TUNL, Durham, NC, USA c Institut f¨ ur Experimentalphysik III, Ruhr-Universit¨ at Bochum, Bochum, Germany 14 N(p,γ ) 15 O is the slowest reaction of the CNO cycle and thus it regulates stellar energy production, the flux of CNO neutrinos from the sun, and plays a role in determining the age of the globular clusters. We present R-matrix fits of the capture data to the ground state, the 6.79 MeV and the 6.18 MeV states in 15 O from two recent and independent direct measurements of the 14 N(p,γ ) 15 O low-energy cross section, together with previous data. The results confirm that the rate of the 14 N(p,γ ) 15 O reaction is a factor of 2 lower than previously accepted for temperatures below 0.1 GK. The recommended astrophysical S-factor at zero energy is S(0) = 1.70 ± 0.07 stat ± 0.10 sys keV-barn. These results could increase the age at the main-sequence turnoff by about 0.5-1 Gy, provided that all other astrophysical parameters are known. 1. INTRODUCTION The conversion of hydrogen into helium through the p-p chain and the CNO cycle is the main energy source in stars. The 14 N(p,γ ) 15 O reaction is the slowest step in the CNO cycle, thus regulating the rate of energy generation in massive main-sequence stars and, consequently, the time scale for their evolution. In addition, the CNO cycle is the main energy source for lower-mass stars near the end of their main-sequence evolution. The power liberated and the helium produced by the CNO cycle affect the luminosity on both the main sequence and horizontal branch and play an important role in determinating the ages of globular clusters [1]. Although the CNO cycle represents a small part of the sun’s energy budget, the rate of the 14 N(p,γ ) 15 O reaction determines the overall flux of CNO solar neutrinos. Accordingly, it is important to understand the 14 N(p,γ ) 15 O cross section at low energies. Among the direct measurements of the 14 N(p,γ ) 15 O cross section available in the litera- ture [2], the results of Schr¨ oder et al. [3] have been largely adopted. However, a reanalysis [4] of these data in the framework of the R-matrix model [5] resulted in an S-factor that was 2 times lower than reported by [3] and, consequently, a reaction rate that differed from the NACRE [2] rates by a factor of about 2 at temperatures below T = 0.15 GK. In addition, a measurement [6] of the lifetime for the 6.79 MeV state in 15 O resulted in a value about 15 times smaller than the previous value [3], which also implied a reduction in the S-factor by about a factor of 2. A direct measurement of the low-energy 14 N(p,γ ) 15 O Nuclear Physics A 758 (2005) 391c–394c 0375-9474/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2005.05.070