A Study of 19Ne Resonances and their Astrophysical Implication for the Detection of Novae

Classical novae are the most common astrophysical thermonuclear explosion and are thought to contribute noticeably to the galactic chemical evolution. As one of the few environments that can be modelled primarily from experimental nuclear data, observations of isotopic abundances would provide a direct test for current hydrodynamic codes. Gamma rays are the only such radiation that can be observed to trace the nucleosynthesis of isotopes directly. Fluorine-18 produced in the runaway is the strongest γ-ray source immediately after the outburst, although reaction rates must be constrained further to predict its intensity and therefore detectability. The 18F(p,α)15O reaction remains the largest uncertainty in constraining these rates as key nuclear state parameters in the compound nucleus, 19Ne, are still not known despite considerable experimental effort. To resolve this, the most important levels close to the proton threshold were populated using the charge exchange reaction 19F(3He,t)19Ne at IPN, Orsay. A Split-pole spectrometer measured the tritons and identified the states of interest, whilst a highly segmented silicon array detected alpha-particle and proton decays from 19Ne over a large angular range and at a high angular resolution. The resonance parameters, extracted from the experimental results, provide evidence for a postulated broad state and produce a spin-parity result for the important -122 keV subthreshold state in direct contradiction to previous measurements of the nucleus. The results, in addition to other recent studies, provided input parameters for a comprehensive set of theoretical R-matrix calculations that have realistically modelled the remaining uncertainty in the reaction rate. The newly proposed rate is discussed, along with implications for future studies of the destruction reaction, both direct and indirect, which are necessary in providing an answer to the γ-ray detectability of classical novae

First measurement of the 34S(p,γ)35Cl reaction rate through indirect methods for presolar nova grains

Sulphur isotopic ratio measurements may help to establish the astrophysical sites in which certain presolar grains were formed. Nova model predictions of the 34S/32S ratio are, however, unreliable due to the lack of an experimental 34S(p,γ)35Cl reaction rate. To this end, we have measured the 34S(3He,d)35Cl reaction at 20 MeV using a high resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. Twenty-two levels over 6.2 MeV<Ex(35Cl)<7.4 MeV were identified, ten of which were previously unobserved. Proton-transfer spectroscopic factors have been measured for the first time over the energy range relevant for novae. With this new spectroscopic information a new 34S(p,γ)35Cl reaction rate has been determined using a Monte Carlo method. Hydrodynamic nova model calculations have been performed using this new reaction rate. These models show that remaining uncertainties in the 34S(p,γ) rate affect  nucleosynthesis predictions by less than a factor of 1.4, and predict a 34S/32S isotopic ratio of 0.014–0.017. Since recent type II supernova models predict 34S/32S=0.026−0.053, the 34S/32S isotopic ratio may be used, in conjunction with other isotopic signatures, to distinguish presolar grains from oxygen-neon nova and type II supernova origin. Our results address a key nuclear physics uncertainty on which recent considerations discounting the nova origin of several grains depend

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 $^{13}$C and $^{15}$N excesses when the shock of a core-collapse supernova passes through its helium shell. This prediction questions the origin of extremely high $^{13}$C and $^{15}$N abundances observed in rare presolar SiC grains which is usually attributed to classical novae. In this context $^{13}$N($\alpha$,p)$^{16}$O the reaction plays an important role since it is in competition with $^{13}$N $\beta^+$-decay to $^{13}$C. The $^{13}$N($\alpha$,p)$^{16}$O 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 $^{13}$N($\alpha$,p)$^{16}$O reaction rate, based on available experimental data. Unbound nuclear states in the $^{17}$F compound nucleus were studied using the spectroscopic information of the analog states in $^{17}$O nucleus that were measured at the Alto facility using the $^{13}$C($^7$Li,t)$^{17}$O alpha-transfer reaction, and spectroscopic factors were derived using a DWBA analysis. This spectroscopic information was used to calculate a recommended $^{13}$N($\alpha$,p)$^{16}$O reaction rate with meaningful uncertainty using a Monte Carlo approach. The present $^{13}$N($\alpha$,p)$^{16}$O 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 $^{13}$C production of a factor of 50. The main source of uncertainty on the re-evaluated $^{13}$N($\alpha$,p)$^{16}$O reaction rate currently comes from the uncertain alpha-width of relevant $^{17}$F states