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I Low mass limit of a neutron star

  1. Dec 1, 2017 #1
    Note: this is QM question, not about stellar science. I am not asking what are the lightest neutron stars found in the Universe.

    The same star (say, 1 sun mass) can exist both in a form of a white dwarf and a neutron star. Both states are stable.

    However, lets say I start to stripe outermost levels of a neutron star one by one (*), making it lighter and lighter. I expect that when neutron star becomes really light, the pressure would push matter outwards, creating an explosion or just increase the volume dramatically, creating a white dwarf back.

    So what is that low mass limit?

    (*) This is just a thought experiment, but you can actually do it by dropping small amounts of antimatter to a neutron star, so the outermost levels would burn to light, which escapes. Of course, you have to do it slowly leaving time for a star to cool down.
     
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  3. Dec 1, 2017 #2

    Vanadium 50

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    I don't believe you can have a neutron star that light.

    Before discussing why something is true, we first need to understand if it is true.
     
  4. Dec 1, 2017 #3
    Whats about 0.86 M☉ ?

    https://en.wikipedia.org/wiki/GW170817

    The masses of the component stars have greater uncertainty. The larger (m1) has a 90% chance of being between 1.36 and 2.26 M☉, and the smaller (m2) has a 90% chance of being between 0.86 and 1.36 M
     
  5. Dec 1, 2017 #4

    PeterDonis

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    AFAIK our current understanding of the equation of state of neutron star matter does not rule this out; I believe it allows neutron stars as light as about 1/10 of a solar mass to be stable. But our current understanding of the equation of state of neutron star matter is not very good.

    See above.
     
  6. Dec 1, 2017 #5

    Vanadium 50

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    J0453+1559 has what might be a NS at M = 1.174 (with a 0.4% uncertainty). It could be a WD, although the orbit better fits a NS. Apart from that, the distribution of neutron stars is in the 1.1-2.0 solar mass range.

    Using a measurement of 1.11 solar masses with a 23% uncertainty as evidence for M < 1 is just silly.

    Peter is right, many EOS's allow for stable neutron star matter at M ~ 0.1. However, these require cold neutron star matter. For hot neutron stars - that is to say, all of them - the limit is close to M = 1. (Indeed, this is one of the reasons we don't know the EOS well. All EOS's under consideration have similar predictions for hot NS matter). So even if a smaller object would be stable, there is no known way to form one, nor do there appear to be any observed.
     
  7. Dec 1, 2017 #6

    PeterDonis

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    More precisely, all of them that we have observed. That might be because all of the neutron stars we have observed are fairly young ones--that is, the supernovas that formed them were fairly recent, on cosmological time scales. (The Crab Nebula pulsar, for example, is about 1000 years old.) As these objects age and grow colder, they will get harder to observe, at least with our current technology.

    However, it's also true that the main process we are aware of for forming neutron stars--a supernova--requires a star significantly more massive than the Sun (somewhere about 2 to 3 solar masses). A fair fraction of that mass gets ejected during the explosion, but that still leaves a remnant of about 1 solar mass or so. So even a cold neutron star that is a remnant from a supernova that happened billions of years ago would not, on cosmological grounds, be expected to be much ligher than 1 solar mass. I don't know if other mechanisms for forming neutron stars have been proposed that could make cold neutron stars significantly lighter than 1 solar mass.
     
  8. Dec 2, 2017 #7
    That sounds strange. What is a temperature which would remove degeneration in the cores of neutron stars? For any temperature much below that scale the temperature should not matter a lot.

    Let me know why I am wrong.
     
  9. Dec 2, 2017 #8
    So when neutron star becomes lighter and lighter (in a thought experiment) the radius would increase, like for the white dwarfs?
     
  10. Dec 2, 2017 #9

    Vanadium 50

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    Sorry, that's - posting nonsense and saying "prove me wrong!" - is not what we do here. Anyway EOS has a temperature dependence to it, and for a neutron star this is important because of chemical potentials and phase transitions. Hot NS core matter behaves differently than cold NS core matter just like ice behaves differently than steam.
     
  11. Dec 2, 2017 #10

    Vanadium 50

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    That's not actually the case. There are young pulsars, like the Crab and Vela, that are in the process of shedding their initial angular momentum. But there are other pulsars (so-called millisecond pulsars, although this term is misleading) that have been spun back up. Dating them is not easy; all we can do is date their companion stars. Typical ages are in the billions of years.

    There are about 200 of these known. None have a mass below 1.1-1.2. So if there are smaller NSs out there, why haven't we seen any spun up?

    As I mentioned earlier, there's no known formation mechanism. This may not adequately explain the difficulty. To make one of these, you need to squeeze it without heating it. Gravity will not do that for you. You need a non-gravitational squeezing mechanism that can radiate energy faster than thermal radiation. Good luck finding one of those.

    Some fraction of these objects will be in binaries, so you have the possibility of SNa 1a equivalents. Nothing like that is visible.
     
  12. Dec 2, 2017 #11

    PeterDonis

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    Yes.
     
  13. Dec 2, 2017 #12

    PeterDonis

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    Hm, interesting.

    Yes, good point.
     
  14. Dec 9, 2017 #13
    Yes, it is true, but the equation of state that provides this low mass neutron stars is the superior limit ( something like ρ= 6-7 x 10^14) of the equation of state to White dwarfs, BBP, which i belive is well know.
     
  15. Dec 9, 2017 #14

    PeterDonis

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    Do you have a reference for this? AFAIK the equations of state of white dwarfs and neutron stars are quite different, because of the different interaction between the particles (strong interaction for neutron stars vs. electromagnetic for electrons).
     
  16. Dec 10, 2017 #15
    There is one natural process to remove mass from a neutron star. When the matter from the surface of neutron star starts to spill over to another neutron star or a black hole, and escape on the other side.

    But because the neutron stars expand on removal of mass, as a neutron star starts losing mass it expands causing further mass to spill over. The timescale of that process? A neutron star is in the region of 10 km across. Speed of sound is in the region of 100 000 km/s. So the timescale is hundreds of microseconds.

    How does the timescale of neutron star expansion compare against timescale of beta decay? Urca process cooling?

    What precisely has a neutron star merger to reveal about equation of state of nuclear matter, cold and hot?

    The smaller neutron star is cooled by adiabatic expansion.

    And decay of neutrons necessarily releases both antineutrinos and heat.

    Heat may go to adiabatic expansion, electromagnetic radiation or Urca process... which unlike beta decay produces both antineutrinos and neutrinos.

    In the milliseconds after GW170817 started spilling over, the bigger neutron star must have immediately got hot and started emitting both thermal electromagnetic and Urca process. On the other hand, a black hole is, well, black, and therefore will NOT produce Urca process neutrinos.

    How efficient is the expanded, spilt-over neutron matter in producing neutrinos by Urca process, as opposed by producing antineutrinos by beta decay?

    Also, a black hole has no hair. A neutron star has hair. Oscillations of an actual matter neutron star with no event horizon should produce gravity waves completely different from the ringdown of a structureless black hole, and sensitive to the actual equation of state of the interior.
     
  17. Dec 10, 2017 #16

    PeterDonis

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    Do you have any references for this?

    Do you mean this?

    https://en.wikipedia.org/wiki/Urca_process
     
  18. Dec 10, 2017 #17
    How else are you claiming a neutron star merger takes place?
    Yes.
    Note: while gravity waves come from bulk movements of mass (thermal production of neutrinos is not negligible, thermal gravitons are negligible even in hot neutron stars) and the escaping electromagnetic radiation comes from surfaces, neutrinos do probe neutron star interior conditions.
     
  19. Dec 11, 2017 #18

    PeterDonis

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    Merging two neutron stars doesn't require any mass to "spill over" from one to the other as you describe (much less to "escape out the other side"). It just requires them to merge.

    If you think "spilling over" takes place as you described, you ought to be able to provide a reference that gives a model of such a merger that shows it taking place. That's what I was asking for.
     
  20. Dec 11, 2017 #19
    A neutron star might spiral in and fall into a black hole without being torn apart.
    However, such a process could not produce R-process and extended optical afterglow.
    The observed kilonova does precisely require mass escaping out the other side.
     
  21. Dec 11, 2017 #20

    PeterDonis

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    @snorkack, either give references to back up your statements or stop posting.
     
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