Evolution of Interstellar Dust

Abstract

This paper presents a review of our current knowledge of interstellar dust. The composition of the interstellar dust is summarized in Table 1. About half of the dust volume consists of amorphous silicates. The other half has to be made up out of a carbonaceous component, such as graphite, amorphous carbon (e.g., soot), and/or organic grain mantles (e.g., mixed polymers). Presently it cannot be decided which of these carbonaceous components dominates the interstellar dust, but future observations which can settle this point are discussed. Some discussion is given of the similarities and differences between graphite and amorphous carbon. Other minor dust components, such as SiC and MgS, are probably also present in the interstellar medium. Inside dense molecular clouds icy grain mantles can be a very important dust component containing up to 40% of the available elemental carbon and oxygen. The evolution of dust in the interstellar medium is described and some important physical processes are outlined. This includes nucleation, condensation and coagulation of Stardust (e.g., silicates, graphite and soot) in the outflows from late-type stars and UV photolysis and transient heating of icy grain mantles forming organic grain mantles in the interstellar medium. The destruction of dust by interstellar shocks is also described. The short destruction timescales which result from analysis of this process form a serious problem for any interstellar dust model based on Stardust alone. Even those models in which the interstellar dust is mainly formed in the interstellar medium may face problems in explaining the measured silicate dust volume. The interrelationship between interstellar and interplanetary dust is briefly described and it is argued that interstellar Polycyclic Aromatic Hydrocarbon molecules (hereafter PAHs) have carried the measured deuterium enhancement of the carbonaceous meteorites into the solar nebula. Finally an unaltered interstellar dust origin for the Ca,Al-rich inclusions in meteorites is rejected. A general description of infrared spectroscopy is given and applied to observations of interstellar icy grain mantles. Recent 5–8µm Observations of compact objects embedded inside dense molecular clouds are described. They show absorption features near 6.0 and 6.85 µm whose shape and peak position vary from source to source. The relatively narrow features observed towards W33 A are identified with the OH and CH deformation modes in H ₂O and alcohols (i.e., CH₃OH). The much broader features observed towards Mon R2-IRS 2 imply that a more complex array of molecular subgroups are present. The observed band shapes indicate that aldehydes (e.g., H₂CO) and possibly ketones (e.g., CH₃COCH₃) are important grain constituents in the grain mantles along the line of sight towards that source. Mineral identifications for the 6.0 and 6.85 µm absorption features are briefly discussed and it is concluded that minerals do not contribute appreciably to these bands. The identification of each of the molecules proposed to be present in interstellar icy grain mantles is reviewed and critical observations required to confirm some of them are pointed out. The molecular composition of icy grain mantles for several sources is summarized in Table 3. While interstellar icy grain mantles have a variable composition, the simplest spectra imply a composition given approximately by H₂O/CH₃OH/CO/NH₃ ≃ 1/0.66/0.05/0.05.