Section 9

Question 1

What will eventually happen to the balance of forces in a cloud while it is rotating?

Answer 1

The centrifugal force will win over the gravitational force and halt collapse along equatorial plane

Question 2

What is the typical range of centrifugal radius for typical cloud?

Answer 2

100-1000 AU

Question 3

What leads to a flattened disk?

Answer 3

When the cloud contracts parallel to the rotation axis leading to a flattened structure

Question 4

What happens to the centrifugal force with increasing latitude?

Answer 4

It decreases as the rotational velocity decreases

it only acts perpendicular to the rotation axis so material can infall

Question 5

What does infalling material flow along?

Answer 5

Streamlines

Question 6

From the graph of z/Fc vs R/r_c what can be deduced about infalling material?

Answer 6

Piles up at the centrifugal radius (around 1) and doesn’t fall equidistantly due to the polar direction

(R/r_c = centrifugal barrier)

Question 7

What does the resulting density distribution show?

Answer 7

The material is concentrated in a disk-like geometry

Question 8

What do molecular clouds possess?

Answer 8

A small amount of angular momentum due to rotation so resulting collapse isn’t completely spherical

Question 9

What will the collapse of even very slowly rotating clouds lead to?

Answer 9

Massive amplification of the rotational velocity

Question 10

What needs to happen for a cloud to collapse?

Answer 10

The system needs to loose a lot angular momentum

Question 11

What are the processes of loosing angular momentum?

Answer 11

Fragmentation/fission: transfer of momentum to a cluster or binary or planet

Transfer of angular momentum through cloud-cloud interactions

Magnetic braking of the star: charged particles couple with the magnetic field and resist angular motion

Mass loss through outflows

Question 12

What does the charged fluid velocity do?

Answer 12

Bends the field and creates a resisting tension force perpendicular to the magnetic field

Question 13

What does a spin up during collapse cause?

Answer 13

It twists the field and increases the local magnetic tension, creating a breaking torque and lowering angular momentum

Question 14

What are the two possible ways in which angular momentum is transferred locally?

Answer 14

Turbulent viscosity

Magnetic fields

Question 15

How is angular momentum transferred outwards?

Answer 15

Collisions between an inner particles and outer particles causes the outer annulus to speed up as angular momentum is transferred

Question 16

What causes the magnetic breaking of annuli?

Answer 16

Stretching due to shear, creating a restoring force

Question 17

What are molecular clouds threaded with?

Answer 17

Magnetic fields

(if isotropic)

Question 17

What is flux freezing?

Answer 17

When charged particles spiral around field lines effectively freezing in the magnetic field into the material, providing support mechanism against collapse

magnetic flux is conserved

Question 18

What happens when the magnetic field is strong enough and uniform?

Answer 18

Expect flattened clouds as the material cannot cross the field lines, it can only move along them

Question 19

Which force is dominant once collapse has started?

Answer 19

gravitational force over magnetic force but they both increase at same rate initially

Question 20

What becomes negligible when magnetic collapse happens?

Answer 20

thermal pressure

Question 21

How is magnetic flux lost?

Answer 21

Through ambipolar diffusion

Question 22

What is ambipolar diffusion?

Answer 22

The relative drift of neutrals (not coupled to field) and ions (well coupled to the field) that allows the neutral to condense, so magnetic flux is lost and collapse can occur

Question 23

What slows down neutrals?

Answer 23

Collisions with ions causing friction

Question 24

Why must ambipolar diffusion occur before collapse?

Answer 24

Its timescale is typically longer than the free-fall time scale