Three-dimensional mesh view of the model of the molecular envelope of BK Vir, which is defined by a sphere whose equatorial plane is inclined by ∼25° with respect to the plane of the sky. Left: view from Earth. The X and Y axes are represented by red and green, respectively (positive directions are in the clockwise direction). Right: resulting synthetic spectra for both models, the oblate spheroid (red) and the sphere (blue), and observation (black) for the ¹²CO J = 2–1 transition toward BK Vir. We note the similarity between the synthetic spectra for both models.