Zoom of the observed 0 km s^-1 feature in various CO lines (histograms). The results from the radex slab are shown as solid lines. To improve visibility, the brightnesses for C¹⁸O (both observations and models) are multiplied by 10. Vertical offsets are applied as follows: +5 K for CO 5–4, −5 K for C¹⁸O 2–1, −10 K for C¹⁸O 1–0 and −15 K for ¹³CO 1–0. The gray dashed line marks the approximate centroid velocity of the absorption feature at + 0.2 km s^-1. The model representations only include a component for the main envelope emission for 5–4 and 3–2, where a background is needed to absorb against. The 2–1 and 1–0 model lines only account for the foreground emission at ~0 km s^-1, hence the discrepancies at V_lsr ≲ −1 km s^-1 where the main envelope of AFGL 2591 dominates.