2021MNRAS.501L..33D

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 X_obs(⁷Be) = 10^–4. This is an order of magnitude larger than theoretically predicted values of X_theor(⁷Be) ∼ 10^–5 for novae. We use an analytical solution of the ⁷Be production equations to demonstrate that X_theor(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 M_☉ 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 X_obs(⁷Be) and X_theor(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 X_theor(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.