7Be abundance in nova V5668 Sgr doesn't contradict theory
Abstract
Resonance lines of 7Be are detected currently in five novae. Available abundances for this isotope estimated from equivalent widths of 7Be\,II and Ca\,II lines are significantly higher compared to predictions of models for the thermonuclear flash. In attempt to pinpoint the reason for this disparity we explore the possibility for the higher 7Be yield via computing kinetics of the thermonuclear burning in the framework of two-zone model and find that even for a favorable choice of parameters 7Be mass fraction does not exceed 3·10-5. This is consistent with known theoretical results and leaves the disparity between the theory and observations unresoved. We find that the contradiction is caused by the assumption that the ionization fraction of Be\,II/Be is equal to that of Ca\,II/Ca, which has been adopted formerly in order to estimate the 7Be abundance. In the case of nova V5668 Sgr the ionization fraction of Be\,II/Be turns out to be at least by a factor of 10 higher compared to Ca\,II/Ca due to the difference of ionization potentials. Our new estimate of the 7Be mass fraction for nova V5668 Sgr does not contradict the theory. The calculated flux of 478 keV gamma-quanta from the 7Be decay is consistent with the upper limit according to INTEGRAL observations.