Population diagrams of CO v = 2–1 based on the spectra extracted at the incremental distances from the central star. The upper and the lower panels are from the spectra recorded with the slit PA = 145° and 325°. Open circles and filled triangles are for R- and P-branches, respectively. The column densities divided by the statistical weights are vertically shifted by arbitrarily amount for the presentation. The uncertainty of the individual column density is on the order of the dispersions of the data points. The red lines are the best-fit LTE model population diagrams assuming Gaussian line width of 6 km s^-1. The blue lines are the same but the column density of CO is fixed to N_CO = 10²⁰ cm^-2.