Chemistry in AGB stars: successes and challenges

T J Millar

doi:10.1088/1742-6596/728/5/052001

Abstract

Emission and absorption line observations of molecules in late-type stars are a vital component in our understanding of stellar evolution, dust formation and mass loss in these objects. The molecular composition of the gas in the circumstellar envelopes of AGB stars reflects chemical processes in gas whose properties are strong functions of radius with density and temperature varying by more than ten and two orders of magnitude, respectively. In addition, the interstellar UV field plays a critical role in determining not only molecular abundances but also their radial distributions. In this article, I shall briefly review some recent successful approaches to describing chemistry in both the inner and outer envelopes and outline areas of challenge for the future.

Highlights

The circumstellar envelopes (CSEs) of AGB stars have long been known to present a rich molecular chemistry dominated by the interaction of external, interstellar FUV photons with parent species formed by thermal equilibrium processes near the photosphere [1, 2, 3, 4, 5, 6]
These papers show that if shocks are strong any molecules formed in thermodynamic equilibrium (TE) are rapidly destroyed in the immediate post-shock gas and that ‘parent’ species available for chemistry in the outer CSE are the end products of shock chemistry coupled with dust nucleation and growth
Shock chemistry induced by stellar pulsations is clearly important as may be the detailed chemistry associated with dust formation and growth

Summary

Introduction

The circumstellar envelopes (CSEs) of AGB stars have long been known to present a rich molecular chemistry dominated by the interaction of external, interstellar FUV photons with parent species formed by thermal equilibrium processes near the photosphere [1, 2, 3, 4, 5, 6]. 1. Introduction The circumstellar envelopes (CSEs) of AGB stars have long been known to present a rich molecular chemistry dominated by the interaction of external, interstellar FUV photons with parent species formed by thermal equilibrium processes near the photosphere [1, 2, 3, 4, 5, 6].

Results

Conclusion