Velocity field vectors (arrows) indicating the expansion of the molecular envelope of τ⁴ Ser, which is defined by an edge-on oblate spheroid. View from the direction defined by west in the plane of the sky (the observer is on the left). The Y and Z axes are represented by green and blue, respectively. As in the case of AQ Sgr, the expansion is predominantly equatorial, with higher velocities in equatorial regions than those in the poles.