Highly enriched 7Be in the ejecta of Nova Sagittarii 2015 No. 2 (V5668 Sgr) and the Galactic 7Li origin

Abstract

We report on the evidence of highly blueshifted resonance lines of the singly ionized isotope of <SUP>7</SUP>Be II in high resolution UVES spectra of Nova Sagittarii 2015 No. 2 (V5668 Sgr). The resonance doublet lines <SUP>7</SUP>Be II at λλ313.0583, 313.1228 nm are clearly detected in several non-saturated and partially resolved high velocity components during the evolution of the outburst. The total absorption identified with Be II has an equivalent width much larger than all other elements and comparable to hydrogen. We estimate an atomic fraction N(<SUP>7</SUP>Be)/N(Ca) ≈ 53-69 from unsaturated and resolved absorption components. The detection of <SUP>7</SUP>Be in several high velocity components shows that <SUP>7</SUP>Be has been freshly created in a thermonuclear runaway via the reaction <SUP>3</SUP>He(α, γ)<SUP>7</SUP>Be during the Nova explosion, as postulated by Arnould & Norgaar, however in much larger amounts than predicted by current models. <SUP>7</SUP>Be II decays to <SUP>7</SUP>Li II with a half-life of 53.22 d, comparable to the temporal span covered by the observations. The non-detection of <SUP>7</SUP>Li I requires that <SUP>7</SUP>Li remains ionized throughout our observations. The massive Be II ejecta result into a <SUP>7</SUP>Li production that is ≈ 4.7-4.9 dex above the meteoritic abundance. If such a high production is common even in a small fraction (≈5 per cent) of Novae, they can make all the stellar <SUP>7</SUP>Li of the Milky Way.