Maps of (a) the excitation temperature derived from the peak intensity of ¹²CO (J = 1–0), (b) the optical depth and (c) the column density of ¹³CO (J = 1–0), (d) the optical depth and (e) the column density of C¹⁸O(J = 1–0), and (f) the ratio of the fractional abundances of ¹³CO and C¹⁸O, |$X_{^{13}\mathrm{CO}}/X_{\mathrm{C}^{18}\mathrm{O}}$|⁠. (Color online)