Hydrogen burning of 17O in classical novae

We report on the observation of a previously unknown resonance at E(lab)(R)=194.1+/-0.6 keV in the 17O(p,alpha)14N reaction, with a measured resonance strength omegagamma(palpha)=1.6 +/- 0.2 meV. We studied in the same experiment the 17O(p,gamma)18F reaction by an activation method and the resonance-strength ratio was found to be omegagamma(palpha)/omegagamma(pgamma) = 470 +/- 50. The corresponding excitation energy in the 18F compound nucleus was determined to be 5789.8 +/- 0.3 keV by gamma-ray measurements using the 14N(alpha, gamma)18F reaction. These new resonance properties have important consequences for 17O nucleosynthesis and gamma-ray astronomy of classical novae.

21Na(p,gamma)22Mg reaction and oxygen-neon novae

The 21Na(p,gamma)22Mg reaction is expected to play an important role in the nucleosynthesis of 22Na in oxygen-neon novae. The decay of 22Na leads to the emission of a characteristic 1.275 MeV gamma-ray line. This report provides the first direct measurement of the rate of this reaction using a radioactive 21Na beam, and discusses its astrophysical implications. The energy of the important state was measured to be E(c.m.)=205.7+/-0.5 keV with a resonance strength omegagamma=1.03+/-0.16(stat)+/-0.14(sys) meV.

Gamma-Ray Emission from Novae Related to Positron Annihilation: Constraints on its Observability Posed by New Experimental Nuclear Data

Classical novae emit gamma-ray radiation at 511 keV and below (with a cutoff at around 20-30 keV), related to positron annihilation and its Comptonization in the expanding envelope. This emission has been elusive up to now because it occurs at epochs well before the maximum in optical luminosity, but it could be detected by some sensitive instrument on board a satellite, provided that the nova is close enough and that it is observed at the right moment. The detection of this emission, which is a challenge for current and future gamma-ray instruments, would shed light into the physical processes that occur in the early phases of the explosion, which are invisible in other lower energy ranges. A good prediction of the emitted fluxes and of the corresponding detectability distances with different instruments relies critically on a good knowledge of reaction rates relevant to 18F destruction, which have been subject to strong revision after recent nuclear spectroscopy measurements. With respect to previous results, smaller ejected masses of 18F are predicted, leading to smaller emitted fluxes in the 20-511 keV range and shorter detectability distances.