2 O and SiO molecules may be observed. SiO is depleted into dust grains except close to the star. Maser strengths scale with the local kinetic energy density or the local IR radiation field, which both depend on the CSE temperature. Masers are quenched above a critical density. As a result the CSE has a layered maser structure, corresponding to subsequent stages in the mass-loss process from the underlying star.
Radiation pressure accelerates the matter to velocities exceeding the escape velocity.
Matter that does not reach the dust formation radius falls back to the star. SiO masers probe the inner dust-free zone, H 2 O masers probe the acceleration zone, and OH masers probe the final stellar wind. The physical conditions in the CSE, and the evolutionary stage of the star can be determined from the presence or absence of different species of masers, and from their photon fluxes. The kinematic structure of the CSE can be determined from the maser line profiles. SiO maser radiation is predominantly amplified tangentially, resulting in a single maser peak within a few kms -1 of the stellar velocity .