# B-field of n-star

1. Jul 28, 2004

### turin

Can someone explain to me how a neutron star (I'm assuming that it has no charge even on the microscopic level) can have a magnetic field? And even if I assume that there is some residual charge left after the collapse, how could the magnetic poles point in significantly different direction from the rotational poles?

My only conjecture that I could come up with is that the quark spins in the neutrons themselves are all alligned (from one neutron to the next).

2. Jul 28, 2004

### Labguy

The first part of an answer is that a neutron star is not composed of just neutrons as most descriptions say. The whole star has a "structure", and a significant portion is other elements starting with an "atmosphere and upper crust".There are lots of other elements (nuclei with free electrons = plasma), so it isn't all neutrons yet. At the top of the crust, the nuclei are mostly iron 56 and lighter elements, but deeper down there is a conductive superfluid which can and does create a huge magneto effect, causing the magnetic field. See the neat chart at: http://www.lsw.uni-heidelberg.de/users/mcamenzi/NS_Mass.html for an example. (I like the chart, but don't agree with the "mass limit" part though)

There is no such thing as "typical" when referring to neutron stars, or any other kind, but in a typical neutron star the surface gravity is ~10^11 times Earth's, and the magnetic field is ~10^12 Gauss. At densities of ~10^6 g/cm^3 the electrons become degenerate causing huge electrical and thermal conductivities because the electrons can travel long distances before interacting. Add this to a high spin-rate and you have a great magneto causing a big-time magnetic field.

3. Aug 2, 2004

### Orion1

Hyperoneutronium...

Independent of the details, Glendenning found a maximum possible mass for neutron stars of only 1.5 solar masses (nucl-th/0009082; astro-ph/0106406).

According to various neutron star core theories, hyperon core saturation (hyperon condensation), results in the reduction of the neutron star maximum mass, resulting in this estimation of the maximum mass of $$1.5 M_\odot$$.

Results from (TOV) equasion of states for spherically symmetric compact stars: (astro-ph/0106406, page 11)

$$M_n = 1.649 M_\odot$$ - neutron star
$$\rho(0) = 7.7n_o$$

$$M_n = 1.789 M_\odot,$$ - hyperstar (pure hyperons)
$$\rho(0) = 5.16n_o$$

When hyperon condensation is included in the (TOV) equasion of state (EoS), the neutron star maximum mass is attained at an earlier central density:
$$M_n = 1.571 M_\odot,$$
$$\rho(0) = 4.49n_o$$

Therefore, the estimate of $$1.571 M_\odot$$ is a spherically symmetric static hybrid (mixed phase) of a neutron star with a hyperon core.

Reference:
http://arxiv.org/abs/nucl-th/0009082
http://arxiv.org/abs/astro-ph/0106406

Last edited: Aug 2, 2004