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Can very compact and very massive neutron stars both exist?

Alessandro Drago1, Andrea Lavagno2, and Giuseppe Pagliara1

  • 1Dipartimento di Fisica e Scienze della Terra dell’Università di Ferrara and INFN Sezione di Ferrara, Via Saragat 1, I-44100 Ferrara, Italy
  • 2Department of Applied Science and Technology, Politecnico di Torino, I-10129 Torino, Italy and INFN Sezione di Torino, I-10126 Torino, Italy

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Phys. Rev. D 89, 043014 – Published 25 February, 2014

DOI: https://doi.org/10.1103/PhysRevD.89.043014

Abstract

The existence of neutron stars with masses of ∼2M⊙ requires a stiff equation of state at high densities. On the other hand, the necessary appearance also at high densities of new degrees of freedom, such as hyperons and Δ resonances, can lead to a strong softening of the equation of state with resulting maximum masses of ∼1.5M⊙ and radii smaller than ∼10  km. Hints for the existence of compact stellar objects with very small radii have been found in recent statistical analyses of quiescent low-mass X-ray binaries in globular clusters. We propose an interpretation of these two apparently contradicting measurements, large masses and small radii, in terms of two separate families of compact stars: hadronic stars, whose equation of state is soft, can be very compact, while quark stars, whose equation of state is stiff, can be very massive. In this respect an early appearance of Δ resonances is crucial to guarantee the stability of the branch of hadronic stars. Our proposal could be tested by measurements of radii with an error of ∼1  km, which is within reach of the planned Large Observatory for X-ray Timing satellite, and it would be further strengthened by the discovery of compact stars heavier than ∼2M⊙.

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Phys. Rev. D 89, 043014– Published 25 February, 2014
  • Received 13 October 2013

© 2014 American Physical Society

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