Measured wind velocities (as determined from the half-line width v_line) versus half of the LAS (representing the not-deconvolved largest detectable scale of each transition) in the left panel and half of the spatial FWHM s_FWHM (representing the dominant line formation region) in the right panel. Note that the value for the LAS is restricted by the sensitivity limit of the observations and the fact that large-scale flux might be resolved out (see Sect. 2.2). The blended lines are indicated with a squared box, the unblended lines with a cross (and include the error bars). In the right panel, the full black line shows the velocity structure as derived by Decin et al. (2010a) from solving the momentum equation; the dashed line indicates the velocity structure as derived by Fonfría et al. (2008) from modelling different J ro-vibrational transitions of C₂H₂, the dashed-dotted line the velocity structure derived by Keady et al. (1988) from the analysis of the near-infrared ro-vibrational CO spectrum, and the thick grey lines the acceleration derived from the ALMA data. The vertical dotted line indicates the dust nucleation region around 5.6 R_⋆, where the sound velocity is ~3 km s^-1.