6.4 Protostellar Evolution and Young Stellar Objects
3 min read•august 9, 2024
Protostellar evolution kicks off with dense cores collapsing in . As they spin and flatten, disks form, feeding the growing star. blast material along the rotation axis, creating cool Herbig-Haro objects.
Young stellar objects come next. are small and feisty, while 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
- 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 g/cm³
- Rotation of the core leads to conservation of
- 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 to 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
- 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
Key Terms to Review (18)
Accretion: Accretion refers to the process of accumulating mass, particularly in astronomical contexts where matter is drawn together by gravitational forces. This process plays a vital role in the formation and growth of celestial objects, such as stars, planets, and black holes, where material gradually gathers to form a more massive entity over time.
Angular Momentum: Angular momentum is a physical quantity that represents the rotational inertia and angular velocity of an object or system. It plays a crucial role in understanding how objects move and interact, especially in celestial mechanics, where it helps explain the behavior of stars, galaxies, and accretion disks. The conservation of angular momentum is a fundamental principle, indicating that in a closed system with no external torques, the total angular momentum remains constant over time.
Class 0: Class 0 refers to the earliest stage of protostar development, characterized by dense and cold regions within molecular clouds where the formation of a star is just beginning. These objects are still deeply embedded in their parent cloud and show minimal radiation, making them difficult to observe directly. This phase is crucial as it sets the foundation for subsequent stages of stellar evolution and ultimately influences the characteristics of the star that will form.
Class I: Class I refers to a specific category of young stellar objects (YSOs) that are in the earliest stages of protostellar evolution. These objects are characterized by their strong infall of material from their surrounding envelopes and are typically still gaining mass from their parent molecular cloud, leading to significant accretion onto the forming star. This stage is critical for understanding how stars form and evolve, particularly regarding the physical processes that govern the growth of stellar mass and the development of circumstellar disks.
Core Accretion Model: The core accretion model describes the process by which gas giant planets form through the gradual accumulation of solid material, forming a dense core that attracts surrounding gas. This model emphasizes the importance of solid bodies, such as planetesimals, which collide and stick together to build up a core, eventually allowing it to capture a substantial gaseous envelope. This mechanism is crucial for understanding how young stellar objects evolve and develop their planetary systems.
Disk instability model: The disk instability model is a theoretical framework that explains how young stellar objects can form as a result of gravitational instabilities in rotating accretion disks. This model suggests that when the density within a disk becomes high enough, it can lead to local regions collapsing under their own gravity, ultimately giving rise to new stars and planetary systems. The dynamics of the disk and its ability to fragment are key features of this model, linking it to the processes of protostellar evolution.
Gravitational Collapse: Gravitational collapse is the process by which an astronomical object loses its structural integrity due to the gravitational forces overwhelming other forces, causing it to contract under its own gravity. This phenomenon is fundamental in the formation of stars and galaxies, as it leads to the birth of new stellar objects from dense regions of molecular clouds and plays a crucial role in the evolution of large-scale structures in the universe.
Herbig AE/BE stars: Herbig AE/BE stars are a class of young, pre-main sequence stars that possess spectral types A and B, indicating they are more massive than typical T Tauri stars. These stars are still in the process of forming and exhibit significant infrared excess due to the surrounding circumstellar material, which plays a crucial role in the study of protostellar evolution and the characteristics of young stellar objects.
Hydrostatic Equilibrium: Hydrostatic equilibrium is the state in which a fluid, such as a star, is balanced under the influence of gravity and pressure gradients. In stars, this balance is crucial as it ensures that the inward pull of gravity is counteracted by the outward pressure from nuclear fusion and thermal energy. This concept is foundational in understanding stellar structure and evolution, as it governs how stars maintain stability throughout their life cycles.
Infrared spectroscopy: Infrared spectroscopy is a technique used to identify and analyze the composition of matter by measuring how substances absorb infrared radiation. It is particularly useful in astrophysics for studying celestial objects, as it can reveal the presence of various molecules and dust in regions where visible light is blocked, such as in protostellar environments. This method allows astronomers to gather crucial information about the temperature, composition, and physical properties of young stellar objects and the surrounding material.
Jets: Jets are highly collimated streams of gas and plasma that are ejected from the regions around young stars and supermassive black holes. They are formed through the interaction of magnetic fields and accretion processes, playing a crucial role in transporting energy and material away from these cosmic objects. Jets can affect their surroundings significantly, impacting star formation in nearby regions and influencing the dynamics of host galaxies.
Luminosity: Luminosity is the total amount of energy emitted by a star, galaxy, or other astronomical object per unit time, typically measured in watts. This fundamental property allows for the comparison of different celestial objects and plays a crucial role in understanding their behavior, evolution, and classification. By knowing the luminosity, astronomers can infer distances and the physical characteristics of these objects, which is essential for grasping the dynamics of the universe.
Mass Function: The mass function is a statistical description that provides the number of stars within specific mass ranges in a given population. It is a crucial tool for understanding stellar formation and evolution, as it helps astronomers characterize how stars are distributed based on their masses in star clusters and galaxies, thereby shedding light on the processes that influence protostellar evolution and the formation of young stellar objects.
Molecular Clouds: Molecular clouds are dense regions of gas and dust in space, primarily composed of hydrogen molecules, where star formation occurs. They are cool, with temperatures typically around 10 to 20 Kelvin, providing the perfect environment for the gravitational collapse necessary to form stars and planetary systems. The characteristics of these clouds are closely tied to various astrophysical processes, including spiral structure dynamics, the lifecycle of protostars, and the rates at which new stars are formed.
Outflows: Outflows refer to the streams of material, such as gas and dust, that are expelled from a celestial object like a star or a galaxy. These outflows play a crucial role in various astrophysical processes, impacting star formation, the evolution of galaxies, and the dynamics of interstellar medium. They can be driven by mechanisms like stellar winds, supernova explosions, or active galactic nuclei, influencing both the immediate environment and larger scale galactic structures.
Radio interferometry: Radio interferometry is a technique that uses multiple radio antennas to observe astronomical objects, combining the signals received by each antenna to create high-resolution images. This method enhances the ability to detect and analyze celestial phenomena by simulating a larger telescope, which allows astronomers to study distant objects with greater detail. It is particularly useful in observing various stages of star formation, analyzing structures in the interstellar medium, and studying the dynamics of astrophysical jets and outflows.
Star-forming regions: Star-forming regions are dense areas within molecular clouds where gas and dust come together under gravity to initiate the formation of new stars. These regions are characterized by their high concentrations of materials, which provide the necessary ingredients for the birth of stars, including hydrogen molecules and dust. The processes occurring in star-forming regions are crucial for understanding how stars evolve and the role they play in galactic dynamics.
T Tauri Stars: T Tauri stars are a type of young, variable star that represents a key stage in the protostellar evolution process, typically found in star-forming regions. These stars are characterized by their low mass and high luminosity relative to their size, as well as strong stellar winds and significant variability in brightness. They play an essential role in our understanding of the formation and early evolution of stars.