6.4 Protostellar Evolution and Young Stellar Objects

Protostellar evolution kicks off with dense cores collapsing in molecular clouds. As they spin and flatten, accretion disks form, feeding the growing star. Outflows blast material along the rotation axis, creating cool Herbig-Haro objects.

Young stellar objects come next. T Tauri stars are small and feisty, while Herbig Ae/Be stars are bigger and brighter. Both types have disks that might form planets. They follow different paths to become full-fledged stars.

Early Protostellar Evolution

Formation and Structure of Protostellar Cores

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• Protostellar cores form within molecular clouds through gravitational collapse
• Dense, cold regions of gas and dust with temperatures around 10-20 K
• Typical masses range from 0.1 to 10 solar masses
• Core density increases as collapse progresses, reaching $10^{-13}$ g/cm³
• Rotation of the core leads to conservation of angular momentum
• Magnetic fields play a crucial role in regulating the collapse process

Accretion Disks and Material Flow

• Accretion disks form around protostars due to conservation of angular momentum
• Disks typically extend 100-1000 AU from the central protostar
• Material from the disk falls onto the protostar, fueling its growth
• Accretion rates vary from $10^{-8}$ to $10^{-6}$ solar masses per year
• Viscous forces within the disk transport angular momentum outward
• Magnetorotational instability drives turbulence and enhances accretion

Outflows and Associated Phenomena

• Bipolar outflows eject material along the protostar's rotation axis
• Outflows can extend several parsecs from the protostar
• Velocities of outflows range from 10 to 1000 km/s
• Herbig-Haro objects form when outflows collide with surrounding material
• HH objects exhibit shock-excited emission lines (hydrogen, sulfur, oxygen)
• Jet-driven bow shocks create characteristic structures in HH objects

Young Stellar Objects

T Tauri Stars: Characteristics and Evolution

• T Tauri stars represent low-mass pre-main sequence stars (< 2 solar masses)
• Strong emission lines, particularly H-alpha, indicate ongoing accretion
• Exhibit irregular variability due to magnetic activity and accretion processes
• Possess strong lithium absorption lines, indicating youth
• Surrounded by circumstellar disks, potential sites for planet formation
• Typical ages range from 1 to 10 million years

Herbig Ae/Be Stars: Properties and Significance

• Herbig Ae/Be stars are intermediate-mass pre-main sequence stars (2-8 solar masses)
• Show strong emission lines, including hydrogen Balmer series
• Exhibit infrared excess due to circumstellar dust
• Often associated with reflection nebulae and molecular clouds
• Evolutionary precursors to A and B type main sequence stars
• Provide insights into the formation of massive stars and their environments

Pre-main Sequence Tracks

Evolutionary Stages and Physical Processes

• Pre-main sequence evolution begins after the protostellar phase
• Stars contract and heat up as they move towards the main sequence
• Deuterium burning occurs early in the pre-main sequence phase
• Convection dominates in fully convective phase for low-mass stars
• Radiative cores develop as stars contract and temperatures increase
• Timescales for pre-main sequence evolution depend on stellar mass

Hayashi Track: Convective Contraction Phase

• Hayashi track represents the fully convective phase of pre-main sequence evolution
• Stars descend nearly vertically on the Hertzsprung-Russell diagram
• Luminosity decreases while temperature remains relatively constant
• Applies to low-mass stars (< 0.5 solar masses) throughout their pre-main sequence life
• Higher mass stars follow the Hayashi track initially before transitioning
• Duration of Hayashi track phase inversely proportional to stellar mass

Henyey Track: Radiative Contraction Phase

• Henyey track follows the Hayashi track for stars > 0.5 solar masses
• Characterized by increasing temperature at nearly constant luminosity
• Radiative energy transport dominates in the stellar interior
• Stars move horizontally across the HR diagram towards higher temperatures
• Contraction continues until hydrogen fusion begins in the core
• Marks the transition from pre-main sequence to main sequence phase