The results of the chemical model with the different cosmic-ray ionization rates of ζ = 1.3 × 10⁻¹⁷, 5.2 × 10⁻¹⁷, and 1.3 × 10⁻¹⁶ s⁻¹, respectively, for Models A, B, and C. 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 dotted line represents the sum of the fractional abundances in the grain mantle and on the grain surface. The region where the model can reproduce the observed values within a factor of 10 is indicated by red and blue for CH₃OH and CH₃NH₂, respectively, and the overlapping region is painted by gray.