Model ¹³CO J=1–0 brightness temperatures as a function of H₂ column density for volume densities 100, 316, 1000 cm⁻³, gas kinetic temperatures 10, 30, 100 K, [H₂/¹²CO]=10⁴, and a line width of 11 km s⁻¹. We assume a ¹²C to ¹³C abundance ratio of 13.25 (Jacob et al. 2020). The horizontal black line marks the 3σ threshold for the observed peak ¹³CO brightness temperature from which upper limits on the H₂ column density for each model are estimated.