¹²C/¹³C isotope ratios from observations in this work and GCE models. The red symbol ⊙ indicates the ¹²C/¹³C isotope ratio of the Sun. The ¹²C/¹³C gradient obtained from C³⁴S in the current work is plotted as a black solid line, with the gray shaded area showing the 1 σ interval of the fit. The red crosses visualize the results from the GCE model of Kobayashi et al. (2011, 2020, see also Sect. 4.9). The dark green, light green, magenta, and pink lines refer to the predicted gradients from models in Table 2 from Colzi et al. (2022).