Intersection of the surfaces of equal line-of-sight (LOS) velocity, v_z, for a collapsing protostellar core, with the plane (p, z) (see Sect. 2). The LOS-velocities are different pairs of negative and positive velocities (v_z = ±2^−1∕2, ±1, ±2^1∕2, ±2). The blue lines are the contours for v_z < 0, and the red lines for v_z > 0. The observer, located to the right, at p = 0, z = −∞, sees the emission coming from the part of the surfaces of equal LOS-velocity facing the observer, traced with thick lines. The dashed lines indicate the position of (z_m, p_m) for each contour, where p_m is the maximum value of p, and z_m the corresponding value of z. The square frame is drawn at p = ±1, z = ±1.