First panel: kinetic energy per unit line as estimated using the same molecular transitions and line colours as shown in Fig. 13. Also plotted is (which is proportional to the potential energy per unit line Ω_line) calculated using Herschel (black solid line) and ¹³CO (1–0) (black dashed line). Second panel: virial ratio as estimated using the same three molecular transitions. The grey shaded region highlights where the gas has a virial ratio between 0.5 and 2. Third panel: escape velocity as estimated using M_line-dust (solid line) and M_line-13CO (dashed line). The purple shaded region indicates the thermal sound speed of ionised gas at T_e = 5600–7600 K. Fourth panel: mass-per-unit-length that can escape the potential calculated using the same three molecular transitions.