¹³CO (J = 2–1)/C¹⁸O (J = 2–1) line intensity ratio calculated with RADEX for a range of temperatures (different colors) and CO column densities. Isotope ratios of [¹²C]/[¹³C] = 77 and [¹⁶O]/[¹⁸O] = 560 are adopted. In the left panel (right panel), open circles indicate that the ¹³CO (C¹⁸O) emission is optically thin (i.e., τ < 1), whereas filled circles are used when the ¹³CO (C¹⁸O) emission is optically thick (i.e., τ > 1). Values below the dotted line are observed toward the L1527 midplane.