Black hole accretion: angular momentum loss

[Aziza]

Why exactly is it necessary for angular momentum to be lost by a mass if it is to accrete around a black hole? the mass is decreasing its radius, so it speeds up: thus angular mometum is conserved. But everywhere it is saying that 99.99% of the angular momentum must be shed for accretion to occur. Exactly why is this the case?

 

[Drakkith]

In order to fall into the black hole the mass must decrease it's orbital radius. To do this it is required that it shed momentum and energy, otherwise it's orbit would remain the same and it would not fall in. This is similar to spacecraft having to fire retrorockets in order to start reentry into Earths atmosphere. The infalling material is moving faster, but the total angular momentum is less. This is compensated by having the lost momentum transferred to material that doesn't fall in.

 

[Aziza]

In order to fall into the black hole the mass must decrease it's orbital radius. To do this it is required that it shed momentum and energy, otherwise it's orbit would remain the same and it would not fall in. This is similar to spacecraft having to fire retrorockets in order to start reentry into Earths atmosphere. The infalling material is moving faster, but the total angular momentum is less. This is compensated by having the lost momentum transferred to material that doesn't fall in.

ohh right! But now why is it necessary for objects to actually fall in? Maybe they just spin around the hole without ever falling in, thus no need for angular momentum loss?

 

[Drakkith]

It's not necessary. If it doesn't happen then nothing falls in. However when material such as a gas or dust cloud falls into the black hole, it's not all falling in at the same speed and radius and stuff. As it orbits around the black hole the material nearer to the black hole is moving faster, and the end result is lots of friction that transfers orbital energy into heat and radiation, with the particles that lose energy falling further towards the black hole.

 

[George Jones]

ohh right! But now why is it necessary for objects to actually fall in? Maybe they just spin around the hole without ever falling in, thus no need for angular momentum loss?

It is true that a particle can orbit, or even a bunch of non-interacting particles, can orbit a black hole.

Suppose that the particle density is high enough that the particles can interact. Dissipative interactions ("friction", radiated "heat") between the particles can rob the particles of energy and angular momentum.

[edit]Drakkith beat me to it![/edit]

 

[Aziza]

It's not necessary. If it doesn't happen then nothing falls in. However when material such as a gas or dust cloud falls into the black hole, it's not all falling in at the same speed and radius and stuff. As it orbits around the black hole the material nearer to the black hole is moving faster, and the end result is lots of friction that transfers orbital energy into heat and radiation, with the particles that lose energy falling further towards the black hole.

So then why do we need models such as magnetorotational instability to explain loss of angular momentum? Cant we just attribute it to frictional energy loss/radiation? Also why do we say that the agn jets are possibly carrying away the angular momentum?

 

[Drakkith]

So then why do we need models such as magnetorotational instability to explain loss of angular momentum? Cant we just attribute it to frictional energy loss/radiation? Also why do we say that the agn jets are possibly carrying away the angular momentum?

I'd guess that friction in this manner isn't the ONLY way to lose angular momentum.

 

[Aziza]

I'd guess that friction in this manner isn't the ONLY way to lose angular momentum.

Yea i guess, from what I am reading it seems that most ppl agree that magnetic field plays stronger role because the particles arent actually that close together to exert significant hydrodynamic pressure on each other, so magnetic pressure is important.

I have another kind of related question: how is it possible for an object emit radiation above its Eddington limit? Wikipedia vaguely says that stars can do this by emission of stellar winds. A study is showing that the higher the redshift of a quasar, the higher is its super-Eddington luminosity...how is this possible? Is this by any chance related to angular momentum transport?