Top panel: location of the opacity transition (solid line) as a function of the value for the α viscous stress parameter and for our fiducial disc model. The dashed line shows the location where the viscous timescale t_dif becomes equal to half of the horseshoe libration timescale (t_lib) whereas the solid-dashed line shows the radius where t_dif equals the U-turn timescale. The intersection between the shaded area and the solid line corresponds to the range of radii where convergent migration for 3  M_⊕ bodies is expected to occur. Lower panel: same but for a disc model ten times more massive than the MMSN.