Routes to formation of a black hole?

[bcrowell]

How many routes are there to the formation of a black hole? A high-mass star can evolve off the main sequence, then end up going supernova and leaving behind a black-hole remnant. In this thread https://www.physicsforums.com/showthread.php?p=3062281 , Radrook and Jim Graber pointed out that there may be mechanisms of collapse that don't involve a supernova. Jim suggested that I search on arxiv.org for "accretion induced collapse," and indeed there were about a gazillion hits. Can anyone sort this out at all? Is anything known about the possible mechanisms of formation of supermassive black holes? At lower masses, is there an accretion/collapse mechanism and a separate collision mechanism?

 

[jimgraber]

Let me give an off the top of my head simple approximation. In a way, it's all accretion. Indeed, the main route to black hole formation is still thought to be when a massive star burns out its fuel and collapses, or "accretes onto its own center" if you will. This is thought to lead to a supernova in most or all cases, but in a minority of cases it just might collapse with only a weak explosion, or no significant explosion at all. The second most common route is when a neutron star in a binary accretes matter from its companion until it exceeds the Chandrasekar limit. Again this is thought to lead to a supernova in some cases, but could lead to a quiet collapse in others. This is still at least somewhat controversial, AFAIK. A third, slightly different case is when two neutron stars merge either by slow inspiral or just possibly, very rarely, by direct collision in a dense environment such as the center of a globular cluster. Once again, there is a possibility of an explosion or a more or less quiet collapse. The calculations are difficult and the final results are still disputed, I think. If some of these alternate scenarios are actually realized, they will be far less common than the main routes. Opinions are still changing, but that's my outsiders take on the current state of play.

Jim Graber

 

[jimgraber]

Actually, that should be Oppenheimer-Volkoff limit, which is largest possible neutron star, rather than Chandrasekhar limit, which is largest possible white dwarf. Sorry.
Jim Graber

 

[bcrowell]

Thanks, Jim. That's very informative!
-Ben

 

[Chronos]

There is evidence suggesting core collapse is not as simple a route to black hole formation as once thought. See:
http://www.space.com/scienceastronomy/massive-mega-star-challenges-black-hole-theory-100818.html
I suspect the majority are merger events.

 

[stevebd1]

Another unlikely but potential way a neutron star might collapse into a black hole is if a large neutron star has very high initial spin meaning the stars shape is an oblate spheroid and the volume of the star is greater than the same neutron star if it was static (the coordinate event horizon radius for a rotating star is also less than that of a static star). As the star slows done over millions of years, the interior density will increase as well the potential coordinate event horizon radius and there's a chance the star could collapse into a black hole, but the star would have to be on the brink of critical mass (~3 sol) with only the spin preventing collapse.