Pitch angles of the primary (ψ_P, left) and secondary (ψ_S, right) spiral arms measured at t = 8 kyr for models ﹩{\dot{M}}_{{\rm{in}}}=1{0}^{-8}\,{M}_{\odot }\,{{\rm{yr}}}^{-1}﹩ (blue), 10⁻⁷ M_⊙ yr⁻¹ (orange), and 10⁻⁶ M_⊙ yr⁻¹ (green). For the primary arm, the dashed curves show fits based on Equation (32) for the ﹩{\dot{M}}_{{\rm{in}}}=1{0}^{-8}\,{M}_{\odot }\,{{\rm{yr}}}^{-1}﹩ model and Equation (33) for the ﹩{\dot{M}}_{{\rm{in}}}=1{0}^{-7}﹩ and 10⁻⁶ M_⊙ yr⁻¹ models. For the secondary arm, the dotted–dashed lines plot the prediction of Equation (35) for the ﹩{\dot{M}}_{{\rm{in}}}=1{0}^{-8}\,{M}_{\odot }\,{{\rm{yr}}}^{-1}﹩ model, while constant pitch angles corresponding to logarithmic spirals are adopted for the ﹩{\dot{M}}_{{\rm{in}}}=1{0}^{-7}﹩ and 10⁻⁶ M_⊙ yr⁻¹ models.