Neutrino trapping in braneworld extremely compact stars
  Extremely Compact Stars (ECS) contain trapped null geodesics. When such objects enter the evolution period admitting geodetical motion of neutrinos, certain part of neutrinos produced in their interior will be trapped influencing their neutrino luminosity and thermal evolution. We study neutrino trapping in the braneworld ECS, assuming uniform distribution of neutrino emissivity and massless neutrinos. We give the efficiency of the neutrino trapping effects in the framework of the simple model of the internal spacetime with uniform distribution of energy density, and external spacetime described by the Reissner-Nordstr\"om geometry characterized by the braneworld "tidal" parameter $b$. For $b < 0$ the external spacetime is of the black-hole type, while for $b > 0$ the external spacetime can be of both black-hole and naked-singularity type. Then the ECS surface radius $R$ can be located also above the unstable (outer) photon circular orbit. Such basically new types of the spacetimes strongly alter the trapping phenomena as compared to the standard case of $b = 0$. It is shown that the neutrino trapping effects are slightly lowered by the presence of physically more plausible case of $b < 0$, as compared to the standard internal Schwarzschild spacetime, while they can be magnified by positive tidal charges if $b < 1$ and lowered for $b > 1$. However, potential astrophysical relevance of the trapping phenomena is strongly enhanced for negative tidal charges enabling a significant enlargement of the ECS surface radius to values coherent with recent observations. 
Zdenek Stuchlik, Jan Hladik, Martin Urbanec
gr-qc astro-ph.SR