Molecular catastrophes and circumstellar SiO masers

Abstract

Understanding the complex SiO maser regions of highly evolved stars can be improved through multiwavelength studies of 'premaser' stars, such as M0-M4 giants and semiregular variables, which can be placed on normal H-R diagrams unlike most of the OH-IR stars. ^I argue that SiO masers are a key part of the transformation of hot stellar plasma into cold circumstellar silicate dust, in the outflows from evolved, oxygen rich stars. ^Evidence for this statement rests on the following: (1) red giant mass loss originates in a stochastic, amsotropic manner; (2) SiO maser maps of Miras and red supergiants show numerous maser spots within a few stellar radii; (3) molecules and dust naturally form in a cooling outflow; (4) the IRAS Low Resolution Spectrometer provided evidence for diverse and variable 10 micron silicate features in Miras, and these shapes correlate well with the proposed maser chronology, suggesting a formation and annealing sequence. ^The theory for the occurrence of SiO masers involving thermal instability, related 'new' physics, recent calculations and a prediction are discussed. ^(Author (revised))