Radiative Shell Formation in Supernova Remnants Is Insensitive to Dissolution Heating

A supernova remnant's evolution is timed by its ambient medium: the blast wave coasts through the Sedov-Taylor phase until the shocked gas can radiate its energy, whereupon a cold dense shell forms and the remnant's dynamics change character. A uniform dissolution heating term, by warming the ambient medium before the shock arrives, would lower shock Mach numbers and could in principle delay shell formation. This paper computes the size of that effect for a 1e51 erg remnant in ambient densities of 0.1 to 10 per cubic centimetre, using the Cioffi, McKee and Bertschinger shell-formation scaling (3.60e4 years at n = 1, with shell radius 20.9 pc and shock speed 227 km/s) and ambient thermal balance including the term Q = f u_L / t_H fed by the surviving vacuum density u_L = 5.25e-09 erg per cubic centimetre. The result is a clean negative. Even at f = 1 — the entire surviving vacuum density handed to the gas once per Hubble time — the ambient temperature of 0.1-per-cubic-centimetre gas rises only from 8,000 K to 12,080 K, the Mach number at shell formation in a Cygnus-Loop-like remnant (ambient density 0.3) falls from 22 only to 21, and driving the Mach number down to the weak-shock value of 3 would require f = 548, an unphysical coupling exceeding unity by nearly three orders of magnitude. At the forest-permitted coupling f below 5.6e-11 the effect vanishes entirely. Supernova remnants keep standard time in a dissolving-vacuum universe; they are a probe that this coupling cannot move. Computed from stated assumptions; not a measurement.