Chapter 6: Chemistry and Dust Formation in Circumstellar Regions and Supernovae

Dust is an important component of the interstellar gas, but it isn't formed in interstellar clouds. It is made in some particular and much denser regions associated with stars, and dust formation follows a period of extensive chemistry in those regions. We describe chemistry and dust formation in two types of region that are believed to make the most significant contributions to dust in the Milky Way. The first regions are the envelopes of fairly cool modest stars (these stars are of about solar mass); these envelopes develop towards the final stages of the stellar evolution. Chemistry, nucleation and then dust deposition may occur in these envelopes, and the amount of dust produced may be sufficient to extinguish the light of the star itself. Newly-formed dust is ultimately driven away from the star by radiation pressure and mixes with pre-existing interstellar dust. The second type of location in which dust is formed is in the explosions that end the lives of much more massive stars; the supernovae. These highly energetic explosions eject large amounts of material from the star, and – unlikely though it may seem – conditions may become favourable for chemistry and for nuclei to form, on which solid dust grains may then be deposited. We discuss the chemistry that leads to nucleation and grain growth in both these scenarios.