Evidence of an asteroid encountering a pulsar

  1. The paper goes on to describe asteroids "formed from supernova fallback material" falling into the pulsar. Why would the asteroids be formed after the supernova? More importantly, how could an asteroid form from supernova remnants?

    If a star with less than ~9 solar masses had planets and asteroids, then those planets and asteroids would still be there after the supernova. Supernova ejecta are charged particles traveling in excess of 10,000 km/s for Type I & II SNe (less than 8,000 km/s for Type Iax SNe). That may be sufficient to strip off layers of atmosphere from planets, but hardly capable of destroying rocky asteroids or planets. Naturally, a star that goes from 9 solar masses to less than 3 solar masses in just a second or two is going to dramatically change the orbits of any object in that solar system, but the supernova would not destroy those objects.

    I do not doubt that the pulsar in question is being pelted by asteroids, as they claim. However, I do dispute that these asteroids were formed after the supernova. I think all planets and asteroids around neutron stars already existed before the star went supernova.

    Planets and asteroids do not form directly from supernova remnants, there must be another trigger involved first. Such as another supernova blast from another star that compresses the extremely disbursed supernova remnant gases. Without that trigger the supernova remnants are too disbursed, and getting further disbursed as it continues to travel, to form anything solid.

    Does anyone think it is possible for a supernova, without any outside influences to trigger a collapse of the supernova remnants, could form asteroids or planets after the supernova? If so, I would be interested in knowing how it would be possible.
  2. jcsd
  3. mfb

    Staff: Mentor

    The first rule of supernovae: it does not matter how you imagine them, they are much more violent.[1]

    A typical supernova emits something like 1044 J. For an object at a distance of 1 AU, this corresponds to 4*1021 J/m^2. If you apply this to an object the size and mass of earth (this would be inside the star, the numbers are just for illustration), you get 7 GJ/kg. Even Jupiter with its larger distance and mass would receive 100 MJ/kg. Asteroids have a larger surface to mass ratio than planets, so they get larger values than planets.

    This is enough to completely vaporize everything, with some orders of magnitude as safety factor.
    Last edited: Feb 22, 2014
  4. For some reason, everyone who quotes this rule, follows it up with an overstatement of supernova power.

    The escape velocity at the surface of Jupiter is 59.5 km/sec. A kg at the surface of Jupiter needs 1/2 mv^2 = (1/2) (59.5*10^3)^2 = 1.7 GJ to escape. It should be clear that with only 100MJ/Kg of energy from the supernova, Jupiter can't evaporate.
  5. I understand that supernovas are extremely powerful. I have also read where they produce 1044 Joules of energy. However, that is 1044 Joules of energy in absolutely every direction, while only a tiny fraction of that energy will be along the ecliptic where the planets form. You are placing the entire force of the supernova against a planet, and while that will assuredly destroy a planet, it simply is not possible to focus the entire supernova's energy into the direction of any one planet.

    7 x 109 Joules is not enough to vaporize Earth. A directed CME will impact Earth with as much as 6 x 1025 Joules of energy, and that barely disturbs the atmosphere.
    Last edited: Feb 22, 2014
  6. SteamKing

    SteamKing 11,044
    Staff Emeritus
    Science Advisor
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    It's not so much the power radiated by the supernova, but what comes out of one. When the core collapses and the envelope surrounding it is blown off, additional nuclear reactions caused by the shock wave propagating thru the envelope create all sorts of heavy nuclei, starting with iron and nickel and proceeding to higher atomic numbers. Iron and nickel, at least based on samples of meteorites recovered on earth, are quite common materials with which to build an asteroid which has a chance of staying in one piece. Sure, this stuff will be hot thermally and radioactively, but 'vaporize' does not mean 'destroy'. This material will cool eventually. After all,

    'We are star dust, we are golden, we are billion year old carbon ...'

    Last edited by a moderator: Sep 25, 2014
  7. Bandersnatch

    Bandersnatch 1,572
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    You misunderstood mfb's calculations. He already took into account the dispersal of the total emitted energy by calculating the energy per square metre at the distance of 1AU. It's the 4*1021J/m2.
    Multiply that by the cross-sectional area of Earth (~1013 square metres) and you end up with ~4*1034Joules of energy absorbed by Earth. The gravitational binding energy of Earth is ~2*1032Joules, so you have a good two orders of magnitude overkill there.

    The 7GJ/kg is the energy received by every kilogram of Earth's mass.
  8. That is a very valid point that I had not considered. That is what really separates a CME from a supernova (besides the amount of energy of course). You are not going to get those heavy elements in a CME because it only consists of the hydrogen surface layer. In a supernova you have the entire iron core, and all those other elements heavier than iron traveling in excess of 10,000 km/s. That is going to do some serious damage.
    Last edited by a moderator: Sep 25, 2014
  9. I did indeed misunderstand. Thanks for the clarification.
  10. SteamKing

    SteamKing 11,044
    Staff Emeritus
    Science Advisor
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    To be clear, the iron core in a supernova star stays at the center of the star while it collapses. Any iron which you would find in the remnants of the star after the stellar envelope has been ejected were produced by the reactions taking place outside of the core. When the core collapses, most of that iron is turned into the degenerate matter which forms the succeeding neutron star. It's not clear if any iron from the collapsing core is spun off into space when the core spins up due to conservation of angular momentum.
  11. mfb

    Staff: Mentor

    You are right, I got the powers of 10 wrong somehow.

    The last parts need less energy than the first parts, but this does not help (~factor of 2):
    2*1035 J hit an object with the same parameters as Jupiter, and its total gravitational binding energy is roughly 2*1036 J.

    Anyway, asteroids are way smaller than Jupiter, so due to the cube/square-scalings there we have the overkill, even if we take into account that the 1044 J include neutrinos (?).
  12. Given that a stereotypical Type I or II SNe that generates 1044 Joules will pretty much destroy anything within 5 AUs. What do you think is the likelihood that supernova ejecta slowed down sufficiently while still in the solar system to reform into solid asteroids which then later fell back into the pulsar? Which is what the paper suggests.

    Would it not be more likely that the objects destroyed by the SNe blast would be the debris falling into the pulsar?

    I have always been under the impression that SNe ejecta will continue, for light years, until some external force (like another nearby SNe blast) causes the nebula gases to collapse.
  13. mfb

    Staff: Mentor

    I don't know. I could imagine that objects far away (like our Kuiper belt or even the Oort cloud) could get influenced significantly, but without getting completely destroyed. If they get highly eccentric orbits, something interesting could happen.
  14. Considering the star just underwent a sudden diet that shed ~6 solar masses, any object orbiting that star would have its orbit significantly disturbed. However, that does not explain the paper's assertion, that the SNe ejecta somehow managed to slow down enough while still within the solar system to form asteroids, which then later crashed back into the pulsar. Does that seem plausible to you?
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