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Title:

Neutron Stars

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Fusion from H to Fe in the core of stars ... Neutron stars are interesting! ... Different n-n models predict different radii of neutron stars ... – PowerPoint PPT presentation

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Title: Neutron Stars


1
Neutron Stars
  • Aree Witoelar

2
What is a neutron star?
  • A collapsed core of a massive star
  • Composed entirely from neutron
  • Incredibly high density

3
Creation of a neutron star
  • Fusion from H to Fe in the core of stars
  • No more fuel - core reaches tremendous density
    and explodes (Supernova)
  • Inverse ßdecay takes place

4
Properties
  • Mass 1.3-1.5 Msun(3.1030 kg)
  • Radius 10 km
  • Density 1014g/cc
  • Magnetic field 1012 Gauss

5
Interior
  • Increasing pressure inwards creates Pasta layers

6
Nuclear Physics of Neutron Stars
  • Neutron stars are like giant neutron supernuclei
  • Testing models of Nuclei-Nuclei Interaction on
    neutron stars
  • Estimating Neutron Star radius
  • Binding Energy
  • Fermi Gas model
  • Nuclei-Nuclei Interaction (first principles)

7
Binding Energy
  • Binding energy for a nucleus
  • Assumptions for a huge neutron nucleus
  • No Coulomb energy
  • Neglect pairing energy
  • Neglect surface term with respect to volume term

8
Binding Energy (2)
  • Simplified Binding energy
  • Bound state exist if binding energy is positive
  • Filling the constants, the result is
  • A 5 x 1055
  • R 4.3 km
  • M 0.045 solar mass
  • Same order of magnitude as observations

9
Fermi Gas model
  • Treat neutron stars as degenerate Fermi gases (of
    neutrons) held by gravity
  • Assume
  • Constant density average pressure
  • No nucleon-nucleon interaction
  • Number of possible states
  • Integrate to Fermi momentum pF

10
Fermi Gas model (2)
  • Calculate from Fermi momentum and/N from gravitational energy
  • Minimize total of kinetic and potential energy
  • The results are
  • R 12 km
  • ? 0.25 nucleons/fm3 (nucleus 0.17
    nucleons/fm3 )
  • Close to experimental values gravitational
    pressure compensated by Fermi pressure and
    nucleon-nucleon repulsion

11
Nucleon-Nucleon Interaction
  • Nuclear force is an interaction between
    colourless nucleons with range of the same order
    of magnitude as the nucleon diameter
  • It is not possible to extract n-n potential
    directly from structure of nucleus
  • Different models with different parameterization

12
General form of n-n potential
  • Quantities to determine interaction
  • Separation of nucleons x
  • Relative momenta p
  • Total orbital angular momentum L
  • Relative orientation of spins s1 and s2
  • Potential is scalar
  • Symmetric under exchange of the two nuclei

13
?-meson theory
  • Nucleons are surrounded by field of massive
    (virtual) particles called ?-mesons (pions)
  • Pion could be absorbed by another nucleon in its
    lifetime
  • Momentum transfer - akin to force (but
    attractive)
  • Direct analogy of EM force but photons have no
    mass, pion have mass of 140 MeV/c2 - finite
    range
  • Heisenberg uncertainty principle

14
Covalent and Meson exchange
  • Covalent bonds (direct q-exchange) are suppressed
    by color restriction

Meson exchange color-neutral
Yukawa potential

15
Equation of State
  • The relations between the density and temperature
    to its pressure and internal energy, specific
    heats, etc.
  • Pure neutron matter is unbound

16
Many-Body Theory
  • Hamiltonian
  • Four-body and higher order interaction are
    neglected

17
Neutron Star Radius
  • Different models have different parameterizations
    of vijR

18
Summary
  • Neutron stars are interesting!
  • Nuclear Physics Approximate Neutron Star radius
    with Binding Energy, Fermi Gas model, or
    Nucleon-Nucleon interaction
  • Nucleon-Nucleon interaction is caused by meson
    exchange (virtual particles)
  • Different n-n models predict different radii of
    neutron stars
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