Evolution of CNO abundance ratios as a function of oxygen abundance for single starburst models with different IMFs, based on the framework of galactic chemical evolution models from Kobayashi et al. (2020). The symbols are as in Fig. 11, with GS-z9-0 in green and other high-z objects observed by JWST from the literature highlighted in yellow. Shaded regions in the C/O vs. O/H and C/N vs. O/H panels mark the range allowed by pure CCSNe based on the yields from Tominaga et al. (2007) and Watanabe et al. (2024), respectively. The blue model assumes a standard IMF (with high-mass end slope x = 1.3), whereas the magenta and red curves assume a top-heavy IMF (with flat slope) for massive stars in the range 30–120 M_⊙ and 100–280 M_⊙, respectively, with the latter including PISNe. For each model, the curve reaches the observed O/H in GS-z9-0 after 4.05 Myr. The combined CNO pattern of GS-z9-0 is better matched by models with underlying top-heavy IMF. In order to reproduce the abundance patterns of GS-z9-0 with a standard IMF, a more tailored double-burst model (with two bursts separated by 100 Myr, dashed orange line) is needed, similar to that proposed by Kobayashi & Ferrara (2024) to explain the nitrogen enhancement in GN-z11.