Comparison of the absorption line profiles of CO v = 2–0 for the lines starting from low J [R(1) and P(3)] and high J [R(21) and R(23)]. The velocityrange of the gas toward CRL 2136 IRS 1 is marked by dotted vertical lines. Two velocity components centered at 23 km s⁻¹ and 27 km s⁻¹ are apparent. The low-velocity component at 23 km s⁻¹ is fitted by aGaussian function, and is separated to apply the curve of growth analysis. The residual absorption is shown as the light blue shaded area.