MASER Shells in Late-Type Stars
Prof. Victor Migenes
Department of Astronomy, University of Guanajuato, Mexico
ABSTRACT
Stars with masses < 8 times the Sun's, evolving along the Asymptotic
Giant Branch become variable giants and supergiants. As they evolve, they
develop large (Rstar ~ 1-3 AU) and cool (T ~ 2000 - 3000 K)
convective photospheres. As they begin to pulsate they produce envelopes
of dust and molecular gas. The pulses (100 - 1000 days) produce shock
waves that generate complex gas motions in the inner regions of the envelope.
As the dust condenses, at radii of a few AU, it becomes momentum-coupled
to the gas and the stellar radiation pressure then accelerates the material
to typical velocities of 10-20 km/s. These conditions favor the formation
of masers. MASERs of various molecular species (mainly OH, H2O
and SiO) have been found in over 500 late-type stars.
Some of the intrinsic properties of MASERs, including their compactness,
ultra-bright emission and polarization, make them very useful probes of
the physical conditions of these evolved envelopes. We present some of
the interesting results obtained by the study of the MASER emission of
these sources.