What is the composition of the crust and atmosphere on a neutron star?

[Jimmy Snyder]

There aren't any electrons on a neutron star right? They all get squooshed into the protons to make the neutrons. Am I right about that?

 

[Bobbywhy]

http://www.astro.umd.edu/~miller/nstar.html

Anyway, imagine starting at the surface of a neutron star and burrowing your way down. The surface gravity is about 10^11 times Earth's, and the magnetic field is about 10^12 Gauss, which is enough to completely mess up atomic structure: for example, the ground state binding energy of hydrogen rises to 160 eV in a 10^12 Gauss field, versus 13.6 eV in no field. In the atmosphere and upper crust, you have lots of nuclei, so it isn't primarily neutrons yet. At the top of the crust, the nuclei are mostly iron 56 and lighter elements, but deeper down the pressure is high enough that the equilibrium atomic weights rise, so you might find Z=40, A=120 elements eventually. At densities of 10^6 g/cm^3 the electrons become degenerate, meaning that electrical and thermal conductivities are huge because the electrons can travel great distances before interacting.

Deeper yet, at a density around 4x10^11 g/cm^3, you reach the "neutron drip" layer. At this layer, it becomes energetically favorable for neutrons to float out of the nuclei and move freely around, so the neutrons "drip" out. Even further down, you mainly have free neutrons, with a 5%-10% sprinkling of protons and electrons.

 

[Jimmy Snyder]

Thanks Bobbywhy. Your answer is far more interesting than I had figured on. What prompted my question was this quote from the book "QFT III: Gauge Theory" by Eberhard Zeidler, page 950.

fermions of the same type (e.g., the electrons in a neutron star) are governed by the Fermi-Dirac statistics

If it were me, I would have said electrons in an atom.

 

[SpiffyKavu]

It is indeed interesting. Beyond the neutron drip density, neutrons tend to clump together. This is analogous to the reason why a water drop likes forming a sphere. So we get bubbles of neutrons floating in otherwise neutron-rich nuclei. As we go deeper we encounter other phases, what are sometimes called "nuclear pasta," before getting to the regime where neutrons dominate. Near the core, there will be more than 10 neutrons for every proton.

http://relativity.livingreviews.org/Articles/lrr-2008-10/

Section 3.3 describes this series of phase transitions and has some pretty pictures.

 

[stevebd1]

There aren't any electrons on a neutron star right? They all get squooshed into the protons to make the neutrons. Am I right about that?

According to most sources, it appears that neutron stars have a crust of electron degenerate iron about 1km thick (with an outer & inner crust) and a plasma atmosphere.

A couple of sources-
http://arxiv.org/abs/0705.2708v2
http://var.astro.cz/brno/perseus4_2002_clanek2.pdf