Relationship between planetesimal radius and gas surface density in a circumplanetary disk for a given value of the non-dimensional gas drag parameter ζ. The cases with ζ = 10 ⁻⁴, 3 × 10 ⁻⁷, and 3 × 10 ⁻⁹ are shown. Lower and upper horizontal axes show the gas surface density at r = r _d (the radial location roughly corresponding to the size of the circumplanetary disk and assumed to be 0.2 r _H ) and r = 10 ⁻² r _H (the vicinity of the radial locations of the Galilean satellites), respectively ( C _D = 1 and ρ _s = 1 g cm ⁻³). Since we assume Σ∝ r ^−3/2, Σ(10 ⁻² r _H )/Σ _d = 20 ^1.5 ≈ 89.