Diagram of the occurrence of temperature as function of mass density in the simulation. The red contours show the fraction of hydrogen atoms bound in H₂ molecules, 2n_H2/(n_H   i + 2n_H2), as function of temperature and mass density, assuming ICE and all hydrogen neutral. The contours are spaced a factor 100 apart, with the labels indicating the exponent a in 10^a. The blue line at T = 1.66 kK specifies the threshold for the ad hoc heating (Eq. (15)).