Sketch of the three thermodynamic zones of an envelope during its formation in a gaseous disk. Radiation dominates heat transport outside the radiative-convective boundary, while the interior is convectively unstable. The two outer regions are composed of nebular gas. In contrast, the inner high-Z region is formed by vapor from impactors and consists entirely of high-Z material until it becomes sufficiently large to mix with significant fractions of hydrogen and helium from the outer layers (see Sect. 2.3.2). We simplify the steep compositional gradient to a step-function at r_vap with a continuous temperature.