Mantle CH₃OH/CO abundance ratio versus time. The red lines refer to Model 1 (E_a = 400 K and E_d/E_b = 0.65), the blue lines refer to Model 2 (E_a = 1450 K and E_d/E_b = 0.5) (see text). Solid, dotted and dashed lines refer to densities 10⁴, 10⁵ and 10⁶ cm^-3 respectively. The grey box with hatching shows the interval of CH₃OH/CO abundance ratio observed towards high- and intermediate- mass protostars, while the green dashed box shows the values (upper limits) observed in low-mass protostars (Table 5).