🚀Astrophysics II Unit 6 – Interstellar Medium and Star Formation

Key Concepts and Definitions

• Interstellar medium (ISM) consists of the matter and radiation that exists in the space between the star systems in a galaxy
• Composition of the ISM includes gas (atomic, molecular, and ionized) and dust particles
• Physical properties of the ISM encompass density, temperature, and magnetic fields which vary across different regions
• Phases of the ISM are determined by the state of the gas (atomic, molecular, or ionized) and the temperature and density of the region
• Molecular clouds are dense, cold regions of the ISM where star formation occurs due to gravitational collapse
• Star formation process involves the accumulation of gas and dust, fragmentation, accretion, and eventual birth of a star
• Observational techniques used to study the ISM and star formation include radio astronomy, infrared astronomy, and spectroscopy
• Jeans instability describes the condition under which a gas cloud will collapse under its own gravity to form stars

Composition of the Interstellar Medium

• ISM is composed of gas (99%) and dust (1%) by mass
• Gas in the ISM exists in atomic, molecular, and ionized forms
  • Atomic gas primarily consists of neutral hydrogen (HI) and helium
  • Molecular gas is dominated by molecular hydrogen (H2) and traces of other molecules (CO, OH, NH3)
  • Ionized gas includes HII regions around hot, young stars and diffuse ionized gas in the galactic halo
• Dust grains in the ISM are composed of silicates, graphites, and ices
• Cosmic rays, high-energy charged particles, permeate the ISM and contribute to its energy density
• Relative abundances of elements in the ISM follow the cosmic abundance pattern, with hydrogen and helium being the most abundant
• Interstellar gas is enriched with heavy elements (metals) produced by stellar nucleosynthesis and distributed via stellar winds and supernovae
• Magnetic fields thread through the ISM, influencing its structure and dynamics

Physical Properties of Interstellar Gas

• Density of the interstellar gas varies widely, ranging from $\sim10^{-4}$ atoms/cm$^3$ in hot, diffuse regions to $>10^6$ molecules/cm$^3$ in dense molecular clouds
• Temperature of the interstellar gas spans from a few Kelvin in cold, dense regions to millions of Kelvin in hot, ionized regions
• Pressure of the interstellar gas is determined by the combination of thermal, magnetic, and cosmic ray pressures
• Turbulence in the ISM is driven by various energy sources, such as supernovae, stellar winds, and galactic shear
• Magnetic fields in the ISM have strengths of a few microgauss and play a crucial role in the dynamics and structure formation
• Ionization state of the interstellar gas depends on the balance between ionization (by cosmic rays and UV radiation) and recombination processes
• Heating mechanisms in the ISM include photoelectric heating by dust grains, cosmic ray heating, and shock heating
• Cooling processes in the ISM involve line emission from atoms and molecules, dust radiation, and gas-dust collisions

Interstellar Dust: Characteristics and Effects

• Dust grains in the ISM have sizes ranging from a few nanometers to a few micrometers
• Composition of dust grains includes silicates, graphites, and icy mantles of water, CO, and other volatile compounds
• Dust grains absorb and scatter UV and optical light, leading to interstellar extinction and reddening
• Interstellar extinction varies with wavelength, with shorter wavelengths being more strongly affected (UV and blue light)
• Dust grains emit infrared radiation as they absorb UV and optical light, contributing to the infrared emission from galaxies
• Dust grains act as catalysts for the formation of molecular hydrogen and other complex molecules in the ISM
• Polarization of starlight by aligned dust grains provides information about the magnetic field orientation in the ISM
• Dust-to-gas ratio in the ISM is typically around 1:100 by mass, but varies depending on the environment and star formation history

Phases of the Interstellar Medium

• Hot ionized medium (HIM) with temperatures of $\sim10^6$ K and densities of $\sim10^{-3}$ atoms/cm$^3$, created by supernovae and stellar winds
• Warm ionized medium (WIM) with temperatures of $\sim8000$ K and densities of $\sim0.1$ atoms/cm$^3$, found in HII regions around hot stars
• Warm neutral medium (WNM) with temperatures of $\sim8000$ K and densities of $\sim0.5$ atoms/cm$^3$, consists of neutral atomic hydrogen
• Cold neutral medium (CNM) with temperatures of $\sim100$ K and densities of $\sim50$ atoms/cm$^3$, contains neutral atomic hydrogen and traces of molecules
• Molecular clouds with temperatures of $\sim10-20$ K and densities of $>10^2$ molecules/cm$^3$, primary sites of star formation
• Coronal gas with temperatures of $>10^6$ K and densities of $<10^{-4}$ atoms/cm$^3$, found in the galactic halo and hot superbubbles
• Transitions between phases occur through heating, cooling, and compression processes, such as supernovae, stellar winds, and gravitational collapse

Molecular Clouds and Star-Forming Regions

• Molecular clouds are dense, cold regions of the ISM where hydrogen is primarily in molecular form (H2)
• Giant molecular clouds (GMCs) have masses of $10^4-10^6$ solar masses, sizes of $\sim50$ parsecs, and densities of $10^2-10^3$ molecules/cm$^3$
• Molecular clouds are highly structured, with clumps, cores, and filaments observed at various scales
• Gravitational collapse of molecular cloud cores leads to the formation of protostars and protostellar disks
• Molecular clouds are often associated with star-forming regions, such as the Orion Nebula and the Taurus Molecular Cloud
• Feedback processes from newly formed stars (stellar winds, radiation, and supernovae) can disrupt the parent molecular cloud and trigger or suppress further star formation
• Turbulence within molecular clouds plays a crucial role in regulating star formation by creating density fluctuations and supporting the clouds against rapid collapse
• Magnetic fields in molecular clouds can provide additional support against gravity and influence the fragmentation process during star formation

Stages of Star Formation

• Gravitational collapse of a molecular cloud core initiates the star formation process
• Prestellar core stage: cold, dense core becomes gravitationally bound and begins to collapse
• Protostellar stage: a central protostar forms, surrounded by an infalling envelope and an accretion disk
  • Class 0 protostars are deeply embedded, with most of the mass in the envelope
  • Class I protostars have accreted much of the envelope, with a significant fraction of the mass in the disk
• Pre-main-sequence stage: the protostar becomes visible as the envelope dissipates, and the star contracts towards the main sequence
  • Class II objects (classical T Tauri stars) have optically thick accretion disks and strong emission lines
  • Class III objects (weak-line T Tauri stars) have optically thin or no disks and weaker emission lines
• Main sequence: the star reaches hydrostatic equilibrium and begins hydrogen fusion in its core
• Stellar feedback (winds, radiation, and outflows) can influence the surrounding environment and the star formation process
• Binary and multiple star systems are common outcomes of star formation, formed through fragmentation or disk instabilities

Observational Techniques and Evidence

• Radio observations (e.g., 21 cm HI line) trace the atomic hydrogen in the ISM and reveal the structure and kinematics of the gas
• Millimeter and submillimeter observations (e.g., CO lines) probe the molecular gas and dust in molecular clouds and star-forming regions
• Infrared observations (e.g., Spitzer, Herschel) detect the emission from dust grains and provide information about the temperature and density of the ISM
• Optical and UV observations (e.g., absorption lines) measure the abundances and physical properties of the gas along the line of sight to background stars
• Polarimetry (e.g., optical, infrared) reveals the magnetic field orientation in the ISM through the polarization of starlight by aligned dust grains
• Spectroscopy (e.g., emission and absorption lines) provides information about the composition, temperature, density, and kinematics of the interstellar gas
• Mapping the distribution and velocities of interstellar gas (e.g., HI, CO) reveals the large-scale structure and dynamics of the ISM in galaxies
• Observations of young stellar objects (YSOs) and their environments provide insights into the star formation process and the evolution of protostellar systems

Applications and Real-World Examples

• Understanding the ISM is crucial for studying the evolution of galaxies, as it is the reservoir for star formation and the medium through which galaxies interact
• The ISM plays a vital role in the chemical evolution of galaxies, as it is enriched by the products of stellar nucleosynthesis and distributes them throughout the galaxy
• Observations of the ISM in different galaxies (e.g., Milky Way, nearby galaxies) provide insights into the diverse physical conditions and star formation histories
• The study of molecular clouds and star-forming regions informs our understanding of the initial mass function (IMF) and the efficiency of star formation in galaxies
• Feedback from massive stars (e.g., supernovae, stellar winds) can trigger or suppress star formation in the surrounding ISM, regulating the star formation rate in galaxies
• The ISM is a source of obscuration for extragalactic observations, and understanding its properties is essential for accurate measurements of galaxy properties (e.g., star formation rates, masses)
• Interstellar dust grains are responsible for the formation of complex organic molecules and may have played a role in the origin of life on Earth
• The study of the ISM and star formation has implications for the search for exoplanets and the habitability of planetary systems, as the composition of the ISM influences the formation and evolution of planets