They don't really, it's a plasma. The plasma is just so dense that it's mechanically indistinguishable from a solid.
It's only indistinguishable in its mechanical aspects, however. Once you start looking at its thermal and electromagnetic behavior it's very different from how solids behave.
That seems a bit excessively nitpicky. Neither you nor the comment you corrected somewhat is really wrong. I'm not sure you're much more right, though. Thomas-Fermi screening and the Coulomb crystal lattice of the outer crust isn't very plasma-like (and differs from the astrophysical plasma in the accretion disc). Perhaps it's better to say that it's a phase of matter unlike the "classical four", and more like one of the dozens and dozens of known condensed matter states. Chamel 2007 is pretty interesting on that front: <https://arxiv.org/abs/0709.3798>.
I would agree with you about the inner crust, and so would Chamel (and many others): "Unlike the situation in ordinary solids (ordinary meaning under terrestrial conditions), the electronic properties in neutron star crust are very simple. The matter density is so high that the Coulomb energy of the electrons is negligible compared to their kinetic energy" [which is essentially your point] "The electrons can thus be treated as a degenerate relativistic Fermi gas." Chamel 2008 is also excellent <https://link.springer.com/article/10.12942/lrr-2008-10>.
Finally, the phys.org article at the top corresponds with Russel et al 2024 <https://arxiv.org/abs/2403.18135> which as far as I can tell has practically nothing to do with this thread of discussion nor the paragraph in the phys.org article that motivated it. Perhaps the phys.org article writer confused accretion "hard state" (relatively low mass accretion rate, p. 3 of Russell et al. 2024) with a stiff (or hard) outer crust equation-of-state which compresses poorly compared to an EOS softened by neutronization (e.g. Haensel, P., Potekhin, A.Y., Yakovlev, D.G. (eds) (2007), chapter 3 <https://link.springer.com/chapter/10.1007/978-0-387-47301-7_...>).
The excellent overview and somewhat different approach to the NS EOS in Sumioshi et al 2022 is pretty neat too <https://arxiv.org/abs/2207.00033>.
Those are some excellent links, thank you. For some reason (misremembering, probably) I had thought that the the limit of any Fermi gas at high density was a plasma but I appreciate the correction.
4 comments
[ 131 ms ] story [ 1955 ms ] threadIt's only indistinguishable in its mechanical aspects, however. Once you start looking at its thermal and electromagnetic behavior it's very different from how solids behave.
I would agree with you about the inner crust, and so would Chamel (and many others): "Unlike the situation in ordinary solids (ordinary meaning under terrestrial conditions), the electronic properties in neutron star crust are very simple. The matter density is so high that the Coulomb energy of the electrons is negligible compared to their kinetic energy" [which is essentially your point] "The electrons can thus be treated as a degenerate relativistic Fermi gas." Chamel 2008 is also excellent <https://link.springer.com/article/10.12942/lrr-2008-10>.
More recently Taverna et al 2022 <https://www.science.org/doi/10.1126/science.add0080> does X-ray polarization spectroscopy on candidate magnetar (Anomalous X-Ray Pulsar AXP) 4U 0142+61 <https://en.wikipedia.org/wiki/4U_0142%2B61> (follow links in [6] therein) to investigate the physical state of the crust, finding the best match is a solid and bare surface.
Finally, the phys.org article at the top corresponds with Russel et al 2024 <https://arxiv.org/abs/2403.18135> which as far as I can tell has practically nothing to do with this thread of discussion nor the paragraph in the phys.org article that motivated it. Perhaps the phys.org article writer confused accretion "hard state" (relatively low mass accretion rate, p. 3 of Russell et al. 2024) with a stiff (or hard) outer crust equation-of-state which compresses poorly compared to an EOS softened by neutronization (e.g. Haensel, P., Potekhin, A.Y., Yakovlev, D.G. (eds) (2007), chapter 3 <https://link.springer.com/chapter/10.1007/978-0-387-47301-7_...>).
The excellent overview and somewhat different approach to the NS EOS in Sumioshi et al 2022 is pretty neat too <https://arxiv.org/abs/2207.00033>.