gas is intercloud or cloud gas with $n < 2.5$ cm$^{-3}$ as the dividing line. All the runs plotted here have $C$ set to 10. \[fig:tevap\]](tevap-2){width="49.00000%"}

![Evaporation timescale factor, $\tau$, of WL vs. remnant age. In this case we use the temperature criterion to differentiate between cloud and intercloud gas with $T > 10^5$ K as the dividing line. This method ensures that the X-ray emitting gas mass (though not the luminosity) for the corresponding WL model matches that for the simulated remnant. The $\tau$ values calculated this way are substantially below those in Figure \[fig:tevap\], primarily because of hot, but still dense, gas generated by shocking clouds close to the explosion center. Such gas is counted as cloud gas (and thus not evaporated) by the method of the previous figure, but should radiate X-rays. \[fig:tau\_eff\]](tau_xray){width="49.00000%"}

Another approach is to calculate the mass of X-ray emitting gas, essentially all gas in the remnant that is hotter than 10$^5$ K, and match that with the X-ray emitting mass, $M_x$, for a given WL model. That approach leads to values of $\tau$ that are considerably lower than for the density based criterion, mostly because of the production of hot, dense gas via 