Escape velocity and mass from the surface as a function of the radial coordinate in the envelope of the progenitor of AT 2018bwo derived from a binary evolution sequence of a 15 M_⊙ donor (initial mass) and a 3 M_⊙ accretor (see Sect. 4.3). The selected envelope profile corresponds to when the donor star was consistent with the location of the AT 2018bwo progenitor in the HR diagram, at which point its total mass was ∼13.1 M_⊙, radius ∼107 R_⊙, and the mass transfer rate ≈ 6 × 10⁻² M_⊙ yr⁻¹.