Ionization-energy density in the reference simulation. The ionization-energy density, ρ_e, ion, is shown at four different times in the orbital plane (top panels, a–d) and perpendicular to the orbital plane (bottom panels, e–h). The gray contours indicate the approximate position of the photosphere at a given time. As marked in the legend of panel (e), the red and green contours enclose a region where the number fraction of ionized hydrogen is between 0.2 and 0.9 and thus illustrate the region where hydrogen recombines. The purple and blue contours correspond to HeI and HeII number fractions of 0.2, i.e., helium recombines within these layers, close to the center of the expanding envelope. In panels (a)–(d), the current, unbound mass fractions as measured by the kinetic criterion are given (cf. Fig. 2).