Search for <SUP>7</SUP>Be in the outbursts of four recent novae

Following the recent detection of <SUP>7</SUP>Be 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 <SUP>7</SUP>Be 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, <SUP>7</SUP>Be II is not detected in ASASSN-17hx and possibly not in Nova Cir 2018, which shows that <SUP>7</SUP>Be is not always ejected in the thermonuclear runaway. Taking into account the <SUP>7</SUP>Be decay, we find X(<SUP>7</SUP>Be)/X(H) ≈ 1.5 × 10<SUP>-5</SUP> and 2.2 × 10<SUP>-5</SUP> in Nova Mus 2018 and ASASSN-18fv, respectively. A value of <SUP>7</SUP>Be/H ≈ 2 × 10<SUP>-5</SUP> is found in five out of the seven extant measurements, and it can be considered as a typical <SUP>7</SUP>Be 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 <SUP>7</SUP>Be/H abundances could be the result of a higher than solar content of <SUP>3</SUP>He in the donor star. The cases with <SUP>7</SUP>Be 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 <SUP>7</SUP>Be/H, or <SUP>7</SUP>Li/H, abundance is ≈ 4 dex above meteoritic abundance, thus confirming the novae as the main sources of <SUP>7</SUP>Li in the Milky Way.