Evaluation of the ^13N(α,p)^16O thermonuclear reaction rate and its impact on the isotopic composition of supernova grains

It has been suggested that hydrogen ingestion into the helium shell of massive stars could lead to high ^13C and ^15N excesses when the shock of a core-collapse supernova passes through its helium shell. This prediction questions the origin of extremely high ^13C and ^15N abundances observed in rare presolar SiC grains which is usually attributed to classical novae. In this context ^13N(α,p)^16O the reaction plays an important role since it is in competition with ^13N β^+-decay to ^13C. The ^13N(α,p)^16O reaction rate used in stellar evolution calculations comes from the CF88 compilation with very scarce information on the origin of this rate. The goal of this work is to provide a recommended ^13N(α,p)^16O reaction rate, based on available experimental data. Unbound nuclear states in the ^17F compound nucleus were studied using the spectroscopic information of the analog states in ^17O nucleus that were measured at the Alto facility using the ^13C(^7Li,t)^17O alpha-transfer reaction, and spectroscopic factors were derived using a DWBA analysis. This spectroscopic information was used to calculate a recommended ^13N(α,p)^16O reaction rate with meaningful uncertainty using a Monte Carlo approach. The present ^13N(α,p)^16O reaction rate is found to be within a factor of two of the previous evaluation, with a typical uncertainty of a factor 2-3. The source of this uncertainty comes from the three resonances at E_r^c.m. = 221, 741 and 959 keV. This new error estimation translates to an overall uncertainty in the ^13C production of a factor of 50. The main source of uncertainty on the re-evaluated ^13N(α,p)^16O reaction rate currently comes from the uncertain alpha-width of relevant ^17F states.