Schematic figure of our model of the expanding shell (gray open circle) to illustrate Eqs. (2) and (3). The expansion (gray arrows) is assumed to be spherical and centered at the position of XZ Tau, labelled as a red star. R_sh is the radius of the expanding shell. θ is the positional offset with respect to XZ Tau on the plane of the sky. ζ is the angle between the radial direction and the line of sight.