Absorption and emission features of $^7$Be{\sc II} in the outburst spectra of V838 Her (Nova Her 1991)

Abstract

High and low resolution $IUE$ spectra of V838 Her in the early outburst stages exhibit a strong absorption feature shortward of $λ$3130. We discuss the nature of this spectral feature and provide convincing evidence that it corresponds to the blue-shifted resonance doublet of singly-ionized $^7$Be, recently discovered in other novae. During the evolution of the outburst the appearance of an emission feature close to $λ$3130 is also identified as $^7$Be II $λ$3132 because the usual identification as the O III $λ$3133.7 Bowen fluorescence line is hardly compatible with both the known oxygen under-abundance in the nova ejecta and the low optical depths in the nebula due to the high outflow velocity. The average $^7$Be abundance relative to hydrogen, estimated by four different methods, i.e. the $^7$Be II / Mg II absorption ratio, and the $^7$Be II / Mg II, $^7$Be II / He II$_{1640}$, and $^7$Be II / H$β$ emission ratios is N(Be)/N(H) $\approx$ 2.5 $\times 10^{-5}$ (by number), i.e. $\approx$ 1.7 $\times 10^{-4}$ by mass. This corresponds to an overproduction of $^7$Be by about 1 dex in comparison with the theoretical models of massive CO and ONe novae. Since $^7$Be all converts into $^7$Li, the $^7$Be/H abundance implies a $^7$Li/H overabundance of about 4 dex over the $^7$/H meteoritic value and indicates a total ejected mass of $^7$Li of $\approx$ 9.5$\times$10$^{-10}$ $M_{\odot}$. These data are in line with previous observations and indicate that large amounts of $^7$Li can be synthesized in a variety of novae, including very fast ONe novae.

MNRAS accepted, 13 pages, 6 figures