🌠Astrophysics I Unit 8 – Star Formation and Protoplanetary Disks

Key Concepts and Definitions

• Star formation occurs in dense, cold molecular clouds composed primarily of hydrogen and helium gas along with trace amounts of heavier elements and dust
• Gravitational collapse initiates star formation when a portion of a molecular cloud becomes denser than its surroundings and begins to contract under its own gravity
• Fragmentation process by which a collapsing cloud breaks up into smaller, denser cores that can each form a star or multiple star system
• Protostar early stage of stellar evolution where a contracting cloud core becomes hot enough to radiate energy but has not yet begun nuclear fusion
• Protoplanetary disk flattened disk of gas and dust surrounding a young star, formed from the conservation of angular momentum during gravitational collapse
  • Provides the raw material for planet formation
• Accretion process by which a protostar or protoplanetary disk gains mass from infalling material, releasing gravitational potential energy as heat and light
• Bipolar outflows jets of material ejected along the rotational axis of a protostar or protoplanetary disk, driven by magnetic fields and accretion processes

Stellar Nurseries and Molecular Clouds

• Molecular clouds are the birthplaces of stars, containing sufficient density and mass for gravitational collapse to occur
• Composed primarily of molecular hydrogen (H2) and atomic helium, with trace amounts of heavier elements and dust grains
• Temperatures in molecular clouds are typically low (10-30 K), allowing for the formation of complex molecules
• Turbulence within molecular clouds creates density fluctuations that can trigger gravitational collapse and star formation
  • Turbulence can be driven by stellar winds, supernovae, or galactic-scale processes like spiral arm density waves
• Magnetic fields play a crucial role in the dynamics and evolution of molecular clouds, providing support against gravity and influencing the formation of filamentary structures
• Dust grains in molecular clouds are essential for shielding the gas from ultraviolet radiation and facilitating the formation of molecules on their surfaces
• Molecular clouds exhibit a hierarchical structure, with smaller, denser cores embedded within larger, more diffuse clouds (ex: Orion Molecular Cloud Complex)

Gravitational Collapse and Fragmentation

• Gravitational collapse occurs when a portion of a molecular cloud becomes denser than its surroundings, causing it to contract under its own gravity
• Jeans instability criterion determines the minimum mass required for a cloud to undergo gravitational collapse, dependent on the cloud's density and temperature
• Fragmentation occurs during the collapse process, as the contracting cloud breaks up into smaller, denser cores
  • Each core can potentially form a single star or a multiple star system
• Hierarchical fragmentation can lead to the formation of stellar clusters and associations, with stars forming in proximity to one another
• The initial mass function (IMF) describes the distribution of stellar masses formed during the fragmentation process, with lower-mass stars being more common than higher-mass stars
• Magnetic fields and turbulence can influence the fragmentation process, either promoting or suppressing the formation of smaller cores
• The efficiency of star formation (fraction of cloud mass converted into stars) is relatively low, typically a few percent, due to the effects of feedback processes like stellar winds and radiation pressure

Protostellar Evolution Stages

• Protostellar evolution begins with the formation of a dense, gravitationally bound core within a molecular cloud
• The core continues to contract and heat up, eventually forming a central protostar surrounded by an infalling envelope of gas and dust
• Four main stages of protostellar evolution:
  1. Class 0: Deeply embedded protostar, accreting mass from surrounding envelope, with bipolar outflows
  2. Class I: Protostar becomes visible as envelope dissipates, accretion continues through protoplanetary disk
  3. Class II: Classical T Tauri star, protoplanetary disk becomes prominent, accretion rate decreases
  4. Class III: Weak-lined T Tauri star, little to no accretion, planetary system may be forming
• Spectral energy distributions (SEDs) used to classify protostars based on the shape of their infrared and submillimeter emission
• Herbig-Haro objects, small patches of nebulosity associated with the jets and shocks from protostellar outflows, signpost early stages of star formation
• Protostellar evolution timescales vary depending on stellar mass, with low-mass stars taking ~1 Myr to reach the main sequence, while high-mass stars evolve more rapidly

Protoplanetary Disk Formation

• Protoplanetary disks form around protostars as a natural consequence of angular momentum conservation during gravitational collapse
• As the cloud contracts, it flattens into a disk perpendicular to the rotational axis, with the protostar at the center
• Disk formation occurs early in the protostellar evolution, with disks observed around Class 0 and Class I sources
• Disk masses are typically a few percent of the protostellar mass, with sizes ranging from 10s to 100s of AU
• Disks are composed of gas (primarily hydrogen and helium) and dust grains, with dust making up ~1% of the total mass
• Dust grains in the disk are responsible for the infrared and submillimeter emission observed in protostellar SEDs
• Disk structure includes a flared outer region and a thinner inner region, with a vertical temperature gradient determined by the balance between stellar heating and radiative cooling
• Accretion from the disk onto the protostar occurs through magnetospheric accretion columns, with material funneled along stellar magnetic field lines

Disk Evolution and Planet Formation

• Protoplanetary disks evolve over time due to a combination of accretion, photoevaporation, and planet formation processes
• Viscous accretion drives the transport of angular momentum outward in the disk, allowing material to spiral inward and accrete onto the central star
• Photoevaporation by high-energy radiation from the central star or nearby massive stars can erode the disk, particularly in the outer regions
• Dust grains in the disk can grow through collisions and sticking, eventually forming larger bodies like pebbles and planetesimals
• Planetesimal formation theories include:
  • Core accretion: Dust grains grow into km-sized planetesimals, which then accrete gas to form planets
  • Disk instability: Gravitational instabilities in the disk lead to fragmentation and direct formation of giant planets
• Terrestrial planet formation occurs through the collisional growth of planetesimals in the inner disk, while giant planets form in the outer disk where volatile ices are present
• Migration processes can cause planets to move inward or outward in the disk, influencing their final orbital positions
• Debris disks, composed of dust produced by collisions between planetesimals, are observed around older stars and provide evidence for ongoing planet formation processes

Observational Techniques and Evidence

• Infrared and submillimeter observations are essential for studying star and planet formation, as they probe the cool dust and gas in molecular clouds and protoplanetary disks
• Spectral line observations of molecules like CO, HCN, and NH3 provide information on the density, temperature, and kinematics of molecular clouds and protostellar envelopes
• Polarimetry measurements of dust emission can reveal the strength and orientation of magnetic fields in molecular clouds and disks
• High-resolution imaging with facilities like ALMA (Atacama Large Millimeter/submillimeter Array) allows for detailed studies of disk structure and substructure (gaps, rings, spirals) that may indicate ongoing planet formation
• Direct imaging of exoplanets in young systems provides insights into the early stages of planet formation and evolution
• Spectroscopic observations of protostellar accretion and outflow signatures (ex: Hα emission, P Cygni profiles) offer evidence for the accretion process and the presence of disks
• Photometric variability of young stars, such as FU Orionis and EX Lupi outbursts, can indicate episodic accretion events or changes in disk structure

Challenges and Open Questions

• The initial conditions for star formation, including the origin of molecular cloud turbulence and the role of magnetic fields, are not fully understood
• The relative importance of different fragmentation mechanisms (turbulent vs. gravitational) in determining the initial mass function is still debated
• The efficiency of angular momentum transport in protoplanetary disks, and the primary drivers of accretion (ex: magnetorotational instability, disk winds), remain active areas of research
• The specific physical processes responsible for disk dispersal (ex: photoevaporation, planet-disk interactions) and their relative timescales are not well-constrained
• The dominant mode of planet formation (core accretion vs. disk instability) and the factors determining the final architecture of planetary systems are still uncertain
• The prevalence and role of episodic accretion in shaping protostellar evolution and disk structure are not fully understood
• The impact of stellar multiplicity on disk evolution and planet formation, particularly in close binary systems, requires further investigation
• Bridging the gap between the small-scale physics of star and planet formation and the larger-scale properties of stellar populations and galaxies remains a challenge for theoretical models and simulations