Composite figure showing one snapshot taken at maximum 5007 Å brightness along each of the four 0.625 M_⊙ sequences (α = 2), with the metallicities indicated over the panels. Details of the models are listed in Table 3. Top panels: structures of the models: run of ion density (thick solid line), flow velocity (thin solid line), and electron temperature (dotted line) with radius. Densities and temperatures are normalised and must be multiplied with the factors given in each panel. The radial positions, R_out, of the nebular outer shock and of the rim, R_rim are also marked in each panel. The central star is at r = 0 cm. The electron temperature spikes seen at larger radii are caused by the ionisation fronts, and the jumps at the inner nebular edges mark the positions of the contact discontinuity/conduction fronts. Middle panels: normalised spatially resolved [O iii] 5007 Å and thermally broadened line profiles (dotted line) as seen through a central numerical aperture with a diameter of 1  ×  10¹⁶ cm (corresponding to 0″̣67 at 1 kpc), and additionally broadened by a Gaussian of 10 km s^-1 FWHM (solid line) in order to simulate a typical instrumental broadening. The maximum velocity behind the outer shock at R_out, V_post, is indicated by vertical dashed lines. Bottom panels: [O iii] 5007 Å line profiles as integrated over the whole nebula (dotted line), and additionally broadened by a Gaussian of 10 km s^-1 FWHM (solid line). Two vertical dashed lines in each panel indicate the HWHM and the shock velocity, Ṙ_out, respectively.