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arXiv · 2608.10991

Ultracompact Anisotropic Stars and Gravastars in General Relativity

Prajwal Hassan Puttasiddappa·Nora Bretón·Santiago Esteban Perez Bergliaffa
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Abstract

We study static, spherically symmetric ultracompact objects in General Relativity with anisotropic stress. Using the covariant anisotropic equation of state and assuming homogeneous density, we construct a mostly analytical model admitting configurations beyond the Buchdahl limit and with compactness arbitrarily close to the black hole value. The solutions exhibit two regimes separated by a critical curve in the parameter space of compactness and anisotropy, associated with the divergence of the central pressure: regular anisotropic configurations with positive central pressure, and singular configurations with negative central pressure for arbitrary anisotropy. In the latter case, we show that introducing a thick shell removes the pressure divergence, yielding regular ultracompact gravastar configurations. We further construct both anisotropic and isotropic gravastar models using this thick-shell construction.

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Prajwal Hassan Puttasiddappa, Nora Bretón, Santiago Esteban Perez Bergliaffa. 2026-08-11. Ultracompact Anisotropic Stars and Gravastars in General Relativity. https://arxiv.org/abs/2608.10991

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