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Register a new userNucleosynthesis in classical novae
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A general review of the relevance of classical novae for the chemical evolution of the Galaxy, as well as for Galactic radioactivity is presented. A special emphasis is put on the pioneering work done by Jim Truran in this field of research. The impact of recent developments in nuclear astrophysics on nova nucleosynthesis, together with the prospects for observability of novae radioactivities with the INTEGRAL satellite are discussed.
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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.
Classical novae are among the most frequent transient events in the Milky Way, and key agents of ongoing nucleosynthesis. Despite their large numbers, they have never been observed in soft $gamma$-ray emission. Measurements of their $gamma$-ray signatures would provide both, insights on explosion mechanism as well as nucleosynthesis products. Our goal is to constrain the ejecta masses of $mathrm{^7Be}$ and $mathrm{^{22}Na}$ from classical novae through their $gamma$-ray line emissions at 478 and 1275 keV. We extract posterior distributions on the line fluxes from archival data of the INTEGRAL/SPI spectrometer telescope. We then use a Bayesian hierarchical model to link individual objects and diffuse emission and infer ejecta masses from the whole population of classical novae in the Galaxy. Individual novae are too dim to be detectable in soft $gamma$-rays, and the upper bounds on their flux and ejecta mass uncertainties cover several orders of magnitude. Within the framework of our hierarchical model, we can, nevertheless, infer tight upper bounds on the $mathrm{^{22}Na}$ ejecta masses, given all uncertainties from individual objects as well as diffuse emission, of $<2.0 times 10^{-7},mathrm{M_{odot}}$ (99.85th percentile). In the context of ONe nucleosynthesis, the $mathrm{^{22}Na}$ bounds are consistent with theoretical expectations, and exclude that most ONe novae happen on white dwarfs with masses around $1.35,mathrm{M_{odot}}$. The upper bounds from $mathrm{^{7}Be}$ are uninformative. From the combined ejecta mass estimate of $mathrm{^{22}Na}$ and its $beta^+$-decay, we infer a positron production rate of $<5.5 times 10^{42},mathrm{e^+,s^{-1}}$, which would make at most 10% of the total annihilation rate in the Milky Way.
The uncertainty in the 29P(p,gamma)30S reaction rate over the temperature range of 0.1 - 1.3 GK was previously determined to span ~4 orders of magnitude due to the uncertain location of two previously unobserved 3+ and 2+ resonances in the 4.7 - 4.8 MeV excitation region in 30S. Therefore, the abundances of silicon isotopes synthesized in novae, which are relevant for the identification of presolar grains of putative nova origin, were uncertain by a factor of 3. To investigate the level structure of 30S above the proton threshold (4394.9(7) keV), a charged-particle spectroscopy and an in-beam gamma-ray spectroscopy experiments were performed. Differential cross sections of the 32S(p,t)30S reaction were measured at 34.5 MeV. Distorted wave Born approximation calculations were performed to constrain the spin-parity assignments of the observed levels. An energy level scheme was deduced from gamma-gamma coincidence measurements using the 28Si(3He,n-gamma)30S reaction. Spin-parity assignments based on measurements of gamma-ray angular distributions and gamma-gamma directional correlation from oriented nuclei were made for most of the observed levels of 30S. As a result, the resonance energies corresponding to the excited states in 4.5 MeV - 6 MeV region, including the two astrophysically important states predicted previously, are measured with significantly better precision than before. The uncertainty in the rate of the 29P(p,gamma)30S reaction is substantially reduced over the temperature range of interest. Finally, the influence of this rate on the abundance ratios of silicon isotopes synthesized in novae are obtained via 1D hydrodynamic nova simulations.
Recurrent novae (RNe) are cataclysmic variables with two or more nova eruptions within a century. Classical novae (CNe) are similar systems with only one such eruption. Many of the so-called CNe are actually RNe for which only one eruption has been discovered. Since RNe are candidate Type Ia supernova progenitors, it is important to know whether there are enough in our galaxy to provide the supernova rate, and therefore to know how many RNe are masquerading as CNe. To quantify this, we collected all available information on the light curves and spectra of a Galactic, time-limited sample of 237 CNe and the 10 known RNe, as well as exhaustive discovery efficiency records. We recognize RNe as having (a) outburst amplitude smaller than 14.5 - 4.5 * log(t_3), (b) orbital period >0.6 days, (c) infrared colors of J-H > 0.7 mag and H-K > 0.1 mag, (d) FWHM of H-alpha > 2000 km/s, (e) high excitation lines, such as Fe X or He II near peak, (f) eruption light curves with a plateau, and (g) white dwarf mass greater than 1.2 M_solar. Using these criteria, we identify V1721 Aql, DE Cir, CP Cru, KT Eri, V838 Her, V2672 Oph, V4160 Sgr, V4643 Sgr, V4739 Sgr, and V477 Sct as strong RN candidates. We evaluate the RN fraction amongst the known CNe using three methods to get 24% +/- 4%, 12% +/- 3%, and 35% +/- 3%. With roughly a quarter of the 394 known Galactic novae actually being RNe, there should be approximately a hundred such systems masquerading as CNe.
We present a preliminary comparison of the post-nova population with that of general cataclysmic variables (CVs). We particularly focus on the mass-transfer rate and its potential relation to the nova eruption. We find that the known post-nova sample exclusively consists of high mass-transfer CVs, but that this is more likely to be due to the shorter recurrent time for those systems, rather than the mass-transfer rate being affected by the eruption. Nevertheless, we find evidence for such an effect for specific post-novae, and that it is potentially related to the binary separation and to presence or absence of an accretion disc.
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