S9 - Rotation and magnetic fields Flashcards
How does a hydrostatic core form into an accretion disk?
A collapsing molecular cloud has angular momentum. The centrifugal force of the rotating cloud increases faster than the gravitational force as r decreases (as cloud collapses) so the centrifugal force will eventually win out over the gravitational force and halt collapse along the equatorial plane (where centrifugal force is largest). However the cloud can still continue to contract parallel to the rotation axis (which is perpendicular to the equatorial plane). The rotational velocity decreases with increasing latitude so the centrifugal force decreases with increasing latitude, and only acts perpendicular to the rotation axis, meaning that material can continue to infall. The maximum centrifugal force is along the equatorial plane (minimum material infall rate) and minimum along the axis of rotation (maximum material infall rate). Infalling material piles up at the equatorial plane, resulting in a disk-like density distribution.
Why must the core system need to lose a lot of angular momentum for the cloud to collapse to form a star?
Because if the angular momentum in the cloud stays the same before and after collapse, the rotational period of stars must be much lower than what they are observed to be.
What mechanisms decrease the angular momentum of a cloud during collapse?
Fragmentation: Transfer of angular momentum to a cluster or a binary, or planets during collapse.
Transfer of angular momentum through cloud-cloud interactions.
Magnetic braking of the star: Charged particles in the cloud couple with a magnetic field and resist angular motion.
Mass loss through outflows: the mass lost carries with it angular momentum.
How is angular momentum transferred locally within the cloud?
Friciton between neighbouring annuli will create torques that act to bring the annuli into co-rotation. The outer (slower) annuli will try to speed up (i.e. gain angular momentum) so angular momentum is transferred outwards.
Why must the cloud lose magnetic flux to collapse?
Because if the magnetic flux is conserved, the magnetic flux of stars must be much higher than they actually are.
What mechanism reduces the magnetic flux of molecular clouds?
Ambipolar diffusion
Colder regions emit _________ wavelength of light.
Colder regions emit longer wavelength of light.
How does ambipolar diffusion work?
Magnetic fields can only act on the ions/electrons in molecular clouds (not the neutrals). Neutrals move towards the centre of the cloud opposed only by collisions with ions: friction slows down the neutrals. Charged particles are moved away along the magnetic field lines making the concentration of charged particles lower, weakening the magnetic field (and magnetic force) supporting the cloud. Charged particle flux becomes more diffuse. Slowly a cloud supported by a magnetic field will expel the field. Eventually it will no longer be able to support itself and will collapse as gravity wins.
What supports the idea that ambipolar diffusion causes collapse?
The timescale for ambipolar diffusion is typically longer than the free-fall time scale so it must occur before collapse.
Why do Jeans critical molecular clouds stick around for a long time (even though their mass suggests that they should collapse) until they eventually collapse?
Because magnetic fields support them but due to ambipolar diffusion, the magnetic field support is eventually weakened sufficiently for collapse to occur.
