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Can white dwarfs have iron cores?

  1. Mar 28, 2015 #1
    Wikipedia only talks about helium and carbon-oxygen white dwarfs... I understand that on a red giant you only get fusion up to oxygen, but I though that maybe if you had a red supergiant which produced iron in its core, and when it exploded as a supernova it threw off enough mass to form a white dwarf with an iron core, would this be possible???
     
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  3. Mar 28, 2015 #2

    Drakkith

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    A supernova is powered by the collapse of the core into a neutron star, so the very fact that it goes supernova means that you cannot have a white dwarf left over.
     
  4. Mar 28, 2015 #3

    mathman

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  5. Mar 28, 2015 #4

    Chronos

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    Mathman is correct. White dwarfs are not massive enough to create iron. The are primarily composed of oxygen and carbon with a sprinkling of other elements..
     
  6. Mar 28, 2015 #5

    Drakkith

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    That wasn't what the OP was suggesting. She was thinking that a core-collapse supernova could throw off enough mass to avoid collapsing into a neutron star and instead form a white dwarf. But that's impossible since the supernova is powered by the collapse of the core in the first place.

    Joanna, the core collapse occurs in a matter of milliseconds. The shockwave that throws off the outer layers takes much, much longer. In fact, the shockwave is produced by two methods; absorption of neutrinos produced during the collapse, and infalling material rebounding off of the surface of the already-formed neutron star.
     
  7. Mar 28, 2015 #6

    Chronos

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    Agreed, Drakkith, I should read more carefully before writing. Actually, the question is interesting and turns out to be plausible. See http://arxiv.org/abs/1208.5069 Failed-Detonation Supernovae: Sub-Luminous Low-Velocity Ia Supernovae and Their Kicked Remnant White Dwarfs with Iron-Rich Cores, for discussion.
     
  8. Mar 28, 2015 #7

    Drakkith

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    Wow, you're right. Joanna, just to be clear, Chronos' link talks about the formation of a white dwarf with an iron-rich core from a failed type 1a supernova. This is not a core collapse supernova like you asked about in your post, which is a type 2 supernova. The formation of a white dwarf from a type 2 supernova cannot happen.
     
  9. Mar 28, 2015 #8

    marcus

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    I disagree with Chronos and Mathman here
    http://en.wikipedia.org/wiki/White_dwarf#Type_Ia_supernovae

    This section 4.4 of the article says that a mechanism involving collision may produce white dwarfs with iron core.
    ==quote==
    Type Ia supernovae, that involve one or two previous white dwarfs, have been proposed to be a channel for transformation of this type of stellar remmant. In this scenario, the carbon detonation produced in a Type Ia supernova is too weak to destroy the white dwarf, expelling just a small part of its mass as ejecta and producing an asymmetric explosion that kicks the star at high speeds as a Hypervelocity star. The matter processed in the failed detonation is re-accreted back by the white dwarf with the heaviest elements such as iron falling to its core and accumulating there.[114]

    These iron-core white dwarfs would be smaller than their carbon-oxygen kind of similar mass and would cool and crystallize faster than them.[115]
    ==endquote==
    I agree with Drakkith that the mechanism that Joanna proposed would likely not produce an iron core white dwarf. But the other mechanism in section 4.4 might.
     
  10. Mar 28, 2015 #9

    marcus

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    http://arxiv.org/abs/1208.5069
    Failed-Detonation Supernovae: Sub-Luminous Low-Velocity Ia Supernovae and Their Kicked Remnant White Dwarfs with Iron-Rich Cores
    George C. Jordan IV, Hagai B. Perets, Robert T. Fisher, Daniel R. van Rossum
    (Submitted on 24 Aug 2012 (v1), last revised 20 Nov 2012 (this version, v2))
    Type Ia supernovae (SNe Ia) originate from the thermonuclear explosions of carbon-oxygen (C-O) white dwarfs (WDs). The single-degenerate scenario is a well-explored model of SNe Ia where unstable thermonuclear burning initiates in an accreting, Chandrasekhar-mass WD and forms an advancing flame. By several proposed physical processes the rising, burning material triggers a detonation, which subsequently consumes and unbinds the WD. However, if a detonation is not triggered and the deflagration is too weak to unbind the star, a completely different scenario unfolds. We explore the failure of the Gravitationally-Confined Detonation (GCD) mechanism of SNe Ia, and demonstrate through 2D and 3D simulations the properties of failed-detonation SNe. We show that failed-detonation SNe expel a few 0.1 solar masses of burned and partially-burned material and that a fraction of the material falls back onto the WD, polluting the remnant WD with intermediate-mass and iron-group elements, that likely segregate to the core forming an WD whose core is iron rich. The remaining material is asymmetrically ejected at velocities comparable to the escape velocity from the WD, and in response, the WD is kicked to velocities of a few hundred km/s. These kicks may unbind the binary and eject a runaway/hyper-velocity WD. Although the energy and ejected mass of the failed-detonation SN are a fraction of typical thermonuclear SNe, they are likely to appear as sub-luminous low-velocity SNe Ia. Such failed detonations might therefore explain or are related to the observed branch of peculiar SNe Ia, such as the family of low-velocity sub-luminous SNe (SN 2002cx/SN 2008ha-like SNe).
    Comments: 6 pages, 2 figures; accepted ApJL
     
  11. Mar 28, 2015 #10

    Drakkith

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    Marcus, you're aware that that's the exact paper Chronos linked, right?
     
  12. Mar 28, 2015 #11

    marcus

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    Wasn't aware! I was responding to earlier posts and was interrupted while writing, didn't see Chronos' post #6. So my posts were superfluous. Point was already made. Sorry.

    BTW it opens up something very interesting. In certain elliptical galaxies (perhaps formed by mergers of spiral galaxies) the motion is more chaotic and the stars are more dense. There are more collisions between stars than there would be in the more orderly spiral galaxies where everybody is revolving in the same plane more or less.

    In some of these galaxies there is much less X-ray emission from accretion discs than one would expect, given the prevalence of Type!a supernovae.
    Assuming that Type1a supernovae occur in binary systems where the white dwarf is accreting mass from its red giant partner.

    The answer may be that in these elliptical galaxies most of the TypeIa supernovae occur as a result of collision of two white dwarf stars RATHER than by accretion up to the 'Chandrasekhar limit, as usually supposed.

    Quite possibly you or Chronos will have already pointed that out and given a link to the paper. Posts often cross in a lively discussion and I'm a slow typer :smile:. But anyway, thought I'd mention it.

    Collision supernovae are just the sort of thing that can result in iron-rich core dwarf, such as Joanna' topic.
     
  13. Mar 29, 2015 #12
    Thank you all! This has been a very interesting thread! :)
     
  14. Mar 29, 2015 #13

    Chronos

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  15. Apr 1, 2015 #14
    Actually, it is quite possible for a supernova to leave the progenitor at least partially intact. It is classified as a Type Iax SNe. However, the progenitors of the Type Iax SNe are also a white dwarf stars, so no iron will be present in the progenitor, but iron will be created in the supernova ejecta.

    Source:
    Type Iax Supernovae: A New Class of Stellar Explosion - arXiv : 1212.2209v2 [PDF]
     
  16. Apr 1, 2015 #15

    Drakkith

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    That's about a carbon-oxygen white dwarf stealing mass from a helium star and undergoing a type 1a supernova. It is not a core collapse supernova.

     
  17. Apr 3, 2015 #16
    Actually, it is a Type Iax supernova, not a Type Ia, and the original poster said nothing about a core collapse supernova. That is entirely your assumption.
     
  18. Apr 3, 2015 #17

    Drakkith

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    On the contrary, that is exactly what the OP asked about, as you can see in the quote.
     
  19. Apr 4, 2015 #18
    As I understand the original post, they were asking whether or not a supernova could leave behind its iron core, not whether the core would collapse. The answer to the original post is "yes, but the core would not be iron." In a Type Iax SNe at least part of the white dwarf remains after the outer helium layer ignites. Iron is produced in the SN ejecta, but not in the white dwarf progenitor.
     
  20. Apr 4, 2015 #19

    Drakkith

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    I'm sorry I can't really understand what you're trying to get at here. A red supergiant can only undergo a core-collapse supernova.
     
  21. Apr 4, 2015 #20
    I understand that, but that was not the question being asked.

    [emphasis added]

    There is only one type of supernova that leaves behind any part of its progenitor, and that is a Type Iax SN. Although, the core is not iron but rather a degenerate form of carbon and oxygen. All other supernovae are either thermal-runaway, core collapse, or pair-instability.
     
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