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I What % of the heavy elements are produced by kilonovas vs. supernovas?

  1. Nov 1, 2017 #1
    So the recent neutron star merger event showed that most of the heavy elements (gold, platinum, uranium, etc.) are produced in kilonovas. But with neutron star mergers so rare, there can't be that many kilonovas. Prior to this I always used to think they were mostly produced in supernovas. The chances that a solar system was seeded by a supernova seem much higher than one seeded by a kilonova. What proportion of heavy elements are produced in kilonovas vs. supernovas?
     
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  3. Nov 2, 2017 #2
    The mergers do not need to be that common. Compare magnesium at 6 x 10-4 abundance in the universe to osmium at 3 x 10-9. Some of the material ejected by core collapse supernovas is still hydrogen and helium.

    The Milkyway's core has several million solar masses. I do not know how much of that came from neutron stars.
     
  4. Nov 4, 2017 #3
    I am at a loss to understand how two objects made of degenerate matter can produce any element by merging. When all the protons and electrons have been combined to make neutrons, how can any other element be made after that?
     
  5. Nov 4, 2017 #4
    Because when you tear apart the neutron star, neutrons would start beta decay.
     
  6. Nov 4, 2017 #5
    Okay, but a merger of two neutron stars does not tear apart either of the neutron stars. Some material may be expelled - and that is certainly true in the case of the GRBs, but the majority of the degenerate matter is going to be combined to either form a larger neutron star or possibly a black hole if there is sufficient mass. It is nothing like supernovae, which do blow themselves apart.
     
    Last edited: Nov 4, 2017
  7. Nov 4, 2017 #6
    Far from all types of supernovas result in the star blowing itself up completely.
     
  8. Nov 4, 2017 #7
    That's an interesting point.
    Even if there were in mergers, unknown conditions which lead to faster neutron decay, all that is going to do is generate hydrogen (I suppose)
     
  9. Nov 4, 2017 #8

    phyzguy

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    You're correct in that the majority of the degenerate matter remains part of the larger neutron star or collapses into a black hole. The estimates of the recently observed NS-NS merger were that only between 0.1% and 1% of the original neutron star matter was expelled. However, that tiny fraction is enough to explain the amount of heavy elements here on Earth. Elements like gold are billions of times less abundant than hydrogen.
     
  10. Nov 4, 2017 #9

    phinds

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    Good point but doesn't address the core of the OP's question.
     
  11. Nov 4, 2017 #10

    phyzguy

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  12. Nov 4, 2017 #11
    ... but neutron decay does not have to mean Hydrogen?
    It could produce any element if unconfined quarks are allowed?
     
  13. Nov 4, 2017 #12

    phyzguy

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    Is this in response to my Post #8? I didn't mean to imply that the decompressed neutron star matter gives rise to hydrogen. It gives rise to heavy elements. My point was that heavy elements like gold are very rare, so if only 0.1% of the neutron star matter is ejected and converted to heavy (r-process) elements, that is enough to explain the observed abundance of these elements.
     
  14. Nov 4, 2017 #13
    Thanks for your link about the r process.
     
  15. Nov 5, 2017 #14
    It is extremely difficult to believe that all the gold, platinum, uranium, etc. in the universe was created only by neutron star mergers. Considering that neutron stars themselves are already rare, and mergers of neutron stars are exceedingly rare, and you are talking about only between 0.1% and 1% of that ejected degenerate material creating everything heavier than iron in the universe. I realize that elements heavier than iron are not common, but even a billion times rarer than hydrogen is still more common than the process I just described. That process could not possibly account for all the elements heavier than iron in the universe.

    FYI, according to the paper below the NS-NS merger ejected between 0.03 to 0.05 solar masses of material, which puts the ejecta slightly higher than 1% of the combined mass.

    Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger - Nature 551, November 2017
     
    Last edited: Nov 5, 2017
  16. Nov 5, 2017 #15
    When the neutron stars merge, the looser upper crusts of these objects are not as tightly coupled with the rest of the object and they have a potential of getting loose into space. These loose pieces or shards of neutrons are no longer gravitationally bound tightly enough to remain as degenerate matter, so they'll (explosively) break up into regular atomic nuclei. Then some of the neutrons will beta decay into protons and electrons along the way too, so they won't stay neutrons forever.

    Each of the neutron shards might be of various sizes, with many different numbers of nucleons per shard. They beta decay, and fission down to the most stable configurations in the end.
     
  17. Nov 5, 2017 #16
    So one of the articles I've recently read is this one:
    Neutron star collisions may have created most of the gold in the universe | Popular Science

    There is a graphic in there, which shows which percentage of elements come from what types of explosions:

    periodic_table.jpg

    There are 6 types of origins mentioned: (1) Big Bang, (2) cosmic ray fission, (3) merging neutron stars, (4) exploding massive stars, (5) dying low-mass stars, and (6) exploding white dwarfs. Now #1 through #4 are pretty clear what they are, but #5 & #6 seems a little unclear, wouldn't these be the same things? The whole graphic is a bit confusing. It says high mass star explosions produce only the elements upto Zr, Zirconium. All heavier elements are produced by either dying low-mass stars, or neutron star mergers! This makes no sense whatsoever!
     
  18. Nov 5, 2017 #17
    Thanks for the explanation. snorkack also explained that process to me. It would certainly appear to be one method for creating elements heavier than iron, but considering the rarity of the event and the small amount of mass they eject during that merger there has to be another process to explain the elements heavier than iron.

    Even supernovae are rare, on average only one supernova per galaxy per century, and yet every neutron star is the result of a supernova. Supernovae eject vast quantities of material (particularly Type II supernovae), many times the mass of our sun and they do so at relativistic speeds. Why does it have to be one or the other, could not both processes contribute to the elements heavier than iron in the universe?
     
  19. Nov 5, 2017 #18
    Neutron star mergers might have been more common in the early universe, maybe? Back in the first generation of stars, pretty much all of them were massive enough to produce neutron stars or black holes after they went supernova. Then I'm guessing that these early galaxies were filled with heavy gas clouds (unlike today's galaxies) that might have provided a braking force against speeding neutron stars, and then they might have gotten caught up within gas clouds and several neutron stars might have found themselves in gravitational locked to each other, and they quickly merged (took millions of years rather than billions). Seeding the early universe quickly.
     
  20. Nov 5, 2017 #19
    By "dying low-mass star" I presume they mean a star smaller than ~3 solar masses, but larger than ~0.75 solar masses, such as our sun. Stars smaller than ~0.75 solar masses haven't come close to dying yet. An "exploding white dwarf" would refer to either a Type Ia or Iax SNe. In the case of the Type Ia SNe the white dwarf is completely obliterated. In the case of Type Iax SNe some of the white dwarf remains.

    It was also my understanding that when "low-mass stars" die they begin to lose their gravitational hold on their outer layers and the material is ejected to form planetary nebula. However, that ejection of material is not at relativistic speeds.
     
  21. Nov 5, 2017 #20
    No. It would generate helium 3.
    A neutron undergoes beta decay - half-life 10 minutes.
    The resulting proton captures a neutron - D is stable.
    D captures a neutron... and T cannot capture another neutron. Because H-4 would be an unbound state - any extra neutrons just bounce off the tritons until they undergo free neutron beta decay and the one proton formed binds another 2 neutrons.
    T is radioactive... with 12 years halflife, eventually forming He-3.
    He-4 is also at dripline - neutrons bounce off with no effect because the third neutron of He-5 is unbound.
    But now suppose that somehow there are 3 protons together. Say stable Li-7.
    What next?
    Li-8 and Li-9 are both bound. Only Li-10 is not.
    The half-lives are Li-8 840 ms, Li-9 178 ms. Drastically faster that the 600 s half-life of neutrons
    And Li-9 forms (50 % of times) stable Be-9.
    Repeat with Be. First unbound species Be-13. So neutron dripline at Be-12. Halflife 21 ms.
    Etc. All the way to over 100 protons.
     
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