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

  1. Jun 4, 2018 #21

    Ken G

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    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.
     
  2. Jun 4, 2018 #22
    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.
     
  3. Jun 5, 2018 #23

    Ken G

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    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.
     
  4. Jun 5, 2018 #24
    Fusion weapons designers would most certainly disagree.
     
  5. Jun 5, 2018 #25
    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.
     
  6. Jun 5, 2018 #26

    Ken G

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    They woudn't disagree that this thread is about core-collapse supernovae, so they would know it has nothing to do with fusion design.
     
  7. Jun 5, 2018 #27

    Ken G

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    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.
    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 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.
     
    Last edited: Jun 5, 2018
  8. Jun 6, 2018 #28
    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?
     
  9. Jun 6, 2018 #29
    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?
     
  10. Jun 6, 2018 #30

    Ken G

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    Because of decades of supernova research, not done by me, but I am aware of it. Perhaps you need to be.
     
  11. Jun 6, 2018 #31
    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.
     
  12. Jun 6, 2018 #32

    Ken G

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    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.
     
  13. Jun 6, 2018 #33

    JMz

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    Yes, but they have no neutrinos to make use of, right?
     
  14. Oct 13, 2018 #34
    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.
     
  15. Oct 15, 2018 #35

    stefan r

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    There is no p+p in any fusion weapons created by humans.
     
  16. Oct 15, 2018 #36
    So what reaction is used in fusion weapons?
     
  17. Oct 15, 2018 #37
    It doesn't have anything to do with getting an explosion, that's the whole point...the failure of the core bounce mechanism to produce an outward going shockwave means there's a void left behind between the newly formed neutron star and the stellar envelope. The outer layers do not fall onto the neutron star immediately; they fall on a free fall timescale. This means there's only a few moments for some mechanism to produce an explosion. Gamma radiation from the formation of the neutron star sounds like a viable possibility.

    All researchers who've done computer simulations have NOT included radiation in their calculations. Not Bethe, not Wilson at Los Alamos, not Stirling Colgate, not Stan Woosley at Santa Cruz, not Arnett, not Rood, not Adam Burrows. Instead, they all assume, without any proof, that 99% of the energy is carried away by neutrinos and they ignore radiation completely.

    There's also another problem with the core collapse scenario - If the inner core collapses and the outer core is blown away by the shock wave, then only a fraction of the 1.4 solar mass iron core is left as a neutron star. Observations of neutron star masses compiled by Lattimer find they vary only from 1.2 to 1.4 solar masses, so the theory doesn't agree with observations.

    Also, if the explosion is going to be powered by the gravitational potential of the neutron star, then the outer lying material has to first access that potential - it needs to fall down the potential well of the neutron star in order to extract that energy.

    Fortunately, that doesn't need to happen. Using a very crude approximation, by the mass-luminosity relation, a 15 solar mass star has a luminosity of 2x10^38 erg/sec. While the diffusion timescale for photons is very roughly 300,000 years. This means there's roughly 10^51 ergs stored in the star's interior that could be released all at once in a supernova explosion without having to access the gravitational potential well. This is also the amount of energy that is observed in supernovae explosions.
     
  18. Oct 17, 2018 #38
    Dutirium and or tritrium
     
  19. Oct 17, 2018 #39

    anorlunda

    Staff: Mentor

    This article is from the 70s, so it may be dated, but it is a wonderful description of the time evolution by Hans Bethe and Gerald Brown.
    http://www.cenbg.in2p3.fr/heberge/EcoleJoliotCurie/coursannee/transparents/SN - Bethe e Brown.pdf

    A couple of interesting points from that article.
    • The time to maximum density in the collapse is not several seconds, it is on the order of 5 ms.
    • Densities are so great that the infalling materials are opaque to neutrinos. Even thermonuclear explosions do not duplicate that condition.
     
  20. Nov 6, 2018 #40
    The article you're referencing is referring to the time for the core to collapse to neutron star densities, NOT the time for the rest of the stellar envelope to freefall onto the newly formed neutron star. That timescale, for a neutron star of 1.5 solar masses and a distance to the envelope of 400,000 km. is given by the formula t=(d^3/(2GM))^1/2 ignoring general relativistic effects, turns out to be about 400 sec.
    While the infalling nuclear material is opaque to neutrinos, all computational simulations thus far have failed to produce an explosion due to neutrino pressure. Thermonuclear explosions don't need to resort to that condition in order to produce an explosion; nikkkom's whole point is that if a thermonuclear explosion can be achieved at temperatures of millions of degrees due to gamma ray heating and photon pressure, why aren't temperatures of billions of degrees relevant for supernova explosions?
    Computer simulations that model the formation of the neutron star from the stellar core occur at timescales of nanoseconds, any modelling of the physics in the stellar envelope would occur on a hydrodynamic timescale of milliseconds. Thus for any one timestep of the envelope, a thousand timesteps of the core would have to be calculated. For a fully three dimensional model that would be increased to a billion, not to mention the increased spatial resolution...
    So out of the many papers modeling the supernova problem, I've only found two that look at the physics in the stellar envelope;
    One, by Stirling Colgate, that found that energies from neutrinos produced a high pressure, low density region in the envelope that might be susceptible to Rayleigh-Taylor overturn instability producing an outward flow, and
    Second, a paper by Stan Woosley which found that a combination of angular momentum conservation and nuclear reactions in the oxygen layer produced an outward motion. This study was done in the early eighties and I thought it was an extremely promising avenue for further investigation, and I thought that with the increase in computing power coming, that he would pursue it further, but for some reason he never did...
     
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