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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 NeNa-MgAl and Si-Ca regions, are also given.

. Because of the lower peak temperatures achieved in CO models, and also because of the lack of significant amounts of seed nuclei in the NeNa-MgAl region, the main nuclear activity in CO novae does not extend much beyond oxygen, as seen from the overproduction plot. In contrast, ONe models show a much larger nuclear activity, extending up to silicon (1.15 M ⊙ ONe) or argon (1.35 M ⊙ ONe). Hence, the presence of significantly large amounts of intermediate-mass nuclei in the spectra, such as phosphorus, sulfur, chlorine or argon, may reveal the presence of an underlying massive ONe white dwarf. Another trend derived from the analysis of the nucleosynthesis accompanying nova outbursts is the fact that the O/N and C/N ratios decrease as the mass of the white dwarf (and hence, the peak temperature attained during the explosion) increases.

Abundance Determinations in the Ejecta from Novae
In order to constraint the models, several works have focused on a direct comparison of the atomic abundances inferred from observations of the ejecta with the theoretical nucleosynthetic output (see [39,83], and references therein). Despite of the problems associated with the modeling of the explosion [86], such as the unknown mechanism responsible for the mixing between the accreted envelope and the outermost shells of the underlying white dwarf [9,21], or the difficulties to eject as much material as inferred from observations [71], there is an excellent agreement between theory and observations as regards nucleosynthesis (i.e., including atomic abundances -H, He, C, O, Ne, Na-Fe-, and a plausible endpoint for nova nucleosynthesis). In some cases, such as for PW Vul 1984, the agreement between observations and theoretical predictions (see [39], Table  5, for details) is really overwhelming. The reader is referred to [24] for an extended list of abundance determinations in the ejecta from novae, and to [68,88] for recent efforts to improve the abundance pattern for QU Vul 1984 and V1974 Cyg 1992, respectively.
Since the nuclear path is very sensitive to details of the evolution (chemical composition, extend of convective mixing, thermal history of the envelope...), the agreement between inferred abundances and theoretical yields not only validates the thermonuclear runaway model, but also poses limits on the (yet unknown) mixing mechanism itself: for instance, if mixing occurs very late in the course of the explosion, the accumulation of larger amounts of matter in the envelope will be favored (since the injection of significant amounts of the triggering nucleus 12 C will be delayed). Hence, one would expect to end up with a more violent outburst, characterized by a higher T peak , exceeding in some cases 4 × 10 8 K, and, as a result, a significant enrichment in heavier species, beyond calcium, in the ejecta from novae involving very massive white dwarfs, a pattern never observed so far.

Presolar Grains: Gifts from Heaven
Infrared [22,24] and ultraviolet observations [70] of the temporal evolution of nova light curves suggest that novae form grains in the expanding nova shells. Both CO and ONe novae behave similarly in the infrared right after the outburst. However, as the ejected envelope expands and becomes optically thin, such behavior dramatically changes: CO novae are typically followed by a phase of dust formation corresponding to a decline in visual light, together with a simultaneous rise in the infrared emission [23,25]. In contrast, it has been argued that ONe novae (that involve more massive white dwarfs than CO novae) are not so prolific producers of dust as a result of the lower mass, high-velocity ejecta, where the typical densities can be low enough to enable the condensation of appreciable amounts of dust. Hints on the condensation of dust containing silicates, silicon carbide, carbon and hydrocarbons have been reported from a number of novae (see [24] for a recent review).
Up to now, the identification of presolar nova grains, presumably condensed in the shells ejected during the explosion, relied only on low 20 Ne/ 22 Ne ratios (attributed to 22 Na decay), but quite recently five silicon carbide and two graphite grains that exhibit isotopic signatures characteristic of nova nucleosynthesis have been identified [1,2]. They are characterized by very low 12 C/ 13 C and 14 N/ 15 N ratios, 30 Si excesses and closeto-or slightly lower-than-solar 29 Si/ 28 Si ratios, high 26 Al/ 27 Al ratios (determined only for two grains) and low 20 Ne/ 22 Ne ratios (only measured in the graphite grain KFB1a-161). Such a promising discovery provides a much valuable source of constraint for nova nucleosynthesis (since contrary to the atomic abundance determinations derived from nova ejecta, measurements provide more accurate isotopic ratios) and opens interesting possibilities for the future.
Theoretical isotopic ratios for a variety of nuclear species, ranging from C to Si, based on hydrodynamic computations of the nova outburst, have been reported by different authors [82,42,44,45]. A more detailed analysis, which focuses on the different chemical pattern expected for CO and ONe novae, will be presented elsewhere [46].

SYNTHESIS OF RADIOACTIVE NUCLEI DURING NOVA OUTBURSTS
Among the isotopes synthesized during classical nova outbursts, several radioactive species deserve a particular attention. Short-lived nuclei, such as 14,15 O and 17 F (and to some extent 13 N) have been identified as the key isotopes that power the expansion and further ejection during a nova outburst through a sudden release of energy, a few minutes after peak temperature [74]. Other isotopes have been extensively investigated in connection with the theoretical gamma-ray output from novae [11,12,55]. Hence, 13 N and 18 F are responsible for the predicted prompt γ-ray emission [29] at and below 511 keV, whereas 7 Be and 22 Na [27], which decay much later, when the envelope is optically thin, are the sources that power line emission at 478 and 1275 keV, respectively. 26 Al is another important radioactive isotope that can be synthesized during nova outbursts, although only its cumulative emission can be observed because of its slow decay. We will briefly focus on the corresponding nuclear paths leading to the synthesis of the abovementioned gamma-ray emitters, with special emphasis on the nuclear uncertainties associated with the relevant reaction rates. We refer the reader to [30] for a review of the current theoretical predictions of the gamma-ray output from novae and of the chances for a nearby future detection using spacecrafts such as INTEGRAL (see also [31]). As for a comprehensive summary of the main uncertainties affecting nuclear reaction rates for nova temperatures, the reader is referred to [15]. Other recent attempts to fully analyze the impact of nuclear physics uncertainties in nova nucleosynthesis involve parametric one-zone calculations with temperature and density profiles extracted from hydrodynamic models [34,35], as well as Monte Carlo simulations [72]. We would like to stress, however, that results based on parametric calculations usually tend to overestimate the effect of a given reaction rate uncertainty, as compared with the outcome from hydrodynamic tests. Hence, although these simplified techniques provide a very valuable tool to potentially identify key reactions through an extraordinary large number of tests, final conclusions have to be confirmed through hydrodynamic calculations. 7 Be-7 Li lope in-place (the first hydrodynamic nova models that properly included the onset of the accretion phase were not available until the 80s), thus neglecting the impact of the accretion phase on the evolution. These results were refuted later on, in terms of parametric one/two zone models [8], pointing out the critical role played by the 9 C(p,γ) reaction, not included in all previous works (i.e., [4,77]), and claiming therefore for an unlikely synthesis of 7 Li in novae. The scenario was recently revisited by [28,39], who performed new hydrodynamic calculations, taking into account both the accretion and explosion stages, and a full reaction network (including 9 C(p,γ)). These studies confirmed that the Be-transport mechanism [10] is able to produce a large overproduction of 7 Li in nova explosions.
Among the issues that affect 7 Li synthesis in novae, one the most critical ones is the final amount of 3 He that survives the early rise in temperature when the thermonuclear runaway ensues. In particular, the different timescales to reach T peak achieved for CO and ONe novae, which deeply depend on the initial 12 C content in the envelope, lead to a larger amount of 7 Be in CO novae (which survives destruction through 7 Be(p,γ) 8 B because of the very efficient inverse photodisintegration reaction on 8 B). No relevant nuclear uncertainties in the domain of nova temperatures affect the corresponding reaction rates.
The puzzling 7 Li synthesis in novae, suggested for many years but elusive up to now, has been apparently confirmed for the first time in the spectra of V382 Vel (Nova Velorum 1999) [18], for which an observed feature compatible with the doublet at 6708 Å of LiI has been reported. Although this identification is not yet confirmed (there is no unambiguous interpretation for such a feature), the lithium abundances inferred are fully compatible with the theoretical upper limits given for a fast nova (i.e., [28,39]).
The potential contribution of classical novae to the Galactic 7 Li content turns out to be rather small (i.e., less than 15%), even if the 7 Li yield from the most favorable case, a massive CO nova [28], is considered. Nevertheless, a nova contribution is required to match the 7 Li content in realistic calculations of Galactic chemical evolution [66,67]. 22

Na
The potential role of 22 Na for diagnosis of nova outbursts was first suggested by [11]. It decays to a short-lived excited state of 22 Ne (with a lifetime of τ = 3.75 yr), which de-excites to its ground state by emitting a γ-ray photon of 1.275 MeV. Through this mechanism, nearby ONe novae within a few kiloparsecs from the Sun may provide detectable γ-ray fluxes [30]. Several experimental verifications of this γ-ray emission at 1.275 MeV from nearby novae have been attempted in the last twenty years, using balloon-borne experiments and detectors on-board satellites such as HEAO-3, SMM, or CGRO, from which upper limits on the ejected 22 Na have been derived. In particular, the observations performed with the COMPTEL experiment on-board CGRO of five recent Ne-type novae [36], as well as observations of standard CO novae, have led to an upper limit of 3.7 × 10 −8 M ⊙ for the 22 Na mass ejected by any nova in the Galactic disk. A limit that poses some constraints on pre-existing theoretical models of classical nova explosions.
A significant nuclear uncertainty affects the 25 Al(p,γ) 26 Si rate [40], which translates into an uncertainty in the expected contribution of novae to the Galactic 26 Al content. A critical issue in order to estimate this contribution is the initial composition of the ONe white dwarf. Whereas calculations by Starrfield et al. [79,83,84,85] assume a core composition based on hydrostatic models of carbon burning nucleosynthesis [3], rather enriched in 24 Mg (with a ratio 16 O: 20 Ne: 24 Mg around 1.5:2.5:1), we adopt more recent values taken from stellar evolution calculations of intermediate-mass stars [65], for which the 24 Mg content is much smaller ( 16 O: 20 Ne: 24 Mg is 10:6:1). Calculations based on the new ONe white dwarf composition [37,39,40,41], suggest that the contribution of novae to the Galactic 26 Al abundance is rather small (i.e., less than 15%), in good agreement with the results derived from the COMPTEL map of the 1809 keV 26 Al emission in the Galaxy (see [20]), which points towards young progenitors (type II supernovae and Wolf-Rayet stars).

F
The predicted gamma-ray emission from novae at and below 511 keV at early epochs after the explosion is basically driven by the amount of 18 F present in the envelope [55,27,29]. The synthesis of 18 F is powered by 16 15 O plus a minor contribution from 18 F(p,γ) 19 Ne. The effect of the nuclear uncertainties associated with some of the rates (i.e., 18 F(p,α) 15 O, 18 F(p,γ) 19 Ne, 17 O(p,α) 14 N and 17 O(p,γ) 18 F) [29,14,15] is, in this case, quite remarkable: they translate into a large uncertainty in the expected 18 F yield, and therefore, in the corresponding gamma-ray flux and maximum detectability distance. Advances to reduce this uncertainty have been reviewed during the Classical Nova Conference [6,19] and elsewhere [16], and involve several nuclear physics experiments performed in Oak Ridge (USA) and Orsay (France).

The endpoint of nova nucleosynthesis
In agreement with the chemical pattern derived from detailed observations of the ejecta, the theoretical endpoint for nova nucleosynthesis is limited to A < 40 (i.e., calcium), in agreement with current theoretical nucleosynthetic estimates, provided that the temperatures attained in the envelope during the explosion remain limited to T peak ∼ (2 − 3) × 10 8 K.
The nuclear activity in the Si-Ca region has been scarcely analyzed in detail in the context of classical nova outbursts [83,33,90,43]. It is powered by a leakage from the NeNa-MgAl region, where the activity is confined during the early stages of the explosion. The main reaction that drives the nuclear activity towards heavier species (i.e., beyond S) is mainly 30 P(p,γ) 31 S, either followed by 31 S(p,γ) 32 Cl(β + ) 32 S, or by 31 S(β + ) 31 P(p,γ) 32 S [43]. The 30 P(p,γ) rate is based only on Hauser-Feshbach estimates, which can be rather uncertain at the domain of nova temperatures. To test the effect of this uncertainty on the predicted yields, we have performed a series of hydrodynamic calculations [43], modifying arbitrarily the nominal rate. Hence, for a high 30 P(p,γ) rate (i.e., 100 times the nominal one), the final 30 Si yields are dramatically reduced by a factor of 30, whereas for a low 30 P(p,γ) rate (i.e., 0.01 times the nominal one), the final 30 Si yields are slightly increased by a factor of 5, whereas isotopes above silicon are reduced by a factor of ∼ 10, with dramatic impact on theoretical estimates ionvolving both the composition of the ejecta and of presolar grains.
The impact of nuclear reaction rate uncertainties on nucleosynthesis calculations points out the need of accurate nuclear physics inputs, and stresses the role played by classical novae as perfect laboratories for nuclear astrophysics.