High lithium abundance in the secondary of the black-hole binary system V404 Cygni

Abstract

THE maximum abundance of lithium in the oldest stars of the Galaxy is a factor of ten less than that of young stars and the local interstellar medium1, prompting an active search for sources of lithium. Type II supernovae, novae and accretion disks around black holes may be important sites of lithium production2,3. Stars in general are unlikely to produce lithium, as it is destroyed in their interiors through (p, α) reactions. Recently we showed that the transient X-ray source V404 Cyg is a low-mass X-ray binary with a black-hole primary4. Here we describe the discovery of lithium in the spectrum of V404 Cyg, as has also been noted independently by Wallerstein5 in our previously published spectra4. We derive a high lithium abundance in the secondary star of V404 Cyg, close to that of very young stars. Without lithium production, the secondary would have to be comparable in age to the Pleiades cluster (∼100Myr). If we are not seeing the system at an early moment in its lifetime, there must have been lithium production, which could be associated either with the supernova explosion that created it, or with the accretion disk now there. Further observations of this and similar systems may reveal that they are important sources of galactic lithium enrichment.