Core growth as function of time, for two values of the initial gas surface density β₀, which has been altered by a factor of two from the standard value used here (β₀ = 500 g cm^-2). Cores are placed on the same orbits as in Fig. 3, and similar labeling is used. The grey lines give the evolution in a disc with a temporally constant gas surface density profile, corresponding to Eq. (35).