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Dynamical response of twin stars to perturbations

Shamim Haque1,*, Luciano Rezzolla2,3,4,†, and Ritam Mallick1,‡

  • *Contact author: shamims@iiserb.ac.in
  • †Contact author: rezzolla@itp.uni-frankfurt.de
  • ‡Contact author: mallick@iiserb.ac.in
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Phys. Rev. D 113, 103044 – Published 29 May, 2026

DOI: https://doi.org/10.1103/99c1-2p2y

Abstract

If a strong first-order phase transition takes place at sufficiently high rest-mass densities in the equation of state (EOS) modeling compact stars, a new branch will appear in the mass-radius sequence of stable equilibria. This branch will be populated by stars comprising a quark-matter core and a hadronic-matter envelope, i.e., hybrid stars, which represent “twin-star” solutions to equilibria having the same mass but a fully hadronic EOS. While both branches are stable to linear perturbations, it is unclear which of the twin solutions is the “favored” one, that is, which of the two configurations is expected to be found in nature. We assess this point by performing a large campaign of general-relativistic simulations aimed at assessing the response of compact stars on the two branches to perturbations of various strength. In this way, we find that, independently of whether the stars populate the hadronic or the twin branch, their response is characterized by a critical-perturbation strength such that the star will oscillate on the original branch for subcritical perturbations and migrate to the neighboring branch for supercritical perturbations while conserving rest mass. Because the critical values are different for stars with the same rest mass but sitting on either branch, it is possible to define as favored the part of the branch that has the largest critical perturbation, thus correcting the common wisdom that stellar models on the twin branch are the favored ones. Interestingly, we show that the binding energies on the two branches can be used to deduce without simulations which of the stellar configurations is more likely to be found in nature.

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Phys. Rev. D 113, 103044– Published 29 May, 2026
  • Received 27 March 2026
  • Accepted 21 April 2026
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