Schematic illustrating the limb brightening of the emission incident on the circumstellar CO envelope amplified by the maser inversion of the foreground CO gas. Left panel: illustrates the continuum emission from the star (solid circle) that is absorbed by the turbulent and inwards/outwards moving CO gas in the extended atmosphere. The dust formation zone is indicated by the dashed circle. The dashed and solid lines represent the lines of sight towards the centre and limb of the star, respectively, for an observer on the right side of the page. Center panel: illustrates the optical depth. Towards the stellar centre the optical depth (τ) peaks towards high velocities representing the inward and outward motions of the gas in the extended atmosphere. In contrast, towards the limb of the star, the projection of the velocity vectors in the extended atmosphere result in the strongest absorption closer to the stellar systemic velocity. Therefore, the centre of the stellar disc is dimmer than the limbs at themaser velocities, as shown in the rightmost scheme. The brightest maser emission is thus expected towards the limb of the star where it amplifies the strongest incident emission.