 ![Plot of plasma temperature $kT_e$ vs. ionization timescale $\tau=n_e t$ for 00 Cas A regions. The upper panel shows the full range of values, and the lower panel zooms in on the cluster of values. Three regions, R0, R00 and R00, show different properties from the rest. Region R0 is represented with two points, representing the mean value (lower left corner in the upper panel) and the upper limits (lower right corner in the upper panel).[]{data-label="fig-kT-tau"}](f00