Schematic view of the geometry and main parameters of our model of the ionized core of M 2-9 (Sect. 6.1 and Table 3). Top-left: 3D view indicating the four major structural components in our model, namely: nucleus (I), inner wind (II), high-velocity spots/shells (HVS, III), outer wind (IV). Colour scale represents density as indicated in the right wedge; arrows represent the velocity field (colour indicate blue or red Doppler shift with respect to the line of sight). The line of sight runs along the z-axis. Top-right: 2D view through the x=0 offset. Bottom panels show the radial distribution of the electron density (left) and radial expansion velocity modulus (right) adopted in our model. Vertical dashed lines indicate the boundaries of regions I-IV.