An evidence-based assumption that helps to reduce the discrepancy between the observed and predicted 7Be abundances in novae

Abstract

ABSTRACT Recent spectroscopic measurements of the equivalent widths of the resonant Be ii doublet and Ca ii K lines and their ratios in expanding nova ejecta indicate surprisingly high abundances of 7Be with a typical mass fraction Xobs(7Be) = 10−4. This is an order of magnitude larger than theoretically predicted values of Xtheor(7Be) ∼ 10−5 for novae. We use an analytical solution of the 7Be production equations to demonstrate that Xtheor(7Be) is proportional to the 4He mass fraction Y in the nova accreted envelope and then we perform computations of 1D hydrostatic evolution of the $1.15\, \mathrm{M}_\odot$ CO nova model that confirm our conclusion based on the analytical solution. Our assumption of enhanced 4He abundances helps to reduce, although not completely eliminate, the discrepancy between Xobs(7Be) and Xtheor(7Be). It is supported by ultraviolet, optical, and infrared spectroscopy data that reveal unusually high values of Y in nova ejecta. We also show that a significantly increased abundance of 3He in nova accreted envelopes does not lead to higher values of Xtheor(7Be) because this assumption affects the evolution of nova models resulting in a decrease of both their peak temperatures and accreted masses and, as a consequence, in a reduced production of 7Be.