a: on-axis spectrum and transverse PV diagram for the reference model viewed at i = 87°, illustratingthe “edge-on” case. The two red dots indicate the intensity peaks in the PV, which have opposite velocity signs in this configuration; their connecting line defines the spatial and velocity separations Δr and ΔV used to estimate the observed specific angular momentum j_obs in Eq. (15). We also plot in black the ellipse and peak positions contributed by the outermost streamline alone, which predict a similar velocity shift ΔV_th (see Eq. (13)) but a much larger spatial shift Δr_th (see Eq. (12)). Other model parameters are: z_cut = 225 au, M_* = 0.1 M_⊙, r_in = 0.25 au, r_out = 8 au, α = − 2, and θ_b = 225 au. Filled rectangles show the spectral and angular resolutions. b: same as (a) but at lower inclination i = 70° (note the change in velocity scale) for α = 0 and θ_b = 225 au (red) and 45 au (blue). Double-peaks in the PV diagram now have the same velocity sign, and can vanish at moderate angular resolution.