S6 - Fragmentation Flashcards
Molecular clouds (10K) are hotter than the cosmic background temperature (2.7K). What is the main heating mechanism of molecular clouds?
Ionisation of molecular hydrogen by cosmic rays.
What is the primary source of galactic cosmic rays?
Supernovae
Molecular clouds are mainly composed of…
…molecular hydrogen.
What is the distribution of cosmic ray ionisation rate throughout the Milky Way?
Decreases with distance from galactic centre (as there are more supernovae closer to the galactic centre).
Where in the Milky Way galaxy does most star formation occur?
The galactic centre.
How are low mass (mass below initial (start of first collapse) Jeans mass) stars created?
By fragmentation during molecular cloud collapse. Jeans mass decreases during the initial, isothermal collapse of a molecular cloud.
Describe the molecular cloud heating mechanism.
High-energy protons (cosmic rays) collide and scatter inelastically with molecular hydrogen, mainly ionising the hydrogen molecules. An electron is produced during every ionisation event which then collides with hydrogen molecules causing them to dissociate. The excess energy from the electron during the dissociation event is distributed to the hydrogen atoms produced. Subsequent collisions quickly disperse this energy throughout the gas. It is a very inefficient process (very low net energy gain) but enough to heat the molecular clouds.
What is the main cooling mechanism of molecular clouds?
Emission by carbon monoxide due to rotational de-excitation from its first excited rotational transition above the ground state.
Why is the initial collapse of a molecular cloud isothermal?
Because the cooling rate of the molecular cloud during its initial collapse is very fast (much faster than its free-fall time).
Explain the process of a molecular cloud forming hydrostatic cores (which eventually become protostars).
A molecular cloud is isothermal, and thus the Jeans mass is only dependent on density (decreases with increasing density). If a large cloud becomes sufficiently dense that its mass exceeds the Jeans mass, it will begin to collapse. As the cooling process is so fast, the cloud remains isothermal during the initial collapse. Eventually, during the collapse, localised regions of the cloud also collapse independently of the molecular cloud (fragmentation). When a fragment can collapse independently without further disruption (further fragmenting), it will collapse until it becomes optically thick (opaque to its own radiation) and thus can no longer radiate thermal energy away efficiently enough to remain isothermal, causing its temperature (and thus Jeans mass) to increase until the mass of the fragment becomes ≥ Jeans mass (internal temperature and thus pressure increase to amount required for hydrostatic equilibrium), halting collapse and becoming a hydrostatic core.
What is Jeans length?
The radius of a molecular cloud at which outward pressure balances gravity.
