Top: impact of a lower column density, namely = 4 × 10²¹ cm⁻² (indicated with l. c.) instead of = 6 × 10²¹ cm⁻², on the ¹³CO(3–2)/¹³CO(2–1) ratio as a function the density. Bottom: impact of a lower FWHM (0.5 km s⁻¹ instead of 0.7 km s⁻¹) on the ¹³CO(3–2)/¹³CO(2–1) ratios as a function of the density. For both the width and the column density, their impact is plotted for 3 different temperatures: 10, 13 and 16 K. This shows that the impact of these two variables is smaller than the uncertainty related to the temperature.