Distribution of the ¹³CO peak line intensities. Full black triangles correspond to ¹³CO(6–5), (8–7) and (10–9) observed intensities. The circle represents the observed C¹⁸O(3–2) flux, the empty black triangle the flux of the C¹⁸O(3–2) line multiplied by X_{¹³CO/C¹⁸O} ~ 8. The red triangles are the best model fit results. The error bars include only calibration uncertainties. The dashed lines represent the best fit ¹³CO ladder.