ratio in the ices as function of the gas and dust temperature for n_H = 10⁴ (dashed), 10⁵ (dotted), and 10⁶ cm³ (solid) as modeled by Bøgelund et al. (2018) and Taquet et al. (2019) using the GRAINOBLE model (Taquet et al. 2012, 2013, 2014). The model results are shown for 0.1 (left), 1 (middle), and 10 (right) times free-fall timescales t_FF. The observed average ratios are indicated in blue and orange for the low-mass and high-mass protostars, respectively. In the right panel, the range of possible desorption temperature of CO ice is indicated with the gray bars for binding energies between 863 K and 1307 K for CO ice deposited on non-porous amorphous solid water (Noble et al. 2012).