Nuclear Ashes: Reviewing Thirty Years of Nucleosynthesis in Classical Novae

One of the observational evidences in support of the thermonuclear runaway model for the classical nova outburst relies on the accompanying nucleosynthesis. In this paper, we stress the relevant role played by nucleosynthesis in our understanding of the nova phenomenon by constraining models through a comparison with both the atomic abundance determinations from the ejecta and the isotopic ratios measured in presolar grains of a likely nova origin. Furthermore, the endpoint of nova nucleosynthesis provides hints for the understanding of the mixing process responsible for the enhanced metallicities found in the ejecta, and reveals also information on the properties of the underlying white dwarf (mass, luminosity...). We discuss first the interplay between nova outbursts and the Galactic chemical abundances: Classical nova outbursts are expected to be the major source of 13C, 15N and 17O in the Galaxy, and to contribute to the abundances of other species with A < 40, such as 7Li or 26Al. We describe the main nuclear path during the course of the explosion, with special emphasis on the synthesis of radioactive species, of particular interest for the gamma-ray output predicted from novae (7Li, 18F, 22Na, 26Al). An overview of the recent discovery of presolar nova candidate grains, as well as a discussion of the role played by nuclear uncertainties associated with key reactions of the NeNaMgAl and Si-Ca regions, are also given. GALACTIC ALCHEMY: THE INTERPLAY BETWEEN NOVA OUTBURSTS AND THE GALACTIC ABUNDANCES The high peak temperatures achieved during nova explosions, Tpeak ∼ (2−3)×10 8 K, suggest that abundance levels of the intermediate-mass elements in the ejecta must be significantly enhanced, as confirmed by spectroscopic determinations in well-observed nova shells. This raises the issue of the potential contribution of novae to the Galactic abundances, which can be roughly estimated as the product of the Galactic nova rate, the average ejected mass per nova outburst, and the Galaxy’s lifetime. This order of magnitude estimate points out that novae scarcely contribute to the Galaxy’s overall metallicity (as compared with other major sources, such as supernova explosions), nevertheless they can substantially contribute to the synthesis of some largely overproduced species (see Table 1, for a sample of publications addressing nucleosynthesis in classical novae). Hence, classical novae are likely sites for the synthesis of most of the Galactic 13C, 15N and 17O, whereas they can partially contribute to the Galactic abundances of other species with A < 40, such as 7Li, 19F, or 26Al [83, 39]. Overproduction factors, relative to solar, corresponding to hydrodynamic calculations TABLE 1. A sample of publications in refereed journals addressing nucleosynthesis in classical