Download The Interior Structure of Neutron Stars

The Interior Structure of Neutron Stars
Evan Keane
Jodrell Bank Centre for Astrophysics
University of Manchester
08/04/2008
3rd Estrela Workshop
Bonn, Deutschland.
Neutron Star - Basics
Bang! When stars, with initial masses in the range 11±1 ->
~25 M will, once they have evolved of the main sequence
and burnt all their nuclear fuel (up to 56Fe), will end their
main lives in a Type II Supernova and leave a neutron star
(NS) remnant.
Properties: From observations of binary pulsar systems
most NSs are seen to have masses around M=1.35M
which is taken as the canonical mass.
Also must spin slower than break-up speed -> gives us
Rmax=16.8km(P/ms).
Requirining vsound<c implies Rmin=1.5RSchwarzschild=6.2km
Take canonical radius of R=10km
A Minimum Energy Argument
The basic method for determining the
composition of the internal structure is to
minimise the energy density of the system as a
whole with respect to A, Z and Yn
Tricky bit: Determining a form for the energy
density of the nucleus (i.e. need to chose a
nuclear physics theory)
Minimum energy configuration at a given ρ, gives
(A,Z) the equilibrium composition at that density.
At the crust the equilibrium configuration is 56Fe
Equilibrium Nucleus down to ρdrip
Measured in lab
Theoretical Models
Equilibrium Nuclei
The Crust
A NS is not a big ball of neutrons!
We start with 56Fe ions in a degenerate electron
gas and no free neutrons.
P=Kρ5/3 (non-rel degenerate electron gas) but
need corrections for Coulomb repulsion.
“Coulomb lattice” of Wigner-Seitz cells.
At ρ=1010kg/m3 electrons now relativistic enough
to penetrate into nuclei and combine with
protons to “neutronise” nuclei via unbalanced
inverse beta decay.
Towards Neutron Drip
Above ρ=1010kg/m3 get more & more neutron-rich in an
electron sea
->The equilibrium nucleus shifts to heavier configs.
At a certain depth nuclei become saturated with neutrons
->More energetically favourable for neutrons to form
outside the nucleus.
This is because we have reached the ρ where the highest
occupied neutron energy level is just at the neutron Fermi
energy. As ρ increases further more neutrons “drip” free of
nuclei & we get heavy nuclei in a neutron and electron sea
Beyond Neutron Drip & Complications
Neutron drip proceeds from ρdrip=(4.3)(1014)kg/m3
until all nuclei have dripped free of nuclei by
ρnuclear=(2.7)(1017)kg/m3
At very slightly higher densities we reach the muon
Fermi energy and these will be created & we have
neutron fluid + proton fluid + electrons + muons
There are other complications also: protons and
neutrons form Cooper pairs just below nuclear
density --> superfluids! --> p fluid is (Type II)
superconductor
More Complications
I have also not mentioned magnetic fields!!
Heavy nuclei are actually stretched out along
direction of magnetic field lines.
Also get quantum vortices in the superfluid interior.
The core composition is unknown but decides
whether or not he have a “hard” or “soft” eos.
Pion/Kaon core -> higher ρcore, lower R, M, “soft”
Quark core -> lower ρcore, higher R, M, “hard”
--> binary pulsar observations can tell us which
models are possible.
The Core?
Thank You