Core collapse of a supernova: the "void" left by the collapsed core?

[nikkkom]

When core collapse occurs, a ~10000km diameter core of the star collapses into neutron star or a black hole. Let's look at NS case here: the resulting neutron star is on the order of 20 km across.

And this happens in a few seconds.

The upper layers of the star cannot immediately fall onto the NS in just a second. Effectively, the collapsed core must be leaving an Earth-sized "void" where it used to be.

It won't be really empty: there won't be a sharply defined spherical surface dividing the core and the rest. The picture should be something like this, if we freeze "core just collapsed" star in time: the surface of the star and many hundreds of thousands of kilometers of upper layers didn't feel a thing yet, but somewhere about 5000-10000km distance from the center the density suddenly starts to decrease, and the gas is no longer stationary - it falls inward. And then there is ~20km diameter ultra-dense, ultra hot ball of neutrons in the center.

This should result in some interesting dynamics wrt release waves on the inner surface of the "void" and radiative pressure on its walls. As gas from the surface expands inward, it exerts an outward pressure on the upper layers.

And newly formed neutron star has a tremendous luminosity, on the order of 10^19 solar. Radiation pressure alone is not negligible from it, but just like in a thermonuclear bomb, even larger effect should be that this light from NS (gamma rays really) should heat up the walls of the "void", causing gas from the walls to expand inward faster, which increases outward pressure.

Has this been modeled in the supernova simulations? (I suppose it was - I can't be the first one to think about it...)

 

[phyzguy]

Of course it has been modeled. There is a whole industry of people working on these simulations, with probably thousands of papers. When the free falling outer layers hit the dense neutron star core (called a "proto-neutron star" in the literature), they bounce and drive a shock wave back out into the star outer layers. Historically the (mostly 1D) simulations show that this shock wave stalls due to the rush of infalling outer material and the star fails to explode. Despite more than 20 years of work, I think it is still not completely clear what drives the explosion. The pressure of the huge flux of outgoing neutrinos is part of the answer, and doing the simulations in 3D rather than 1D or 2D is more likely to result in an explosion because apparently the explosion is not even close to being spherically symmetric. If you Google
"core collapse supernova simulations" you will find more stuff than you can read.

 

[nikkkom]

Of course it has been modeled. There is a whole industry of people working on these simulations, with probably thousands of papers. When the free falling outer layers hit the dense neutron star core (called a "proto-neutron star" in the literature), they bounce and drive a shock wave back out into the star outer layers.

Bouncing off a neutron star means rising from a surface with about 10 billion G surface gravity. No wonder it's not that efficient.

It looks to me that a (another?) shock may be generated: a shock may be created by the photons impinging on the wall of the "void" left by just-collapsed core. This "wall" is not significantly moving yet, and it is dense (compared to the "void"), thus it will absorb EM radiation from NS, heat up and try to expand. It can easily expand inwards (because the "void" is below it). This will create a reaction force, like in rocket engine, pushing even higher layers outward. Newly born neutron star has a very high EM luminosity, right?

 

[phyzguy]

This article has a decent summary. Christian Ott is very active in the field. https://stellarcollapse.org/~cott/oldnews.html also has a lot of info.

 

[nikkkom]

This article has a decent summary.

Thanks! Excellent. Exactly what I looked for!

According to the article, the idea they pursue is that the shock propagates in the material which falls on the proto-neutron star. The difficulty is that it does not work with simple calculations, and scientists work on making the calculations which account for more factors, use smaller grids, do not simplify to 1D or 2D, etc.

I have a slightly different idea. What if _that_ shock does not indeed "succeed" to separate and propagate outward? What if that material successfully accretes on the PNS, without blowing up? (And let's suppose the star is not massive enough for black hole, we do end with a NS).

The infalling material does not fall uniformly - the material in "outer core" (see article) falls quickly since it's close to the center, material farther away from center falls progressively slower, up to star surface which does not fall at all. That's what forms what I termed "the void" in place of former core, a low-density region ~10000 km is diameter between NS and the rest of the star matter.

My theory is what if the explosion results from NS pumping this region full of photon gas?

 

[Ken G]

Photons are already in the model, so what you're saying should be in the models too. So it's importance will have been considered. To be a new effect not already included, you would need to include some new physical effect that the simulations either leave out, or are incapable of resolving accurately.

 

[phyzguy]

Thanks! Excellent. Exactly what I looked for!

According to the article, the idea they pursue is that the shock propagates in the material which falls on the proto-neutron star. The difficulty is that it does not work with simple calculations, and scientists work on making the calculations which account for more factors, use smaller grids, do not simplify to 1D or 2D, etc.

I have a slightly different idea. What if _that_ shock does not indeed "succeed" to separate and propagate outward? What if that material successfully accretes on the PNS, without blowing up? (And let's suppose the star is not massive enough for black hole, we do end with a NS).

The infalling material does not fall uniformly - the material in "outer core" (see article) falls quickly since it's close to the center, material farther away from center falls progressively slower, up to star surface which does not fall at all. That's what forms what I termed "the void" in place of former core, a low-density region ~10000 km is diameter between NS and the rest of the star matter.

My theory is what if the explosion results from NS pumping this region full of photon gas?

As Ken G said, the effects you are talking about are already in these simulations. You can't do these things qualitatively, you have to model what is happening as exactly as you can using detailed computer simulations. If you want to understand more about it, you would have to dig into the details of these simulations. Many of the codes are open source and publicly available.

 

[nikkkom]

Photons are already in the model, so what you're saying should be in the models too. So it's importance will have been considered. To be a new effect not already included, you would need to include some new physical effect that the simulations either leave out

They may well be leaving it out. They are concentrating on modeling the inner region, basically NS and its neighborhood only, maximum to ~1000km out from center. (Not surprising, of course, since making grid larger costs a lot in compute resources.)

 

[phyzguy]

They may well be leaving it out. They are concentrating on modeling the inner region, basically NS and its neighborhood only, maximum to ~1000km out from center. (Not surprising, of course, since making grid larger costs a lot in compute resources.)

I strongly doubt they are leaving out the effects you mentioned, but you would have to dig into the simulations to be sure.

 

[alantheastronomer]

I think nikkkom may be on to something here. All simulations concentrate on the inner region surrounding the neutron star in an attempt to find a way to revive the stalled shock. Neutrino transport and magnetohydrodynamic effects are mentioned repeatedly yet no mention is made of the energy released in gamma rays from the formation of the neutron star. In any case, the shock, even if revived, cannot bridge the vacuum of the gap that nikkkom points out is left by the formation of the neutron star. It could very well be that the supernova explosion occurs after the formation of the neutron star rather than being caused by it!

 

[phyzguy]

I think nikkkom may be on to something here. All simulations concentrate on the inner region surrounding the neutron star in an attempt to find a way to revive the stalled shock. Neutrino transport and magnetohydrodynamic effects are mentioned repeatedly yet no mention is made of the energy released in gamma rays from the formation of the neutron star. In any case, the shock, even if revived, cannot bridge the vacuum of the gap that nikkkom points out is left by the formation of the neutron star. It could very well be that the supernova explosion occurs after the formation of the neutron star rather than being caused by it!

Like I said, I strongly doubt that the people who are doing these simulations are leaving out these effects. Also, there is not a "vacuum" left behind when the neutron star collapses. The very high pressure gas behind will follow the surface of the collapsing neutron star. There may briefly be a lower pressure region, but it won't last long. But feel free to set up your own simulation. If you and nikkkom are right, there is a major publication to be had!

 

[nikkkom]

Like I said, I strongly doubt that the people who are doing these simulations are leaving out these effects. Also, there is not a "vacuum" left behind when the neutron star collapses. The very high pressure gas behind will follow the surface of the collapsing neutron star. There may briefly be a lower pressure region, but it won't last long.

You are right. It's far from being a vacuum. I'd wager it will not even fall as low as ~Earth's air density. However, there definitely will be a lower-density region:

The collapse starts when the very center of the star reaches conditions when "neutronisation" (p+e -> n+ve) causes loss of electron degeneracy pressure. So, the very center (a small region, maybe 50 km across) starts shrinking (due to pressure of overlaying matter, not due to its own gravity, which is relatively negligible).

Layers above the center start "falling" onto the shrinking central region - which _inevitably_ means that the density above these layers decreases.

But feel free to set up your own simulation. If you and nikkkom are right, there is a major publication to be had!

Tempting, but I'm not planning a sudden career change from Linux hacker to physicist... I love my current job/hobby...

 

[Ken G]

I think nikkkom may be on to something here. All simulations concentrate on the inner region surrounding the neutron star in an attempt to find a way to revive the stalled shock. Neutrino transport and magnetohydrodynamic effects are mentioned repeatedly yet no mention is made of the energy released in gamma rays from the formation of the neutron star. In any case, the shock, even if revived, cannot bridge the vacuum of the gap that nikkkom points out is left by the formation of the neutron star. It could very well be that the supernova explosion occurs after the formation of the neutron star rather than being caused by it!

There are a few claims here I don't understand. First of all, radiation is generally included in simulations, so I don't understand why you would say the gamma rays are not included. Instead, I think it is found they are not important. Also, I don't see why you would say the shock needs to bridge any gaps-- the point of the shock is to turn downward falling gas into upward moving gas, so by the time the shock would reach very low density gas it would have already done its job, no need to "bridge" anything. Finally, the energetics required to blast the material out into space at high speed can only be provided by the gravitational energy of forming the neutron star, so I cannot see why it could be claimed that the formation of the neutron star does not "cause" the supernova.

 

[nikkkom]

There are a few claims here I don't understand. First of all, radiation is generally included in simulations, so I don't understand why you would say the gamma rays are not included.

I'm sure gamma rays are included... as long as they are in the simulated volume in the center of the star. What simulation assumes to happen when they escape from this volume?

We know the rest of the star is transparent to neutrinos which are leaving the simulated volume (thus simulation can just ignore them), but it's definitely not true for gamma gays.

Also, I don't see why you would say the shock needs to bridge any gaps-- the point of the shock is to turn downward falling gas into upward moving gas, so by the time the shock would reach very low density gas it would have already done its job

Not necessarily. Shocks cause material to accelerate in the direction of the shock, yes, but here material is infalling. If shock is not strong enough, it may fail to reverse the material's velocity as it passes through it. The material may still fall down, just slower.

 

[alantheastronomer]

There are a few claims here I don't understand. First of all, radiation is generally included in simulations, so I don't understand why you would say the gamma rays are not included. Instead, I think it is found they are not important. Also, I don't see why you would say the shock needs to bridge any gaps-- the point of the shock is to turn downward falling gas into upward moving gas, so by the time the shock would reach very low density gas it would have already done its job, no need to "bridge" anything. Finally, the energetics required to blast the material out into space at high speed can only be provided by the gravitational energy of forming the neutron star, so I cannot see why it could be claimed that the formation of the neutron star does not "cause" the supernova.

We may be talking about the same thing here. Correct me if I'm wrong, but when you talk about the shock wave, do you mean a shock wave formed by the material falling onto the neutron star? All numerical simulations of supernova explosions are concerned with shockwav e es moving outward from within the forming neutron star as the means for the explosion. Early simulations had found that the "accretion shock" of infalling material from the envelope would stall above the surface of the neutron star and would not move outward in radius to provide an explosion, rather piling up on the surface of the neutron star and eventually forming a black hole. So the gap I was referring to is the void left by the contraction of the core into a neutron star. The overlying layers of the envelope do not infall immediately even though they are no longer supported against gravity, they are supported by radiation pressure from infrared radiation released by the gravitational binding energy of the neutron star, similarly to the layers pushed out in the formation of a red giant star. A shockwave needs a medium to propagate in, without it the shockwave produced by the formation of the neutron star finds the void produced by the formation of the neutron star insurmountable. Now while the energetics of the neutron star's gravitational binding energy is comparable to the energy of the supernova explosion, they aren't necessarily connected. For instance, in the aforementioned simulations, nuclear reactions in the infalling material, incredible as it may seem, were not included. It could be that there were computing constraints, but I can only surmise.

 

[nikkkom]

The overlying layers of the envelope do not infall immediately even though they are no longer supported against gravity, they are supported by radiation pressure from infrared radiation released by the gravitational binding energy of the neutron star

Just formed neutron star's emissions peak not in the IR, rather somewhere at about 10 MeV, which are gamma rays.

Now while the energetics of the neutron star's gravitational binding energy is comparable to the energy of the supernova explosion, they aren't necessarily connected. For instance, in the aforementioned simulations, nuclear reactions in the infalling material, incredible as it may seem, were not included.

Nuclear reactions are releasing (or in some cases, absorbing) on the order of 1%-0.1% of rest mass-energy of their ingredients, whereas released gravitational binding energy during NS formation is ~15% of its rest mass-energy.

 

[stefan r]

... The material may still fall down, just slower.

The statement is ambiguous. Clearly a lot of mass falls down. The overall average is a drop.

This video uses the example of a stacked basketball, bouncy ball, and golf ball. The stack's mass is lower after the bounce. The golf ball, however, can be seen up at 8.5 meters.

 

[Ken G]

aEarly simulations had found that the "accretion shock" of infalling material from the envelope would stall above the surface of the neutron star and would not move outward in radius to provide an explosion, rather piling up on the surface of the neutron star and eventually forming a black hole.

Yes, but that shock would have nothing to do with any gap left behind by the infalling gas. The trick is to get the infalling gas to turn into outflying gas, which is what they need the shock to do. What happens to the gas above the infalling gas is of little energetic significance, the energy scale is set by the neutron star gravity and is all happening very deep down where the density is very high.

Now while the energetics of the neutron star's gravitational binding energy is comparable to the energy of the supernova explosion, they aren't necessarily connected.

They have to be connected, they are the only energies at that scale in the entire problem.

For instance, in the aforementioned simulations, nuclear reactions in the infalling material, incredible as it may seem, were not included.

The nuclear physics in this process is neutronization and photodisintegration of the nuclei, which is a sink for energy, not a source, and is included in the simulations. You can think of it as basically undoing all the fusion the star has undergone up to that point. The energy for that undoing, as well as the energy from the explosion, all comes from the gravitational energy of creating the neutron star. It's the only energy of significance.

 

[Ken G]

I'm sure gamma rays are included... as long as they are in the simulated volume in the center of the star. What simulation assumes to happen when they escape from this volume?

The action of importance is happening where the energy scale is important, which is way down deep where the neutron star forms and the density is high. What is happening much farther out in the gravitational potential is a paltry energy in comparison, and is of no particular significance to getting an explosion.

Not necessarily. Shocks cause material to accelerate in the direction of the shock, yes, but here material is infalling. If shock is not strong enough, it may fail to reverse the material's velocity as it passes through it. The material may still fall down, just slower.

Yes, but all of that is happening to the infalling gas, not to any void left behind by the infalling gas.

 

[nikkkom]

The action of importance is happening where the energy scale is important, which is way down deep where the neutron star forms and the density is high. What is happening much farther out in the gravitational potential is a paltry energy in comparison,

Yes.

and is of no particular significance to getting an explosion.

No. If formation of NS fails to create an outgoing shock wave and movement of matter from its surface, it's not the end of the story. Now we need to examine what happens to the rest of the star. Maybe there is _another_ process which will make the rest of the star explode?

Let's look at "paltry energy".

Luminosity. The newborn NS temperature is upwards from 100 billion Kelvins. This means that surface luminosity, assuming blackbody radiation, is (1e11/6000)^4 times higher than Sun's surface. The surface of NS is smaller though. NS radius is ~10km, Sun is ~700k km, ratio of surface area is (700000/10)^2. Thus, NS luminosity is (1e11/6000)^4 / (700000/10)^2 = 1.57e19 solar, if NS is at 100 billion K. If it's at the upper end of the range, 1000 billion K, then its a ten thousand times more. That's quite large amounts of "paltry" energy.

Radiation of these intensities drive radiation-dominated shock waves ("Marshak waves") in matter by itself - it does not need any "rebound shock wave" to do so. Essentially, it's a photon shock wave, where matter is merely "along for the ride". It will propagate away from NS (much) faster than local speed of sound through a low-density region, until it impacts on a not-yet disturbed, dense layers of the star (walls of the "void").

(On a much smaller energy scale, this is used in thermonuclear weapons to drive implosion of the secondary. The X-rays from the primary, of paltry 100 million K, impinge on a cylinder of uranium, evaporate its surface, creating ablation-driven shock, about 50-75 times stronger than radiation pressure alone. The achievable shock velocities are on the order of 500 km/s.)

Marshak wave velocity is approximately proportional to T^2. To get about 50000 km/s wave velocity, the temperature of the wave hitting the walls of the "void" needs to be at least ~1 billion K. Looks very plausible to me.

See
http://nuclearweaponarchive.org/Nwfaq/Nfaq3.html (particularly section 3.5.6 Thermal Waves with Hydrodynamic Flow)
http://nuclearweaponarchive.org/Nwfaq/Nfaq4-4.html

 

[Ken G]

No. If formation of NS fails to create an outgoing shock wave and movement of matter from its surface, it's not the end of the story.

Again, whether or not there is an outgoing shock has to do with the infalling gas, and has nothing to do with the void left behind by the falling gas. That's the point here.

Luminosity. The newborn NS temperature is upwards from 100 billion Kelvins. This means that surface luminosity, assuming blackbody radiation, is (1e11/6000)^4 times higher than Sun's surface.

The luminosity of a supernova is paltry, it is not a significant part of the energy balance. It has nothing to do with the explosion.

 

[alantheastronomer]

Again, whether or not there is an outgoing shock has to do with the infalling gas, and has nothing to do with the void left behind by the falling gas. That's the point here.
The luminosity of a supernova is paltry, it is not a significant part of the energy balance. It has nothing to do with the explosion.

nikkkom is referring to the luminosity of the neutron star, not the supernova. The idea is that the high energy radiation from the newly formed neutron star forms an ablation shock in the infalling gas.

 

[Ken G]

That isn't what happens. The shock is not formed by any luminosity, it is formed from kinetic energy, neutrino energy, and perhaps some magnetic effects. It remains difficult to understand the shock, but several processes, including radiation, can be ruled out as unimportant. There does not seem to be any reason to think that radiation, or the void left behind by the infalling gas, play any key role in a supernova explosion-- instead, follow the energy.

 

[nikkkom]

The shock is not formed by any luminosity

Fusion weapons designers would most certainly disagree.

 

[alantheastronomer]

That isn't what happens. The shock is not formed by any luminosity, it is formed from kinetic energy, neutrino energy, and perhaps some magnetic effects. It remains difficult to understand the shock, but several processes, including radiation, can be ruled out as unimportant. There does not seem to be any reason to think that radiation, or the void left behind by the infalling gas, play any key role in a supernova explosion-- instead, follow the energy.

So far all efforts to model the explosion by these means have failed to produce an outgoing shock wave. The void is left behind by the formation of the neutron star, not by the infalling gas. It's important because it defeats any shock wave from getting from the neutron star out to the inner layers of the rest of the star. Simple geometry attenuates any shock wave.

 

[Ken G]

Fusion weapons designers would most certainly disagree.

They woudn't disagree that this thread is about core-collapse supernovae, so they would know it has nothing to do with fusion design.

 

[Ken G]

So far all efforts to model the explosion by these means have failed to produce an outgoing shock wave.

That's not entirely true, some simulations have succeeded in getting an outgoing shock and an explosion using that physics. Rotation seems to matter, perhaps magnetic fields matter, certainly neutrinos matter. They don't think they are missing any important luminosity effects, because the energy is way short, though I agree it is still strangely difficult to get it to work.

The void is left behind by the formation of the neutron star, not by the infalling gas.

Well, the neutron star is formed by infalling gas, the shock merely decides what of that infalling gas ends up in the neutron star and what ends up getting blasted out. The void left behind comes outside all the action of interest, including the shock.

It's important because it defeats any shock wave from getting from the neutron star out to the inner layers of the rest of the star.

It isn't the void that prevents that, voids don't really do much of anything. The defeat of the shock wave happens inside the infalling gas, it's all over by the time it gets to the void. The difficult part is to keep the shock from turning into a standing accretion shock, which is a stalled shock that cannot make headway against the gas falling into it. It requires some additional push from underneath to get it going again, so that it can advance through the infalling gas and energize the stellar envelope. The leading idea is that this additional push comes from neutrino heating, but it doesn't work in one dimension because of the need for instabilities. This seems to be the problem that hampered 1D simulations, they didn't realize the crucial role of higher-dimensional instabilities and convection.

 

[nikkkom]

They woudn't disagree that this thread is about core-collapse supernovae, so they would know it has nothing to do with fusion design.

If in fusion design a ~100 million kelvin photon gas can drive a ~1000km/s shock that compresses LiD to 1000 times its STP density, why do you completely discount effects of ~10 billion kelvin photon gas on star's internal layers?

 

[nikkkom]

The defeat of the shock wave happens inside the infalling gas, it's all over by the time it gets to the void.

Ok, please try to answer this. Suppose the shock is "defeated" and all the dense core material ends up falling on the proto-NS and all of it ends up being the material of the newly-created NS. This new NS is more than 100 billion kelvins hot. Above it, there is a relatively low density gas, which continues to fall on it from the farther-up, less dense layers of the star.

What will be happening in the next ~10 seconds?

 

[Ken G]

If in fusion design a ~100 million kelvin photon gas can drive a ~1000km/s shock that compresses LiD to 1000 times its STP density, why do you completely discount effects of ~10 billion kelvin photon gas on star's internal layers?

Because of decades of supernova research, not done by me, but I am aware of it. Perhaps you need to be.

 

[nikkkom]

Because of decades of supernova research, not done by me, but I am aware of it.

So am I aware of it too.

The simulations have computing power restrictions. The simulated volume can't be too big; the time the simulation runs can't be too long.

Researchers make sensible decisions based on the availability of CPU power.

If simulating a (400km)^3 cube for 1 second takes 2 days, then simulating (4000km)^3 cube for 10 seconds on the same hardware would take 5000 days (~15 years). It makes sense to _not_ try that as the first (or second, or tenth) attempt to figure out why simulation does not match expectations. "Maybe we overlooked something. Maybe it's magnetic fields?" etc. Completely sensible. I'd do the same. I don't want to wait 15 years for one test run! And I'm not ready to give up on my codes simply because they didn't work in the first few tries. Bugs are a fact of life.

However, maybe the simulations _were_ mostly correct. Maybe they do show what really happens in (400km)^3 cube for 1 first second. Maybe star's explosion is not generated in this volume.

I'd be happy to hear this was looked at, and shown not to be the case, by people who did work on it.

 

[Ken G]

So am I aware of it too.

The simulations have computing power restrictions. The simulated volume can't be too big; the time the simulation runs can't be too long.

Of course, but none of that justifies an expectation that a region where there is no material and no energy has anything to do with getting an explosion. Also, none of that justifies thinking that radiation is important, when simulations include radiation and find that it is not important.

 

[JMz]

Fusion weapons designers would most certainly disagree.

Yes, but they have no neutrinos to make use of, right?

 

[alantheastronomer]

Yes, but they have no neutrinos to make use of, right?

Of course they do...p+p->d+e*+v, but the neutrinos aren't dense enough to make a difference in fusion weapons, while they are thought to be critical in creating supernovae explosions.

 

[stefan r]

Of course they do...p+p->d+e*+v, but the neutrinos aren't dense enough to make a difference in fusion weapons, while they are thought to be critical in creating supernovae explosions.

There is no p+p in any fusion weapons created by humans.