CO vibrational ratio, v2∕v1, for differenttemperatures and columns from the RADEX models using a 750 K radiation field with a dilution factor W of 0.01 (left) and 0.3 (right). The area between the blue and white lines shows where both the vibrational ratio and the v1 flux of the low vibrational ratio sources are reproduced. For the v1 flux an emitting area with a radius of 5 AU is assumed. If a smaller emitting area is assumed the blue lines would shift in the direction of the blue arrows. High vibrational ratio sources can either be explained by gas with a high column (N ≳ 10¹⁸ cm⁻²) or a high temperature (T > 2000 K).