Nucleosynthesis in Classical Novae and Its Contribution to the Interstellar Medium

Abstract

ABSTRACTClassical novae, explosions that result from thermonuclear runaways (TNRs) on the surfaces of white dwarfs (WDs) accreting hydrogen‐rich matter in close binary systems, are sporadically injecting material processed by explosive hydrogen‐burning nucleosynthesis into the interstellar medium (ISM). Although novae probably have processed less than ∼0.3% of the interstellar matter in the Galaxy, both theoretical and observational evidence suggests that they may be important sources of the nuclides 7Li, 15N, and 17O, as well as the radioactive isotopes 22Na and 26Al. The latter nuclides are astrophysically important in that they may have been involved in the production of the 22Ne (Ne‐E) and 26Mg enrichments identified in meteoritic inclusions, the composition of which is thought to be representative of the chemical and mineral contents of the primitive solar nebula. These inclusions may be partially composed of dust condensed in nova outbursts. We review theoretical expectations for the yields of various isotopes in nova outbursts and conclude that any of the heavy isotope anomalies attributable to novae are most likely produced by the approximately 25%–33% of novae that occur in systems containing massive (M*>1.2 M⊙) oxygen‐neon‐magnesium (ONeMg) WDs. We attempt to place quantitative constraints on the degree to which classical novae participate in the production of chemical anomalies, both in the primitive solar system and on a Galactic scale. Diffuse Galactic γ‐ray fluxes provide particularly important clues to and constraints on the 22Na and 26Al yields from novae. Ultraviolet (UV), optical, and infrared (IR) emission‐line spectra of classical novae reveal the abundances of some of the gas‐phase elements present in the ejecta; recent results are reviewed. We describe how IR observations of novae reveal dust formation and gas‐phase line emission and how they distinguish the temporal development of nova explosions on carbon‐oxygen (CO) WDs (CO novae) from those on ONeMg WDs (ONeMg or “neon” novae). Recent studies show that the ejecta in some novae can be strongly cooled by near‐ and mid‐IR forbidden‐line radiation from highly ionized (“coronal”) atomic states. We compare the abundances deduced from recent UV, optical, and IR observations with theoretical predictions, and we suggest that future studies of IR coronal emission lines may provide additional key information. Novae produce only about 0.1% of the Galactic “stardust” (dust condensed in stellar outflows), but IR observations show that it may be some of the more interesting dust. Novae appear capable of producing astrophysical dust of virtually every known chemical and mineral composition. We summarize recent IR observations of the dust production scenario in novae and argue that neon novae may lead to the formation of dust grains that carry the Ne‐E and 26Mg anomalies.