Neutron Star Below the Chandrasehkar Limit - Properties & Atomic Matter?

[fhqwgads2005]

Say a core collapse event of a massive star occurs, forming neutron star matter in its core. However, the explosion manages to eject enough core mass such that the remnant is below the Chanrasehkar limit. At this point, gravity would no longer be able to overcome electron degeneracy pressure, decreasing the density, so wouldn't the star's material return to some atomic state? What would be the properties of such a star? Or another question, if a 1.3 Ms white dwarf can be composed entirely of atomic matter, then shouldn't a 1.3 Ms (at least former) neutron star be composed of atomic matter as well?

 

[Drakkith]

The explosion does not eject material from the collapsed core. The explosion only happens because the infalling matter impacts the collapsed core and rebounds off of it. (In addition to some unknown events that we are still searching for)

However, assuming that a neutron star can lose enough mass through some mechanism I would expect the neutrons to decay with their normal half-life until the star is composed mostly of normal degenerate matter. But that's just a guess really. I wonder how much energy 1 solar mass of decaying neutrons would put out...

 

[fhqwgads2005]

The reason I ask is because I was reading a text on pulsars and it mentioned several pulsars with masses of around 1.3 or less. So it is definitely an observed phenomenon.

For example, see this list:

http://www.johnstonsarchive.net/relativity/binpulstable.html

several neutron stars are 1.3Ms.

 

[Drakkith]

I don't think the line between White Dwarf and Neutron Star is clear cut. I believe it is a gradual shift in the makeup of the star. A low mass neutron star only has a core of Neutrons, with the rest of the star being composed of a mix of ions, electrons, and neutrons, with the outer crust being mostly ions and electrons and neutrons increasing in number as you get closer to the core and the pressure becomes greater.

See here: http://en.wikipedia.org/wiki/Neutron_star#Structure

Perhaps it's possible that a high mass white dwarf can act similar to a low mass Neutron star?

 

[Chronos]

There is considerable overlap between the observed masses of neutron stars and white dwarfs. Most neutron stars have masses well below the Chadrasekhar limit. The key factor appears to be progenitor star mass. The progenitor star must be massive enough to overcome electron degeneracy pressure in the core.

 

[Drakkith]

Are you saying that to be considered a Neutron star the progenitor needs to have undergone a supernova?

 

[phyzguy]

Along these lines, there's an interesting hypothesis that the neutron-rich heavy elements (the so-called r-process elements - Uranium, for example) are actually formed from neutron star material that is ejected into space during the merger of two neutron stars. The neutron star matter, relieved of the huge pressure of gravity, then breaks down into heavy, neutron-rich atomic nuclei. It's a very interesting idea, especially since it is hard to account for the r-process elements by other means. Here's a reference:

http://arxiv.org/abs/1107.0899

 

[twofish-quant]

At this point, gravity would no longer be able to overcome electron degeneracy pressure, decreasing the density, so wouldn't the star's material return to some atomic state?

Off the top of my head, probably not. Even though you've removed the pressure, the pure neutron state still is a lower energy state that the electron/proton mix, so it's likely to stay that way. If you remove enough pressure, you may get into a situation where the proton/neutrons are able to turn back into an iron core.

Or another question, if a 1.3 Ms white dwarf can be composed entirely of atomic matter, then shouldn't a 1.3 Ms (at least former) neutron star be composed of atomic matter as well?

No because, the white dwarf isn't in the lowest possible energy state. Remember that the white dwarf is made of carbon, and it can't under go fusion because the temperature/pressure isn't high enough. Once you start burning the carbon, you end up making irreversible changes.

 

[twofish-quant]

I don't think the line between White Dwarf and Neutron Star is clear cut.

It is. White dwarfs are mostly unburned carbon. Neutron stars start out as iron, but there is enough pressure so that the iron nuclei fuse into one giant nucleus.

A low mass neutron star only has a core of Neutrons

The core is likely to be something weird like quark soup.

Perhaps it's possible that a high mass white dwarf can act similar to a low mass Neutron star?

No. Since white dwarf stars are made of unburned carbon.

 

[twofish-quant]

It's a very interesting idea, especially since it is hard to account for the r-process elements by other means.

It's easy to form r-process elements. In fact it's *too easy* to form r-process elements. The outer core of a neutron star is really really rich in r-process, but the trouble is that if you blow off more than a tiny amount of the neutron star, you get ten thousand times more r-process than we actually see.

 

[twofish-quant]

Are you saying that to be considered a Neutron star the progenitor needs to have undergone a supernova?

Yes. If it doesn't go supernova, the carbon doesn't burn, and so you end up with something that is 3000km wide rather than 10km wide.

 

[Drakkith]

Ohh, i get it now Twofish. I forgot that a white dwarf is MUCH MUCH bigger than a neutron star.

 

[fhqwgads2005]

Let's say the Fe core reaches the Chandrasekhar limit. It begins to collapse, and at some point, the very center of the core will reach the neutron degeneracy limit. However, some parts of the core must continue to collapse inwards, until they too reach the neutron degeneracy pressure limit. At some point though, the outer layers of the core must "bounce" off (the supernova event), creating a shockwave. This is the only way to explain how the final resulting neutron star could have a mass below the Chandrasekhar limit, unless I'm totally wrong here.

My question is, at what radius does the core stop collapsing, and bounce? Surely the entire inner core does not become neutron matter at the same time, so there shouldn't be a single specific time when the core suddenly becomes neutron degenerate, it should happen starting from the center and moving outwards.

 

[twofish-quant]

My question is, at what radius does the core stop collapsing, and bounce?

In the calculations I've done, the bounce radius is typically about 0.6-0.7 solar masses. The shock goes out and then stalls at 1.2 solar mass, and then something magic happens to revive the shock.

Surely the entire inner core does not become neutron matter at the same time, so there shouldn't be a single specific time when the core suddenly becomes neutron degenerate, it should happen starting from the center and moving outwards.

Yes. The center is always at velocity zero. Once the core starts collapsing, the velocity looks like a V shape with the center at zero, the velocity goes down and back up again. Once the center starts hitting neutron degeneracy densities, the pressure increases suddenly and the velocity goes to zero. When the velocity changes fast enough so that the material can't react, then you get a shock wave. This happens at 0.6-0.7 solar masses.

All of this happens very fast. Once you set the pressure to zero, then you have the inner core go from 100 KM to 10 KM radius in about 100 milliseconds. The speed of the material of some of the infalling material can reach several percent of the speed of light.

What's interesting is what happens if you try to simulate a black hole. The way that the computer program simulates general relativity is that you insert a time dilation factor and a space dilation factor. When a part of the simulation starts to turn into a black hole, the time dilation factor goes to zero, and the computer simulation "freezes" that part of the simulation. This is not a real effect, but just part of the computer simulation since you are running a computer program that isn't designed to simulate black holes, but you can tell what parts of the simulation are "black hole" by looking at the time dilation factor.

Most people run these sorts of simulations using Newtonian gravity. You run the simulation once using general relativity and then you find that it doesn't make much of a difference, and then you run everything Newtonian to save computer time.

Also the fact that neutron stars seem to be close to the Chandrasekar mass seems to be an "interesting coincidence". There's no obvious reason why neutron stars have to be that mass, although maybe there is a non-obvious reason.

 

[PatrickPowers]

It is. White dwarfs are mostly unburned carbon. Neutron stars start out as iron, but there is enough pressure so that the iron nuclei fuse into one giant nucleus.

The core is likely to be something weird like quark soup.

The outer 1km or so of a neutron star is composed of extremely dense iron. The quark soup model was ruled out when a neutron star of 1.97 stellar masses was discovered.

 

[fhqwgads2005]

In the calculations I've done, the bounce radius is typically about 0.6-0.7 solar masses.

How does this relate to the final mass of the neutron star? Does the star accrete another .7 solar mass of matter after the supernova explosion? Or during it at some point?

Also, can anyone point me towards up to date information regarding the current status of neutron star EOS's, like, which ones have been more or less ruled out by mass observations (too soft)?