Distribution of ¹²C/¹³C ratios from 93 sources in the Milky Way. The background image is the structure of the Milky Way from the artist's impression [Credit: NASA/JPL-Caltech/ESO/R. Hurt]. The ¹²C/¹³C isotope ratios with corrections for optical depth from C³⁴S/¹³C³⁴S in this work are plotted as circles. The triangles, pentagons, stars, squares, and diamonds indicate the ¹²C/¹³C ratios derived from CN/¹³CN (Savage et al. 2002; Milam et al. 2005), C¹⁸O/¹³C¹⁸O (Langer & Penzias 1990; Wouterloot & Brand 1996; Keene et al. 1998), (Henkel et al. 1980, 1982, 1983, 1985; Yan et al. 2019), CH⁺/¹³CH⁺ (Ritchey et al. 2011), and CH/¹³CH (Jacob et al. 2020), respectively. The red symbol ⊙ indicates the position of the Sun. The colorbar on the right-hand side indicates the range of the ¹²C/¹³C ratios.