4.2 Chemical processes in collapsing clouds

Star formation involves complex chemical processes in collapsing molecular clouds. As clouds condense, their composition changes dramatically. Density increases lead to more frequent collisions, while rising temperatures affect reaction rates and molecular stability.

Chemical evolution in these clouds is driven by gas-phase reactions, dust grain surface chemistry, and destruction processes. The interplay of these factors shapes the cloud's composition, influencing the eventual formation of stars and planetary systems.

Chemical Evolution in Collapsing Clouds

Composition Changes During Collapse

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• As a molecular cloud collapses, its density and temperature increase, leading to significant changes in its chemical composition over time
• The initial composition of a molecular cloud is primarily atomic and molecular hydrogen, with trace amounts of other elements such as helium, carbon, oxygen, and nitrogen
• As the collapse progresses and temperature rises, some molecules may dissociate due to the increased energy of collisions, while others may become more stable

Factors Influencing Chemical Evolution

• The increasing density allows for more frequent collisions between atoms and molecules, facilitating the formation of more complex molecules
• The formation of molecules is dependent on the availability of reactive atoms and the presence of a third body (such as dust grains or another molecule) to stabilize the newly formed molecule
• The chemical evolution during the collapse is influenced by factors such as:
  • The initial composition of the cloud
  • The rate of collapse
  • The presence of external radiation sources (nearby stars)

Formation and Destruction of Molecules

Gas-Phase Reactions

• Molecule formation in collapsing clouds occurs primarily through gas-phase reactions, where atoms or molecules collide and react to form new species
• Common gas-phase reactions include:
  • Ion-neutral reactions: collision of an ion and a neutral atom or molecule, resulting in the formation of a new ion and neutral species
  • Neutral-neutral reactions: two neutral atoms or molecules collide and react to form new molecules
  • Dissociative recombination: an electron recombines with a molecular ion, causing the molecule to dissociate into neutral fragments

Destruction Processes

• Molecules can be destroyed through various processes, such as:
  • Photodissociation: dissociation due to absorption of high-energy photons (UV radiation)
  • Collisional dissociation: dissociation due to high-energy collisions between molecules
  • Chemical reactions that break down complex molecules into simpler ones
• The balance between formation and destruction processes determines the overall abundance of a given molecule in the collapsing cloud

Surface Chemistry on Dust Grains

• Surface chemistry on dust grains also plays a crucial role in molecule formation, as grains act as catalysts for reactions that may be inefficient or impossible in the gas phase
• Dust grains provide surfaces for atoms and molecules to adsorb, react, and desorb back into the gas phase, enabling the formation of complex molecules (methanol, formaldehyde)

Dust Grains in Chemical Processes

Catalytic Role of Dust Grains

• Dust grains in collapsing molecular clouds serve as catalysts for chemical reactions, providing surfaces on which atoms and molecules can adsorb, react, and desorb back into the gas phase
• Grains are particularly important for the formation of H2, the most abundant molecule in the universe, as the gas-phase formation of H2 is inefficient at low temperatures

Factors Affecting Grain Surface Reactions

• The efficiency of grain surface reactions depends on factors such as:
  • The grain composition (silicates, carbonaceous material)
  • Surface temperature
  • The sticking coefficients of the adsorbing species
• As the cloud collapses and warms up, molecules formed on dust grains can be desorbed back into the gas phase, enriching the chemical composition of the cloud

Shielding Effect of Dust Grains

• Dust grains can also shield molecules from high-energy radiation that might otherwise cause photodissociation, thus helping to preserve some molecular species in the cloud
• This shielding effect is crucial for the survival of complex organic molecules (amino acids, sugars) that are sensitive to UV radiation

Temperature and Density Impact on Reactions

Collision Frequency and Molecular Complexity

• As the cloud collapses, the increasing density leads to more frequent collisions between atoms, molecules, and dust grains, which can facilitate chemical reactions
• Higher densities also favor the formation of more complex molecules, as the increased collision rates allow for the stabilization of these species before they can dissociate

Temperature Effects on Reaction Rates

• The temperature of the cloud affects the kinetic energy of the atoms and molecules, with higher temperatures leading to more energetic collisions
• At low temperatures, gas-phase reactions may be slow due to the lack of energy to overcome reaction barriers, while grain surface reactions can still proceed efficiently
• As the temperature increases during the collapse, some gas-phase reactions may become more efficient (formation of CO), while others may be suppressed due to the increased likelihood of collisional dissociation

Formation of Distinct Chemical Regions

• The interplay between temperature and density can lead to the formation of distinct chemical regions within the collapsing cloud, with different molecular abundances and reaction pathways dominating in each region
• Understanding the impact of temperature and density on chemical reactions is crucial for interpreting observations of molecular emission from collapsing clouds and for modeling the chemical evolution of these systems