Neutron Star Masses

The maximum mass and radii of neutron stars largely depend on the composition of the central core. Hyperons, as the strange members of the baryon octet, are likely to exist in high density nuclear matter. The presence of hyperons, as well as of a possible K-condensate, affects the limiting neutron star mass (maximum mass). 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).

Figure: Neutron stars are complex stellar objects with an interior composition that is largely unknown. In particular, the composition of the central core is uncertain (a dense neutron fluid including protons, electrons, muons and hyperons - or a quark core ?). Source: Homepage Dany Page

Masses and radii are fundamental parameters for stellar-like objects. Despite various attempts, it was not yet possible to measure the radii of neutron stars. Their values are expected in the range from 10 - 13.5 km. The masses of neutron stars, on the other hand, are the best known masses in Astronomy. As usual for mass determination, binary systems are used.

Figure: Neutron star masses for various binary systems, measured with relativistic timing effects. The upper 5 systems consist of a radio pulsar with a neutron star as companion, the lower systems of a radio pulsar with a White Dwarf as companion. All the masses seem to cluster around the value of 1.4 solar masses.

All these results seem to indicate that the presently measured masses are very close to the maximum possible mass. This could indicate that neutron stars are always formed close to the maximum mass.