Two-dimensional plot of the mass fraction at given density and pressure (top) and at given density and Mach number (bottom), before ionization is turned on in the simulation with a mean Mach number of one. Blue to black contours are increasing mass-fraction contours (blue 1E-6, green 1E-5, yellow 5E-5, orange 1E-4, red 5E-4, black 2E-3). The dashed-red line at 4.8 × 10⁴ cm^-3 is the maximum density achieved in a plane-parallel 1D simulation for a homogeneous density of 500 cm^-3.