The $${}^{\mathbf{7}}$$Be Abundance in Nova V5668 Sgr Is Consistent with the Theory

Abstract

By now the resonance lines of the isotope $${}^{7}$$ Be have been detected in five novae. The known estimates of the relative abundance of this isotope from the equivalent widths of $${}^{7}$$ Be II and Ca II lines lead to the conclusion that the $${}^{7}$$ Be abundance is considerably higher than the predictions of thermonuclear burning models during a nova outburst. In an attempt to understand the reason for the discrepancy, we have investigated the possibility of an increase in the $${}^{7}$$ Be yield during nova outbursts based on nuclear burning kinetics within the framework of a two-zone model and shown that the mass fraction of this isotope does not exceed $$3\times 10^{-5}$$ even for the most favorable set of parameters. This agrees with the known theoretical results and leaves the contradiction between the theory and observations unresolved. The reason for the contradiction is shown to be the assumption that the relative fractions of the ions Be II/Be and Ca II/Ca are equal, which was made when determining the $${}^{7}$$ Be abundance. For Nova V5668 Sgr the relative fraction of the Be II ions in the ejecta turns out to be higher than the relative fraction of the Ca II ions at least by an order of magnitude because of the difference between the ionization potentials. Our new estimate of the $${}^{7}$$ Be mass fraction is consistent with the theory. The calculated gamma-ray flux at 478 keV from the $${}^{7}$$ Be decay agrees with the upper limit from INTEGRAL satellite observations.