Envelope mass M_XY as a function of core mass M_Z for in situ simulations. The green, blue, and red lines correspond to isolation masses of about 3, 5.5, and 13 M_⊕, respectively. For the latter two, different f_opa are shown, with smaller values of f_opa leading to larger M_XY in phase I. The two blue lines show f_opa = 0.003 and 0.1, while the three red lines show f_opa = 1, 0.1, and 0.01. In phase II, the f_opa lines converge. Thin black lines (partially covered) show Eq. (4). The thick gray line shows the crossover point where M_XY = M_Z.