Revision of the ¹⁵N(p, γ)¹⁶O reaction rate and oxygen abundance in H-burning zones

¹ Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Padova, via Marzolo 8, 35131 Padova, Italy
e-mail: caciolli@pd.infn.it
² Dipartimento di Scienze della Terra, Università di Siena, 53100 Siena; and Centro di GeoTecnologie CGT, 52027 San Giovanni Valdarno, Italy
³ Università degli Studi di Milano and INFN, Sezione di Milano, 20133 Milano, Italy
⁴ Helmholtz–Zentrum Dresden–Rossendorf, Bautzner Landstr. 400, 01328 Dresden, Germany
⁵ Università di Genova and INFN Sezione di Genova, Genova, 16146 Genova, Italy
⁶ Institute of Nuclear Research (ATOMKI), 4026 Debrecen, Hungary
⁷ INFN, Laboratori Nazionali del Gran Sasso (LNGS), 67010 Assergi ( AQ), Italy
⁸ Dipartimento di Fisica Sperimentale, Università di Torino and INFN Sezione di Torino, 10125 Torino, Italy
⁹ Dipartimento di Scienze Fisiche, Università di Napoli Federico II, and INFN Sezione di Napoli, 80126 Napoli, Italy
¹⁰ Dipartimento di Fisica, Università degli studi di Perugia and INFN, Sezione di Perugia, 06123 Perugia, Italy
¹¹ Institut für Experimentalphysik, Ruhr–Universität Bochum, 44780 Bochum, Germany
¹² INAF – Osservatorio Astronomico di Collurania, 64100 Teramo, Italy
¹³ Seconda Università di Napoli, 81100 Caserta; and INFN Sezione di Napoli, 80126 Napoli, Italy
¹⁴ CNR IDASC SENSOR Lab and Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali, Università di Brescia, Brescia, Italy

Received: 14 June 2011
Accepted: 22 July 2011

Abstract

Context. The NO cycle takes place in the deepest layer of a H-burning core or shell, when the temperature exceeds T ≃ 30  ×  10⁶ K. The O depletion observed in some globular cluster giant stars, always associated with a Na enhancement, may be due to either a deep mixing during the red giant branch (RGB) phase of the star or to the pollution of the primordial gas by an early population of massive asymptotic giant branch (AGB) stars, whose chemical composition was modified by the hot bottom burning. In both cases, the NO cycle is responsible for the O depletion.

Aims. The activation of this cycle depends on the rate of the ¹⁵N(p, γ)¹⁶O reaction. A precise evaluation of this reaction rate at temperatures as low as experienced in H-burning zones in stellar interiors is mandatory to understand the observed O abundances.

Methods. We present a new measurement of the ¹⁵N(p, γ)¹⁶O reaction performed at LUNA covering for the first time the center of mass energy range 70−370 keV, which corresponds to stellar temperatures between 65  ×  10⁶ K and 780  ×  10⁶ K. This range includes the ¹⁵N(p, γ)¹⁶O Gamow-peak energy of explosive H-burning taking place in the external layer of a nova and the one of the hot bottom burning (HBB) nucleosynthesis occurring in massive AGB stars.

Results. With the present data, we are also able to confirm the result of the previous R-matrix extrapolation. In particular, in the temperature range of astrophysical interest, the new rate is about a factor of 2 smaller than reported in the widely adopted compilation of reaction rates (NACRE or CF88) and the uncertainty is now reduced down to the 10% level.