Interstellar and circumstellar grain formation and survival

Abstract

Dust formation is primarily associated with stars in their dying throes, e.g. when low–mass stars reach the red–giant or asymptotic–giant branch (AGB) phase of their evolution, or when massive stars explode as supernovae (SNe). While the contribution of AGB stars to the galactic dust budget is significant, both in terms of variety and quantity, that due to SNe is not yet clear. AGB stardust formation includes grains of amorphous and crystalline silicates, hydrogenated carbons, silicon carbide and graphite. However, not all of these materials have yet been detected in circumstellar regions or in the interstellar medium (ISM). The derived lifetimes for these materials in the ISM appear to be short compared with the time–scale for the formation of new dust. Thus a grain lifetime and propagation problem is posed. Apparently, it is also necessary to reform and grow grains in the ISM, through accretion and coagulation processes, in order to explain interstellar dust observations. This paper discusses dust formation in circumstellar and interstellar environments, dust sources and their contributions to the galactic dust budget, and dust survival and propagation in the ISM.