DUST FORMATION, PROPAGATION AND SURVIVAL IN THE ISM

摘要

Dust formation in the interstellar medium (ISM) is predominantly associated with evolved stars, i.e., 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). The contribution of AGB stars to the galactic dust budget is important, in terms of the variety and quantity of dust, but that due to SNe is not yet well-determined. AGB stars, through their stellar winds, are known to inject amorphous and crystalline silicates, hydrogenated carbons, silicon carbide, graphite and other rarer materials into the ISM. However, not all of these materials are detected ob-servationally, in circumstellar regions or in the ISM, but are to be found only among the pre-solar grains. The lifetimes of the major dust materials (i.e., carbons and silicates) in the ISM, against destruction in SN shock waves, appear to be short compared to the time-scale for the formation of new dust around evolved stars. A grain lifetime problem is therefore evident. Apparently, it is necessary to re-form and grow grains in the ISM, through accretion and coagulation processes, in order to explain interstellar dust observations. However, SN-generated shock waves with velocities less than 200 km s~(-1) destroy, by erosion, less than half of the dust mass that they interact with. Therefore SN shocks can be an efficient means of propagating dust through the ISM.