The Black Hole Binary Nova Scorpii 1994 ( GRO J 1655 - 40 ) : An improved chemical analysis ⋆

Context. The chemical analysis of secondary stars of low mass X-ray binaries provides an opportunity to study the formation processes of compact objects, either black holes or neutron stars. Aims. Following the discovery of overabundances of α-elements in the Keck I/HIRES spectrum of the secondary star of Nova Scorpii 1994 (Israelian et al. 1999), we obtained VLT/UVES high-resolution spectroscopy with the aim of performing a detailed abundance analysis of this secondary star. Methods. Using a χ2-minimization procedure and a grid of synthetic spectra, we derive the stellar parameters and atmospheric abundances of O, Mg, Al, Ca, Ti, Fe and Ni, using a new UVES spectrum and the HIRES spectrum. Results. The abundances of Al, Ca, Ti, Fe and Ni seem to be consistent with solar values, whereas Na, and especially O, Mg, Si and S are significantly enhanced in comparison with Galactic trends of these elements. A comparison with spherically and non-spherically symmetric supernova explosion models may provide stringent constraints to the model parameters as mass-cut and the explosion energy, in particular from the relative abundances of Si, S, Ca, Ti, Fe and Ni. Conclusions. Most probably the black hole in this system formed in a hypernova explosion of a 30–35 M ⊙ progenitor star with a mass-cut in the range 2–3.5 M ⊙. However, these models produce abundances of Al and Na almost ten times higher than the observed values.