Evolutionary tracks computed at solar metallicity (Z = 0.02) and selected to satisfy the inferred temperature and luminosity of progenitor as described in Sect. 5 and delineated by the box. Left panel: single star models from the STARS code. 9−11 M_⊙ models matched the progenitor properties at its endpoint. The 8 M_⊙ and 12 M_⊙ tracks are shown for comparison. The inset shows a zoom-in of a finer grid of models with masses as indicated in the legend. Right panel: evolutionary tracks resulting from binary evolution computed using the BPASS code. In total, 253 tracks matched the progenitor properties. These had primary stars in the mass range 6−10 M_⊙, and 50% had a 10 M_⊙ primary. Shown here is a 10 M_⊙ primary with a 9 M_⊙ secondary on a long period orbit (∼4000 d) with little interaction (blue) and an early merger between stars in a small orbit(∼10 d), which initially consisted of a 6 M_⊙ primary and 4.8 M_⊙ secondary resulting in a star with a mass between 10 M_⊙ and 11 M_⊙ (red). The red star indicates the position on the isochrone at which the merger was completed.