Velocity of the forward shock as a function of explosion energy calculated using the theoretical model of Truelove & McKee (1999) for a shock expanding into a uniform medium with density ρ₀, and assuming a power law density profile for the ejecta with power law index n = 7. The upper (lower) panel refers to a SNR radius R_SNR = 3.6 pc (R_SNR = 3.2 pc). The thick curve is obtained matching the observational constraints derived from the remnant radius (3.6 pc for the upper panel and 3.2 pc for the lower panel) and age (400 ± 50 yr). The mass of the ejecta is fixed at M_ej = 1.4 M_⊙. The hatched area shows the results allowing for the uncertainty in the age of the remnant (the lower side corresponding to 450 yr while the upper to 350 yr). The filled diamond shows the shock velocity calculated using the best fit parameters obtained by Badenes et al. (2008); i.e., E_SN = 1.4 × 10⁵¹ erg and ρ₀ = 10^-24 g cm^-3.