Phase transitions in compact stars. Problem of micro and macro stability
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hase Transitions in Compact Stars. Problem of Micro and Macro Stability1 J. L. Zdunik N. Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, PL00716 Warszawa, Poland email: [email protected] Abstract—Problem of microscopic stability of dense matter in the case of a phase transition to the new, dense phase (for example quark matter) is discussed. An analysis of the constraints resulting from the observations of 2M䉺 pulsars and the consequences for a stiffness of a dense phase are presented. This may lead to the insta bility with respect to reconversion to the basic nuclear phase before reaching maximum mass. Macroscopic stability is defined as a dynamical stability with respect to radial oscillations (in the case of rotat ing star—axisymmetric pulsations). The conditions for the stability of the star with a small, dense core of new phase are discussed and the universality of the stability properties of the families of rotating neutron stars is presented. DOI: 10.1134/S1063779615050317 1
1. INTRODUCTION
Many models of dense matter predict existence of phase transitions at high pressure and density, i.e. con ditions typical to the interior of neutron stars. In partic ular, strangeness of dense matter is postulated through appearance of hyperons and deconfinement of quarks. Phase transitions to other exotic phases of matter, as pion or kaon condensates were also considered [1]. Observations of neutron stars and measurements of their parameters (mass, radius) seem to be the only pos sibility to study properties of cold, catalyzed matter at density ρ ~ 1015 g cm–3. Recent determinations of the mass of two pulsars (2M䉺) put a constraint on the dense matter properties allowing only for a relatively stiff equation of state (EOS). From the other side, phase transitions and appearance of new particles (for exam ple hyperons) always result in softening of the EOS. As a consequence constructing models of neutron stars with phase transition in their center requires strong tun ing of the dense matter model. In the present paper we discuss two aspects of a sta bility properties of the neutron stars in the case of a phase transition in the interior. For catalyzed matter the microscopic stability correspond to the minimum of the thermodynamical potential. If two phases of dense matter are possible, the stable phase minimizes the Gibbs thermodynamical function at a given pres sure. The relation between the observational con straint (Mmax > 2M䉺) and the stiffness of the EOS at high density is discussed in Section 2. The softening of the EOS could lead to the interest ing properties of the pulsar evolution. The backbend ing phenomenon, a temporary spinup era as an iso 1 The article is published in the original.
lated pulsar looses its energy and angular momentum, was proposed by Glendenning et al. [2] as a signature of a phase transition to quark phase in the center of neutron star. However, if phase transition is connected with relative strong softening it results in a dynamical destabilization o
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