The Heriot-Watt Astrochemistry Research Group

Background: Gas-Dust Interactions

The dense clouds in the interstellar medium consist primarily of hydrogen and helium gas, remaining from the formation of the universe. Other atomic and molecular gases such as NH₃, HCN, CO and CO₂ are also present, having formed in the cloud itself, or having been ejected from a nearby star. The solid-state species only constitute around 1% of the total matter!

Dust grains in the interstellar medium are so cold (10 K) that most debris accretes icy mantles, either from freeze-out of gas-phase reaction products or by retention of atomic species in collision with the grains

Two types of icy mantle are thought to exist:

polar ices (in regions where there is more atomic Hydrogen than molecular Hydrogen (H₂))
a-polar ices (in regions where there is less atomic Hydrogen than molecular Hydrogen (H₂))

The grains are only a few microns diameter, so the ice mantle thickness does not exceed 10 nm. Both amorphous and crystalline ices have been identified in the interstellar medium, from a number of different molecular species.

Why is it important to understand gas-dust interactions?

Astronomy Viewpoint

to model the role of icy dust in the evolution of dense molecular clouds
to study star formation

Chemistry Viewpoint

to explain the existence of complex molecular species in the interstellar medium
to understand the nature of the icy mantles and dust cores
to learn about the bonding and interactions at the gas-ice interface
to study molecular species adsorbing, desorbing and scattering at ice surfaces
to ecludiate the rates and mechanisms of chemical reactions on ice surfaces

Almost nothing is known about how the dust grains form, or the role they play in the chemical evolution of the interstellar medium. However, when atmospheric chemists started to study the interactions between gases and ices in the upper atmosphere they found a feasible way to explain why the ozone hole existed. Similarly, heterogeneous chemistry will help astronomers to explain the production rates for molecular hydrogen and explain the diversity of molecular species that exist in the interstellar medium. By understanding gas-dust interactions we can start to solve some of the key questions facing 21st century astronomers...