Growth timescale t_growth for a pebble of radius s = 30 cm as function of planetesimal radius R at 3 AU. The black dashed curve shows the default model (as in Fig. 4). The other curves consecutively give the effects of a colder disk resulting in a headwind of v_hw = 28 cm s^-1 (blue dashed line) and a threefold increase in gas density (green curve) with a −0.5 power-law index for the surface density (Eq. (3)). The crosses represent numerical results and the solid (and dashed) lines represent fits.