Four different CO ro-vibrational transitions, from top left to bottom right: v(1–0)R20, v(2–1)P1, v(2–1)R20, and v(3–2)R20. In each of the three panels, the optical depth, τ, of the line (blue) and the continuum (black) (upper panel), the radial cumulative line flux (middle panel), and the CO density (bottom panel) are shown. The vertical dashed lines through all three panels show the radial region where 70% of the line flux originates (calculated from vertical escape probability).