🌠Astrochemistry Unit 3 – Interstellar Medium

What's the Interstellar Medium?

• Consists of the matter and radiation that exists in the space between the star systems in a galaxy
• Includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays
• Accounts for ~15% of the visible matter in the Milky Way, with the remaining ~85% found in stars and planets
• Serves as the repository for matter ejected by stars through stellar winds and supernovae explosions
• Provides the raw material for new star formation when dense regions collapse under their own gravity
• Plays a crucial role in the evolution and structure of galaxies across cosmic time
• Exhibits a wide range of physical conditions, from cold and dense molecular clouds to hot and diffuse ionized regions
  • Cold molecular clouds have temperatures around 10-20 K and densities of 10^3-10^6 particles/cm^3
  • Hot ionized regions can reach temperatures of 10^6 K and densities of 0.1-1 particles/cm^3

Key Components of the ISM

• Atomic hydrogen (HI) is the most abundant element, making up ~70% of the ISM by mass
  • HI regions are typically found in diffuse clouds with temperatures around 100 K and densities of 10-100 particles/cm^3
• Molecular hydrogen (H2) is the second most abundant molecule, found in dense molecular clouds
  • H2 is difficult to observe directly due to its lack of a permanent dipole moment
• Ionized hydrogen (HII) regions surround hot, young stars and are created by the ionizing radiation from these stars
• Dust grains, composed of silicates, graphite, and ices, make up ~1% of the ISM by mass but have a significant impact on its properties
  • Dust grains absorb and scatter UV and optical light, leading to interstellar extinction and reddening
  • They also serve as catalysts for the formation of complex molecules on their surfaces
• Cosmic rays, high-energy charged particles, permeate the ISM and contribute to its ionization and heating
• Heavy elements, such as carbon, oxygen, and nitrogen, are present in trace amounts but play crucial roles in the chemistry of the ISM

Physical Conditions in Space

• Density: The ISM spans a wide range of densities, from ~10^-4 particles/cm^3 in hot, diffuse regions to >10^6 particles/cm^3 in dense molecular clouds
  • The average density of the ISM in the Milky Way is ~1 particle/cm^3
• Temperature: The ISM exhibits a wide range of temperatures, from ~10 K in cold molecular clouds to >10^6 K in hot, ionized regions
  • The temperature of the ISM is determined by the balance between heating and cooling processes
• Radiation field: The interstellar radiation field (ISRF) consists of starlight, cosmic background radiation, and emission from the ISM itself
  • The ISRF plays a crucial role in the ionization, heating, and chemistry of the ISM
• Magnetic fields: The ISM is permeated by magnetic fields with strengths ranging from a few μG to several mG
  • Magnetic fields can influence the structure and dynamics of the ISM, particularly in dense regions where they can provide support against gravitational collapse
• Turbulence: The ISM is highly turbulent, with a wide range of scales and velocities
  • Turbulence plays a key role in the structure, dynamics, and mixing of the ISM, and can influence star formation processes

Chemical Reactions in the ISM

• Gas-phase reactions: Include ion-neutral, neutral-neutral, and electron recombination reactions
  • Example: The formation of H2 through the reaction H + e- → H- + hν, followed by H- + H → H2 + e-
• Grain-surface reactions: Occur on the surfaces of dust grains, which act as catalysts for the formation of complex molecules
  • Example: The formation of water through the reaction H + OH → H2O on grain surfaces
• Photochemistry: Driven by the interstellar radiation field, which can dissociate and ionize molecules
  • Example: The photodissociation of H2 by UV photons, H2 + hν → H + H
• Cosmic-ray-induced reactions: Cosmic rays can ionize and dissociate molecules, initiating chemical reaction chains
  • Example: The formation of H3+ through the reaction H2 + CR → H2+ + e-, followed by H2+ + H2 → H3+ + H
• Temperature-dependent reactions: Some reactions have energy barriers that can be overcome at higher temperatures
  • Example: The formation of CO through the reaction C + OH → CO + H, which becomes efficient at temperatures >100 K
• Isotopic fractionation: Chemical reactions can preferentially incorporate certain isotopes, leading to isotopic anomalies in the ISM
  • Example: The enrichment of deuterium in molecules such as HD and DCO+ compared to the cosmic D/H ratio

Dust and Gas Interactions

• Dust grains provide surfaces for gas-phase species to accrete onto, enabling the formation of ices and complex molecules
  • Example: The formation of water ice mantles on silicate dust grains in cold molecular clouds
• Gas-phase species can collide with and stick to dust grains, depleting them from the gas phase
  • Example: The depletion of refractory elements like Si, Fe, and Mg in the diffuse ISM due to their incorporation into dust grains
• Dust grains can absorb and scatter UV and optical light, affecting the radiation field and the ionization state of the gas
• Photoelectric heating: UV photons can eject electrons from dust grains, which then heat the gas through collisions
  • This process is a major heating mechanism in the neutral ISM
• Dust grains can shield dense regions from UV radiation, allowing for the formation of molecules like H2 and CO
• Dust grains can be destroyed or altered by shocks, sputtering, and high-energy radiation
  • Example: The destruction of dust grains in supernova shock waves, releasing refractory elements back into the gas phase

Observing the ISM

• Radio observations: Used to study the 21 cm line of HI, rotational transitions of molecules (e.g., CO, NH3), and continuum emission from ionized gas and dust
  • Example: The mapping of giant molecular clouds using the CO(1-0) transition at 2.6 mm
• Infrared observations: Used to study the emission from dust grains, polycyclic aromatic hydrocarbons (PAHs), and rovibrational transitions of molecules
  • Example: The detection of H2 in shocked regions using the 2.12 μm rovibrational transition
• Optical and UV observations: Used to study absorption lines from atoms and molecules in the diffuse ISM, as well as emission lines from ionized gas
  • Example: The study of interstellar extinction and elemental abundances using absorption lines in the spectra of background stars
• X-ray observations: Used to study hot, ionized gas in supernova remnants, stellar winds, and the hot phase of the ISM
  • Example: The detection of the 0.65 keV line of O VII in the hot ISM, indicating the presence of gas with temperatures ~10^6 K
• Gamma-ray observations: Used to study the interaction of cosmic rays with the ISM, as well as the emission from radioactive decay of unstable isotopes
  • Example: The mapping of the Galactic plane in the 1.809 MeV line of 26Al, a radioactive isotope produced in massive stars and supernovae

Impact on Star and Planet Formation

• Molecular clouds, the densest regions of the ISM, are the sites of star formation
  • Gravitational collapse of dense cores within molecular clouds leads to the formation of protostars and protoplanetary disks
• The initial mass function (IMF) of stars is thought to be influenced by the properties of the ISM, such as turbulence, magnetic fields, and chemical composition
• The chemical composition of the ISM determines the initial elemental abundances in protoplanetary disks, which in turn affects the composition of planets
  • Example: The C/O ratio in the ISM can influence the formation of carbon-rich or oxygen-rich planets
• Feedback from young stars (e.g., radiation, winds, and supernovae) can disrupt the surrounding ISM and trigger or suppress further star formation
  • Example: The expansion of HII regions around massive stars can compress nearby molecular clouds, triggering the formation of new stars
• The presence of short-lived radionuclides (e.g., 26Al, 60Fe) in the early Solar System suggests that the Sun formed in a region enriched by recent supernovae
• The efficiency of planet formation may be affected by the properties of dust grains in the ISM, such as their size distribution and chemical composition
  • Example: The growth of dust grains in protoplanetary disks depends on the initial size distribution inherited from the ISM

Current Research and Open Questions

• The role of magnetic fields in the structure and evolution of the ISM, particularly in the formation and support of molecular clouds
  • How do magnetic fields influence the star formation process, and what is the relative importance of magnetic support compared to turbulence?
• The origin and evolution of interstellar dust grains, including their formation in stellar outflows, processing in the ISM, and incorporation into planets
  • What are the dominant dust production mechanisms, and how do dust grains evolve in different environments?
• The chemical complexity of the ISM and the formation of prebiotic molecules
  • How complex can molecules become in the ISM, and what role do they play in the origin of life?
• The impact of stellar feedback on the ISM and the regulation of star formation on galactic scales
  • How do the various feedback mechanisms (e.g., radiation, winds, supernovae) interact, and what is their relative importance in different environments?
• The role of the ISM in galaxy evolution, including the relationship between star formation and the properties of the ISM over cosmic time
  • How does the ISM evolve in response to the changing star formation history of galaxies, and what role does it play in the quenching of star formation?
• The properties and evolution of the ISM in extreme environments, such as in the early Universe, in starburst galaxies, and around supermassive black holes
  • How do the physical conditions and chemical composition of the ISM differ in these environments, and what can they tell us about the evolution of galaxies and the Universe as a whole?