The results of Model B (the standard model). (a) The simulated fractional abundances, X, of CH₃OH, CH₃NH₂, and CH₂NH in the gas phase are shown by red, blue, and green solid lines, respectively. The vertical axis represent the ages of the core since the beginning of warm-up. We present the CH₃NH₂ abundance predicted by Model B’, where we switch off the series of hydrogenation processes of HCN and CH₂NH to form CH₃NH₂, by the purple line. The dotted line represent the sum of the fractional abundances in the grain mantles and on the grain surfaces. (b) The gas-phase CH₃NH₂/CH₃OH ratio. With horizontal lines, we compare the observed CH₃NH₂/CH₃OH ratio of our sources. The abundance ratios are calculated from Table 5.