Leftover (i.e. ejected and/or accreted) mass fractions of hydrogen (X_surf) and helium (Y_surf) in the last time-step of the selected BPASS models, with contributions from different metallicities determined by the mean metallicity spread at z < 0.4 (Langer & Norman 2006). The colourbar indicates the number of events per 10⁶ M_⊙ of star formation, with contributions from different metallicities following the fiducial case as shown in Figure 2, along with the model weightings as defined in Section 2.