Correspondence between the Hα evolution (left column; only H I lines are included in the displayed range) and the dynamical properties in the spectrum formation region (right column) for model ekin1p2_mdot0p01_rcsm6e14. The shading differentiates emission from the unshocked CSM (blue) and the shocked material (red), which corresponds here to the dense shell. The spectrum forms preferentially over regions in which log τ is between −1 and 1, except for the narrow-line core arising from regions at log τ of about −2. Here, V8 refers to the velocity in units of 10⁸ cm s⁻¹ and T4 is the gas (or electron) temperature in units of 10⁴ K (log ρ is the base-10 logarithm of the density, shown with an additive constant for better visibility).