Surface composition at the onset of core-collapse for the PPI+CC and CC models. ⁴He, ¹²C, ¹⁶O are shown as red filled circles, cyan triangles, and blue crosses, respectively. The dot-dashed horizontal lines of the same color mark the initial mass fraction for these elements for our Z = 0.001 model grid. The colored background indicates the approximate evolution of the star (cf. Fig. 2 and Sect. 3).