Hyperons in neutron-star cores and a 2 M_⊙ pulsar

I. Bednarek², P. Haensel¹, J. L. Zdunik¹, M. Bejger¹ and R. Mańka^2⋆

¹ N. Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
e-mail: ilona.bednarek@us.edu.pl, bejger@camk.edu.pl; haensel@camk.edu.pl; jlz@camk.edu.pl
² Department of Astrophysics and Cosmology, Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland

Received: 1 December 2011
Accepted: 16 April 2012

Abstract

Context. A recent measurement of the mass of PSR J1614-2230 rules out most existing models of the equation of state (EOS) of dense matter that is subjected to the high-density softening caused by either hyperonization or a phase transition to either quark matter or a boson condensate.

Aims. We attempt to resolve the apparent differences between the predictions derived from up-to-date hypernuclear data, which include the appearance of hyperons at about three nuclear densities and the existence of a M = 2.0 M_⊙ neutron star.

Methods. We consider a non-linear relativistic mean field (RMF) model involving the baryon octet coupled to meson fields. An effective Lagrangian includes quartic terms in the meson fields. The values of the model parameters are obtained by fitting the semi-empirical parameters of nuclear matter at the saturation point, as well as potential wells for hyperons in nuclear matter and the strength of the Λ − Λ attraction in double-Λ hypernuclei.

Results. We propose a non-linear RMF model that is consistent with up-to-date semi-empirical nuclear and hypernuclear data and allows for neutron stars with hyperon cores and M > 2 M_⊙. The model involves hidden-strangeness scalar and vector mesons, coupled only to hyperons, and quartic terms involving vector meson fields.

Conclusions. Our EOS involving hyperons is stiffer than the corresponding nucleonic EOS (in which hyperons are artificially suppressed) above five nuclear densities. The required stiffening is generated by the quartic terms involving the hidden-strangeness vector meson.