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I learned about the Chandrasekhar limit as being the UPPER limit, in terms of mass, for a white dwarf...

But, I have never heard of a neutron star being BELOW that mass, so I have wondered, recently, if that is also a LOWER limit for neutron stars...

I suppose that the squished-together protons and electrons that the neutron star is made of might lose their 'degeneracy', if the gravitational potential is (or becomes) low enough, and the star might 'poof-out' (or proof back out) to a white dwarf...

Has this ever been known to happen?

Can a neutron star experience Hawking radiation and actually lose mass, like a black hole?

If black holes can, theoretically at least, exist at very small masses as long as they are small enough voluminously (compacted within their Swarzschild or Kerr radii), then why can't neutron stars?

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    $\begingroup$ NS-BH binaries can cause the NS component to lose mass to below the Chandrasekhar limit. See this Physics SE question. $\endgroup$
    – WarpPrime
    Commented May 24, 2021 at 22:55
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    $\begingroup$ You need an event horizon for Hawking radiation. OTOH, hot neutron stars radiate EM like any hot body does, and they can lose energy via electromagnetic interactions with plasma in the neighbourhood, but of course that energy is pretty small relative to their rest mass. $\endgroup$
    – PM 2Ring
    Commented May 25, 2021 at 1:45
  • $\begingroup$ There's quite an informative answer below, and so far I don't see any comments that suggest otherwise. I've started on a campaign to search through my old questions that have answers but none accepted yet to see if I can bring up my acceptance rate (personal goal). See my answer to Why don't question-askers accept answers? $\endgroup$
    – uhoh
    Commented Aug 10, 2021 at 0:01

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A number of neutron stars in binary systems have measured masses below $1.44 M_\odot$ (e.g. a pulsar of mass $1.251 \pm 0.021 M_\odot$, McKee et al. 2020). I think the current lowest mass contender is $1.174 \pm 0.004 M_\odot$ (Martinez et al. (2015). See the plot below with a pictorial representation of the current state of neutron star mass measurements from Horvath et al. (2020). Conveniently, the vertical line is I think at about 1.4 solar masses.

from Horvath et al. (2020)

The Chandrasekhar mass for a ball of iron at the centre of a core collapse supernova is more like $1.15 M_\odot$ because ionised iron has 2.15 mass units per electron, rather than the 2 of carbon or oxygen, and because the electron energy required to neutronise iron is much lower than for carbon and oxygen.

The theoretical lower limit to the neutron star mass is about $0.1-0.2M_\odot$, but none have been observed below $1M_\odot$ and there isn't any known astrophysical mechanism to produce them. A much fuller answer to this last part, which I won't cut and paste here can be found at Physics SE. In brief, the lower limit arises because the adiabatic index of material in the interior falls below 4/3 (because of inverse beta decay, the formation of neutron-rich heavy nuclei and the disappearance of some free neutrons) and the material is too compressible to form a stable star with a negative binding energy.

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    $\begingroup$ How strongly dependent is the theoretical lower mass of NSs on the (uncertain) nuclear equation of state? And do you have a source for this theoretical lower bound? $\endgroup$
    – Daddy Kropotkin
    Commented May 25, 2021 at 14:08
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    $\begingroup$ @DaddyKropotkin the link to the much fuller answer I gave on Physics SE contains many references to the relevant journal papers. It has very little to do with the neutron star EOS because the "neutron star" density is well below that of nuclear matter at these masses. $\endgroup$
    – ProfRob
    Commented May 25, 2021 at 16:03
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    $\begingroup$ @DaddyKropotkin or at least the EOS uncertainties are not the same ones that are problematic in determining NS radii or the maximum mass if neutron stars. They are uncertainties in the EOS of neutron star "crust" material. Also, rotation would play a role. $\endgroup$
    – ProfRob
    Commented May 25, 2021 at 16:07
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    $\begingroup$ ProfRob, then the 1.4 limit refers to first population? In other words the limit depends on composition not really to just the mass, and this because of the different behaviour of the elements? $\endgroup$
    – Alchimista
    Commented May 26, 2021 at 9:53
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    $\begingroup$ @Alchamista The "Chandrasekhar mass" has always depended on composition - even in the original 1930s papers. It depends upon the number of electrons you have per mass unit in the gas. However, the real upper limit may also depend on energy thresholds for inverse beta decay, photodisintegration or pyconuclear reactions, all of which take place at finite density at a slightly lower mass (e.g. 1.38 solar masses for a carbon composition). $\endgroup$
    – ProfRob
    Commented May 26, 2021 at 10:02

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