SiO masers in red giants

Abstract

In recent years, it has become possible to trace rapid structural changes occurring in the extended atmospheres of red giant stars through VLBI monitoring of SiO masers. The observations reveal a complex picture. Firstly, material is predominantly outflowing, but also infalls towards the star, in a pulsating inner envelope periodically disrupted by shocks. Secondly, circular polarization measurements may indicate that the inner circumstellar envelope is permeated by a magnetic field strong enough to be of dynamical importance. Finally, towards a few stars, observations also show evidence for rotation of the SiO maser region. Current SiO maser models can reproduce many of the key features of circumstellar SiO maser emission, and have successfully predicted e.g. the infall of SiO masing gas and the observed spatial separation ofv= 1 andv= 2 43 GHz masers. However, both stellar hydrodynamical and maser codes need to increase in complexity in order to model fully these regions. In this review I will discuss the current developments in circumstellar SiO maser observations and models.