Kinematics and structure of circumstellar envelopes

Summary

Abstract

This review discusses the kinematics, the overall spatial structure, and the more detailed structure of envelopes around AGB-stars, post-AGB objects, and PNe, as deduced from molecular radio line emission. A possible scenario for the evolution of a circumstellar envelope as the star evolves from an AGB-star to the white dwarf stage is presented.

Introduction

Red giant stars on the asymptotic giant branch (AGB) lose considerable amounts of matter in a slow wind, and a circumstellar envelope (CSE) of gas and dust is formed. The CSE gradually becomes thicker as the star evolves towards the end of the AGB. The mass loss decreases substantially as the star leaves the AGB and the CSE detaches itself from the star. Eventually, the central post-AGB object becomes hot enough to ionize the inner regions of the remnant AGB–CSE and a planetary nebula (PN) forms. Thus, the AGB–CSE provides a common link through this evolutionary sequence, and hopefully much can be learnt about the late stages of stellar evolution as well as the metal enrichment of the interstellar medium through the study of its properties. Unfortunately, space does not permit a discussion of CSEs around supergiants (see e.g. Knapp & Woodhams 1993).