Results 1 to 4 of 4

Thread: White Dwarf Stars And Low-mass Neutron Stars

  1. #1
    Join Date
    Jul 2003
    Posts
    2,784
    During my discussions on the Iron Sun Theory I came across some interesting information regarding low-mass neutron stars and the possibility a white dwarf could collapse into one.

    Most neutron stars masses are determined by measuring the effect of the neutron star on an orbiting companion. Such stars are found to cluster in the ~1.35Sm (Solar mass) range, with a very few instances where the mass range is ~ 1Sm.

    The Chandrasekhar Limit says that electron degeneracy in the core of a massive star is overcome once the iron core exceeds ~1.4SM, at which point it collapses. It does not appear to me that such a star could form a neutron star of less mass than that, so the question is, where do low mass neutron stars come from?

    Current thinking is that an accreting white dwarf star that exceeds the Chadrasekhar Limit violently explodes, creating a Type Ia supernova that completely disrupts the star. I wonder, however, if a low-mass neutron star might also arise from the accreting material pushing the white dwarf past the Chandra Limit.

    In this paper, entitled Odd Binary Stars and The Cycle of Star Formation they mention the possibility that a white dwarf could collapse like this, and I have seen other papers where there is an attempt to look at this possibility more closely.

    My thought is that when some white dwarfs reach 1.4Sm, instead of blowing up they suddenly collapse. However, as they are very low mass as compared to a core collapse supernova (Type II), and have no envelope to plow through, the release of neutrinos blows off some of the outer material, allowing it to escape and reducing the neutron star mass somewhat.

    Comments? Is this even possible?

  2. #2
    Join Date
    Jul 2005
    Location
    Massachusetts, USA
    Posts
    21,961
    On this subject, a few things come to mind. Generally there are four sets of issues that seem able to provide some variation in the formation of a white dwarf.

    1. Total mass of the progenitor. This seems to determine the Carbon/Oxygen/Neon ratio of the final body, with heavier stars producing more Oxygen and Neon.
    2. Total rotational momentum of the star. Some stars are spinning very fast, and are quite oblate. here is a question about how these stars can shed angular momentum as they collapse. Neutron stars slow down through magnetic braking against the media in the accretion disk, and the wind nebula. What do white dwarfs do? Something similar I imagine.
    3. Magnetic field. I don't know how this is affected as the core collapses, but I understand there are some powerful effects possible.
    4. Outside accretion and spin-up.

    Anyway, there is a fair amount of room for variety.
    Forming opinions as we speak

  3. #3
    Join Date
    Jul 2003
    Posts
    2,784
    1. Total mass of the progenitor. This seems to determine the Carbon/Oxygen/Neon ratio of the final body, with heavier stars producing more Oxygen and Neon.
    Yes, but I am talking about a white dwarf that has already gone through its red giant stage and shed its outer atmosphere. One that is in orbit around (with?) a companion star that is dropping mass onto the white dwarf.

    For arguments sake, lets say a white dwarf that is ~1.2 Sm.


    The question of the magnetic field is an interesting one. Should a white dwarf collapse into a neutron star, would it develope a magnetic field similar in strength to one that formed in a Type II SN? Seems to me that the process is similar enough that the answer is "probably"

    Spin would have to be a factor, as this would also affect the rate of accretion. I would think the spin would have to be slow to allow for fast enough accretion to cause the dwarf to pass the Chandra limit and collapse. Otherwise, the accreted material would simply fuse.

    That's also why I'm thinking it has to be a fairly massive dwarf, one that is already pretty close to the Chandra limit.

  4. #4
    Join Date
    Jul 2003
    Posts
    2,784
    This paper by Qian and Wasserburg Stellar Sources for Heavy r-Process Nuclei discusses the possibility of some light r-process nuclei which seem only able to form as a result of a white draft star collapsing into a neutron star.

    I don't see anything in the paper about the size of the remnant though, but it suggests that white dwarfs can, in fact, collapse.

Similar Threads

  1. White Dwarf Stars Consume Rocky Bodies
    By Fraser in forum Universe Today
    Replies: 0
    Last Post: 2011-Aug-15, 08:30 PM
  2. [Do neutron stars lose mass?]
    By astromark in forum Space/Astronomy Questions and Answers
    Replies: 6
    Last Post: 2011-Mar-28, 11:06 PM
  3. New Type of White Dwarf Stars Discovered
    By Fraser in forum Universe Today
    Replies: 15
    Last Post: 2008-May-02, 10:39 PM
  4. White dwarf stars and the Sun
    By suitti in forum Space/Astronomy Questions and Answers
    Replies: 1
    Last Post: 2007-Feb-28, 04:31 PM
  5. White Dwarfs, Neutron Stars, and Black Holes
    By Sue-Ann in forum Space/Astronomy Questions and Answers
    Replies: 8
    Last Post: 2004-Nov-28, 12:23 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •