Measured C¹⁸O J = 2−1 extent versus current bolometric luminosity. The blue symbols show models in quiescent state (no burst) with protostellar luminosities L_∗ of 1, 10 and 100L_⊙. The red symbols are for strong burst models with burst luminosities of L_{∗ ,B} = 100 × L_∗. The orange symbols show weak burst models with L_{∗ ,B} = 10 × L_∗. For both groups of burst models the results in the post-burst phase are shown. The edges of the symbols indicate the time past since the end of the burst of 1000 yr (black edge), 3000 yr (no edge) and 10 000 yr (grey edge). The different shapes of the symbols indicate different inclinations of 0° (square), 45° (diamond) and 90° (thin diamond). For all models a beam size of 1.81″ × 1.65″ was used, except for the models indicated by the dark blue symbols where a beam size of 0.5″ × 0.5″ (high res.) was used. The grey solid and dashed lines are the model results from Jørgensen et al. (2015) for a 1D spherical model of a Class 0 protostellar envelope with varying protostellar luminosities.