Comparison of the observed correlation of [C/O] − [O/H] for Galactic halo stars (open squares: Fabbian et al. 2009a) with the theoretical nucleosynthesis [C/O] ratio from massive stars as a function of the predicted [O/H]. The other symbols are the same as in Fig. 2. From each model, SN models that reside in the range of  −1 <  [C/O]  <  + 0.2 and  [O/H]  >  −3 are plotted. The [O/H] ratio is calculated from the ratio of the mass of O ejected from a SN to that of hydrogen swept by the SN remnant (see the text). For the theoretical model by Woosley & Weaver (1995), the IMF-weighted [C/O] ratio of 12 M_⊙ to 40 M_⊙ SN models is denoted by the dashed line.