Strangeness in Stellar Environments

Contributor: M. Prakash.

A possible site where large amounts of strangeness-bearing matter may be found is in the interior of a neutron star. Relative to matter involving only nucleons, properties of neutron stars such as their masses and radii are affected by the presence of strangeness (hyperons, or a K- condensate, or strange quarks [22,23]). Furthermore, the thermal and structural evolution of a neutron star depends sensitively on the strangeness baryon content of its interior. Some examples of phenomena that occur due to the presence of strangeness are listed below.

1.

Typically, neutron stars with kaon condensates or strange quark matter in the star's core have substantially smaller radii, and maximum masses, than those without these exotic components. However, the addition of strangeness through hyperons, although lowering the maximum mass, does not greatly reduce the stellar radii [24]. There are no direct measurements of the radius or mass of any neutron star outside a binary system. The most promising possibility for a firm radius measurement is the isolated neutron star RX J185635-3754 [25].

2.

Metastable neutron stars are possible with strangeness-bearing matter and allow the formation of a low-mass black hole during the deleptonization stage of a protoneutron star [23,26,27]. This may be confirmed by the abrupt cessation of neutrino emission during the first 10-20 seconds of the evolution of a protoneutron star.

3.

The long-term, up to 106 years, thermal evolution of a neutron star is also affected by strangeness-bearing components [28,29]. Multi-wavelength photon observations of neutron stars (ROSAT, HST, AXAF, and XMM) have been planned to elucidate the thermal histories of neutron stars. Laboratory studies such as phase-shift analyses of nucleon-hyperon, nucleon-meson and hyperon-hyperon interactions are crucial both for many-body calculations of dense matter and for studies of superfluidity and superconductivity.

NASA is actively supporting both theoretical and observational efforts under it's Astrophysical Theory and Long-Term-Space-Astrophysics Programs in order to unravel the properties of matter under extreme conditions of density and temperature through observations of neutron stars. The NSF and DOE are also actively supporting theoretical studies of the role of strangeness in the equation of state in nuclear astrophysics and in relativistic heavy ion collisions. Strong support of laboratory studies aimed at the determination of hyperon and kaon interactions both in free space and in nuclei is therefore extremely important.

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