Large Velocity Gradient (LVG) model fit to the ¹²CO SLED of HerBS-89a (after correcting for lens magnification). Top panel: data are shown as black dots and the solid curve shows the best fit from the single component (MCMC) model corresponding to the maximum posterior possibility (see Sect. 6.3 for details). Bottom panel: two-components fit, including an additional cold and extended component (darker, dot-dashed line).