Molecular clouds, the birthplaces of stars, are thought to form through various mechanisms, with one prominent hypothesis involving colliding flows of gas.
Colliding Flows: A Key Mechanism
Numerous models suggest that molecular clouds arise from colliding flows. According to these models, the initial gas density is typically low enough that the gas exists in a warm neutral (atomic) phase. (Ballesteros-Paredes, Hartmann & Vázquez-Semadeni; Heitsch et al.; Vázquez-Semadeni et al.)
The Process Explained:
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Initial State: Diffuse, warm, and neutral atomic gas is present in interstellar space.
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Colliding Flows: These gas flows collide, driven by processes such as:
- Supernova explosions
- Galactic spiral arm dynamics
- Magneto-rotational instability
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Compression and Cooling: The collision compresses the gas, increasing its density. As the gas density increases, it begins to cool efficiently.
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Molecular Formation: As the temperature drops, atoms combine to form molecules, primarily molecular hydrogen (H2).
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Cloud Formation: The gas becomes dense and cold enough to shield itself from dissociating radiation, allowing molecules to survive and form a molecular cloud.
Challenges and Considerations:
- Magnetic Fields: Magnetic fields can play a significant role, either hindering or facilitating cloud formation depending on their orientation and strength.
- Turbulence: Turbulence within the gas can both promote and inhibit cloud formation, influencing the density distribution and stability.
- Feedback: Stellar feedback, such as radiation and stellar winds from newly formed stars, can disrupt the cloud and influence further star formation.
In summary, colliding flows offer a compelling explanation for the formation of molecular clouds, emphasizing the importance of compression, cooling, and molecular formation in a dynamic interstellar environment.
