The Black Hole Binary Nova Scorpii 1994 (GRO J1655-40): An improved chemical analysis

TL;DR

This study presents a detailed chemical abundance analysis of the secondary star in Nova Scorpii 1994, revealing element enhancements and constraining the black hole's formation process through supernova modeling.

Contribution

It provides the first detailed abundance analysis using high-resolution spectra and compares results with supernova models to infer the black hole's formation mechanism.

Findings

Enhanced O, Mg, Si, S abundances compared to Galactic trends

Most elements have solar-like abundances, except for certain alpha-elements

Supernova models suggest a hypernova origin with specific explosion parameters

Abstract

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. Following the discovery of overabundances of $\alpha$-elements in the HIRES/Keck spectrum of the secondary star of Nova Scorpii 1994 (Israelian et al. 1999), we obtained UVES/VLT high-resolution spectroscopy with the aim of performing a detailed abundance analysis of this secondary star. Using a $\chi2$-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.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. Most probably the black hole in this system formed in a hypernova explosion of a 30--35 \Msun progenitor star with a mass-cut in the range 2--3.5 \Msun. However, these models produce abundances of Al and Na almost ten times higher than the observed values.