Dust from AGB stars

Dust is formed in the expanding atmosphere during late stages of stellar evolution. Dust influences the dynamics and thermodynamics of the stellar atmosphere by its opacity. The dust opacity depends both on the optical properties of the grain material as well as on the amount of dust present. A rich source of information on some mineral phases of dust in AGB stars comes from the study of presolar grains from meteorites. This paper presents a short overview of presolar grains studies and describes how the optical properties of dust grains are obtained in the laboratory. The formation of molecules and dust grains is strongly dependent on the time-dependent dynamical processes that affect the atmospheres of AGB stars, i.e. pulsations and propagating shock waves. Two basic condition must be fulfilled in the AGB star atmosphere for dust formation to succeed, relatively low temperature (< 2000 K) combined with high density. The sufficiently low temperature is present above the photosphere. As a consequence of the pulsation, the outer atmosphere is levitated, resulting in a temporary reservoir of relatively dense gas at a certain distance from the photosphere, and thus increasing the efficiency of dust formation. Observations indicate that dust is a significant ingredient of the outflows from AGB stars. It has therefore long been assumed that the mass loss from AGB stars is driven by radiation pressure on dust grains (e.g. Heras & Hony 2005), however, recent frequency-dependent numerical models of M-type stars have not yet been able to support this (see Höfner and Woitke in these proceedings). In this paper the focus is on what we know about the dust properties from studies of presolar AGB dust grains from meteorites, and from laboratory studies of dust analogues. 2. Dust from AGB stars-Presolar grains extracted from meteorites While most of the material that went into the making of the Solar System was throughly processed and mixed, thus losing isotopic heterogeneity and all memory of its origin, small quantities of refractory dust grains survived the events that led to the formation of the Solar System and such grains have been 1