Cassiopeia A: An Expansion and Energy Budget Test of the Sedov Description at 350 Years

This paper computes the energy, density and acceleration budgets of Cassiopeia A from a small set of adopted, published input values, using the relations stated in the Methods and a deposited script that reproduces every number. Inserting the adopted radius and age gives a Sedov shock speed of 2.83e+03 km/s against a free-expansion speed of 7.07e+03 km/s, so the deceleration parameter is 0.4: the shock has slowed appreciably but is nowhere near the momentum-conserving regime. The implied explosion energy is 0.254 in units of 10^51 erg, a factor 0.127 of the value near 2 x 10^51 erg commonly derived from ejecta modelling, which is the expected direction for a remnant whose reverse shock has not yet reached the centre. The calculation is a computation from stated assumptions, not a measurement, and each headline result is reported together with the diagnostic that tests whether the assumed evolutionary phase applies to this object.