Critical density n_crit vs. the belt width ΔR. Lines represent the critical density n_crit at which the gas transition from a disk to a wind vs. ΔR. The solid line is for a model where n_crit is given by Eq. (5) and assumes that protons collide with ions, C or O. The dashed line is for collisions between protons and CO. We indicate in capital letters whether the gas would be more DISK- or WIND-like in different regions of the plot. We note that under the black lines, winds are very likely to behave as stated in the main text. The blue points show the gas density n_d estimated for the different systems shown in the plot. A vertical line joins two blue dots when a range of values are given in the literature for the gas densities or masses. Data are given in Table H.1.