3.3 Dust grains and their role in astrochemistry

Dust grains are tiny particles in space that play a huge role in astrochemistry. They're made of silicates, carbon, and ice, and come in different sizes. These grains act like little chemistry labs, helping form molecules in space.

Dust grains affect how light travels through space and how energy moves around. They can absorb starlight, get heated up, and then release that heat as infrared light. This process is key to how the interstellar medium works and evolves.

Interstellar Dust Grain Properties

Composition and Size Distribution

• Interstellar dust grains are composed of various materials:
  • Silicates
  • Carbonaceous materials (graphite, polycyclic aromatic hydrocarbons)
  • Ices (water, CO, CO2, and other volatile species)
• Dust grain sizes range from a few nanometers to a few micrometers
  • Grain size distribution follows a power-law (Mathis-Rumpl-Nordsieck or MRN distribution)
  • Smaller grains are more abundant than larger grains

Physical Properties and Interactions

• Dust grain shapes are often approximated as spherical or elongated
• Grains can have varying porosity depending on their formation and processing history
• Optical properties of dust grains (absorption and scattering cross-sections) depend on grain composition and size
• Dust grains can be electrically charged (positively or negatively) due to interactions with:
  • Cosmic rays
  • UV photons
  • Collisions with gas-phase ions and electrons
• Charged dust grains can interact with magnetic fields, leading to grain alignment
  • Aligned grains cause polarization of starlight and emission from the grains themselves

Dust Grain Formation Mechanisms

Formation in Evolved Stars

• Dust grains can form through gas-phase condensation in the cooling ejecta of evolved stars:
  • Asymptotic giant branch (AGB) stars
  • Supernovae
• In evolved star ejecta, dust grains nucleate from the condensation of refractory elements (Si, Mg, Fe, C) as the gas cools
  • Different elements condense at different temperatures, leading to a sequence of dust grain formation

Grain Growth and Processing

• Dust grains can grow through accretion of gas-phase species onto existing grain surfaces in dense interstellar environments (molecular clouds)
  • Accretion efficiency depends on the sticking coefficients of the accreting species
  • Grain growth is more efficient in higher-density regions
• Dust grains can be processed and destroyed by various mechanisms:
  • Shocks
  • Sputtering
  • High-energy radiation
• Processing and destruction occur in different astrophysical environments (supernova remnants, diffuse interstellar medium)

Dust Grains in Molecular Formation

Surface Chemistry Catalysis

• Dust grain surfaces act as catalysts for molecule formation in the interstellar medium
  • Grains provide a platform for atoms and molecules to meet and react
• Hydrogen atoms adsorb onto dust grain surfaces and react to form H2 molecules through:
  • Langmuir-Hinshelwood mechanism: Two adsorbed H atoms migrate on the surface and react
  • Eley-Rideal mechanism: An incoming H atom from the gas phase directly reacts with an adsorbed H atom
• Complex organic molecules (COMs) can form through hydrogenation of simple ice species (CO, CO2, HCOOH) on grain surfaces

Factors Affecting Surface Chemistry

• The efficiency of surface chemistry depends on several factors:
  • Dust grain temperature
  • Grain composition
  • Binding energies of the adsorbed species
• Molecules formed on dust grain surfaces can be desorbed back into the gas phase through:
  • Thermal desorption: Occurs when the grain temperature is high enough to overcome the binding energy
  • Photodesorption: Induced by UV photons
  • Cosmic-ray-induced desorption: Caused by the impact of energetic cosmic rays

Dust Grains and Interstellar Medium

Extinction and Thermal Balance

• Dust grains absorb and scatter UV and optical radiation from stars
  • Contributes to the extinction and reddening of starlight in the interstellar medium
  • The absorbed energy heats the grains, which then re-emit the energy as thermal infrared radiation
• Thermal emission from dust grains is a major coolant in the interstellar medium
  • Particularly important in regions where gas-phase coolants (C+, O) are depleted
• Dust grains contribute to the opacity of the interstellar medium
  • Affects the transfer of radiation and thermal balance of gas in molecular clouds and protostellar envelopes

Opacity and Grain Properties

• The dust opacity depends on several factors:
  • Grain composition
  • Grain size distribution
  • Wavelength of the radiation
• Different types of grains have different opacity curves:
  • Silicate grains
  • Carbonaceous grains
• The interplay between dust opacity, radiation transfer, and gas thermal balance influences the structure and evolution of interstellar clouds and star-forming regions