A. M. S. Richards

Turbulent, steamy red supergiant winds

Rapidly-evolving red supergiants (RSG) lose half or more of their mass before ending their lives as supernovae. Masers allow us to study the mass loss from 4 nearby RSG in AUscale detail using MERLIN and EVN/global VLBI. The water maser clouds are over-dense and over-magnetised with respect to the surrounding wind. In most cases, the brighter an individual maser component is the smaller its apparent (beamed) FWHM appears, as predicted for approximately spherical clouds. Individual water maser features have a typical half-life of 5-10 yr, but comparison with single dish monitoring suggests that the water vapour clouds themselves survive many decades (the water maser shell crossing time), within which the local masers wink on and off. OH mainline masers are found in the tenuous surrounding gas, overlapping the water maser shell, surrounded by OH 1612-MHz masers at a greater distance from the star.

A Study of OH and H2O Masers in the Circumstellar Envelopes Around Miras

We study the mass loss phenomenon which is though to be generally seen in the evolution of late type stars. We are using radio interferometry European VLBI Network (EVN) and Multi-Element Radio Lined Network (MERLIN) to study maser emission from gas molecules of H2O and OH. This enable us to obtain 10-30 mas resolution images of mass-loss in AGB stars

Models of masers in AGB stars

Coupling maser models to dynamical stellar envelope models provides a new step towards the full understanding of the physics of mass-losing stars. Here, we describe how such numerical simulations of H2O and SiO maser emission yield maser variability and the proper motions of maser clumps during a stellar pulsation cycle. These calculations place important constraints on stellar models and provide new insights on the conditions leading to, and predominantly affecting, stellar maser emission. This work also constitutes the foundations towards our final goal: the development of dynamical models of AGB stars which calculate accurately and self-consistently the time-dependent intensity and location of SiO, H2O and OH maser emission, in addition to the inclusion of time- dependent chemistry and dust formation. Such models will provide a ‘big picture’ view of mass-losing stellar envelopes throughout their pulsation cycles