Search for 7Be in the outbursts of four recent novae

Following the recent detection of ⁷Be II in the outburst spectra of classical novae, we report the search for this isotope in the outbursts of four recent bright novae by means of high-resolution Ultraviolet and Visual Echelle Spectrograph (UVES) observations. The ⁷Be II λλ313.0583, 313.1228 nm doublet resonance lines are detected in the high-velocity components of Nova Mus 2018 and ASASSN-18fv during outbursts. However, ⁷Be II is not detected in ASASSN-17hx and possibly not in Nova Cir 2018, which shows that ⁷Be is not always ejected in the thermonuclear runaway. Taking into account the ⁷Be decay, we find X(⁷Be)/X(H) ≈ 1.5 × 10^-5 and 2.2 × 10^-5 in Nova Mus 2018 and ASASSN-18fv, respectively. A value of ⁷Be/H ≈ 2 × 10^-5 is found in five out of the seven extant measurements, and it can be considered as a typical ⁷Be yield for novae. However, this value is almost one order of magnitude larger than predicted by current theoretical models. We argue that the variety of high ⁷Be/H abundances could be the result of a higher than solar content of ³He in the donor star. The cases with ⁷Be not detected might be related to the small mass of the white dwarf (WD) or to relatively little mixing with the core material of the WD. The ⁷Be/H, or ⁷Li/H, abundance is ≈ 4 dex above meteoritic abundance, thus confirming the novae as the main sources of ⁷Li in the Milky Way.