Contour lines mark the core mass M_c (in units of Earth masses M _⊕ ), as function of the dimensionless mass unit and orbital radius r in AU. The red contour line indicates the assumed minimal mass of a gas-giant core. The horizontal red dashed lines indicate the expected initial seed core mass from the streaming instability after planetesimal formation by gravitational collapse, μ_SI, and the transition mass μ_t (see Sect. 3.2).