Model predictions of methanol and acetaldehyde after a shock passage. Upper panel: CH₃ OH/CH₃CHO abundance ratio: solid line refers to a model where acetaldehyde is synthesised in the gas phase, whereas the dashed line refers to predictions assuming that CH₃ OH is injected into the gas phase directly from the grain mantles. The hatched green zone is defined by the value (including uncertainty) measured towards the B1b peak, with a kinematical age of ≃ 1100 yr (Podio et al. 2016, revised according to the new distance of 352 pc by Zucker et al. 2019). Lower panel: acetaldehyde (CH₃ CHO, red) and methanol (CH₃OH, blue) abundances with respect to H nuclei as a function of time from the passage of the shock. The hatched blue and red regions show the measurements towards B1b (see text).