Introduction

The lifecycle of interstellar dust and the processes that play a role are summarized in Fig. 13.1. Dust is formed at high densities and temperatures in the ejecta from stars. This leads to the formation of high temperature condensates such as silicates, graphite, and carbides. Stardust grains with an isotopic composition betraying their birthsites have been isolated from meteorites (Chapter 5). In the interstellar medium, dust cycles many times between the intercloud and cloud phases. In the warm neutral and ionized intercloud media, dust is processed by strong shocks driven by supernova explosions (Section 12.3). The hot gases in the shock can sputter atoms from the grains. Also, high velocity collisions among grains can lead to vaporization, melting, phase transformation, and shattering of the projectile and target. These processes have been discussed in Section 5.2. In the denser media – diffuse and dense clouds – gas-phase species can accrete onto grains forming a mantle. In diffuse clouds, the accreted species may be predominantly bound by chemisorbed forces – partly because physisorbed species will be rapidly photodesorbed by the high flux of FUV photons (Section 10.6.1). In molecular clouds, the accretion process leads to the formation of an icy mantle consisting of simple molecules such as H2O, CO, CO2, and CH3OH (see Section 10.5). These ices may be processed by UV photons and high-energy cosmic rays into larger, more complex species, which could be more tightly bound to the cores.