Quark Matter In Compact Stars Research Articles

Lifting the Veil on Quark Matter in Compact Stars with Core g-mode Oscillations

Abstract Compact stars containing quark matter may masquerade as neutron stars in the range of measured mass and radius, making it difficult to draw firm conclusions on the phase of matter inside the star. The sensitivity of core g-mode oscillations to the presence of a mixed phase may alleviate this difficulty. In hybrid stars that admit quark matter in a mixed phase, the g-mode frequency rises sharply due to a marked decrease in the equilibrium sound speed. Resonant excitation of g-modes can leave an imprint in the waveform of coalescing binary compact stars. We present analytic and numeric results to assess the sensitivity displayed by g-mode oscillations to quark matter in a homogeneous or mixed phase and also compute relevant damping times in quark matter due to viscosity.

Implications of the measurement of pulsars with two solar masses for quark matter in compact stars and heavy-ion collisions: A Nambu–Jona-Lasinio model case study

The precise measurement of the high masses of the pulsars PSR J1614-2230 (M_{1614}=1.97 +- 0.04 solar masses) and PSR J0348-0432 (M_{0348}=2.01 +- 0.04 solar masses) provides an important constraint for the equation of state of cold, dense matter and is suited to give interesting insights regarding the nature and existence of the possible phase transition to deconfined quark matter in the cores of neutron stars. We analyze the stability and composition of compact star sequences for a class of hybrid nuclear - quark-matter equations of state. The quark matter phase is described in the framework of a standard color superconducting 3-flavor Nambu-Jona-Lasinio model and the hadronic phase is given by the Dirac-Brueckner-Hartree-Fock equation of state for the Bonn-A potential. The phase transition is obtained by a Maxwell construction. Within this model setup we aim to constrain otherwise not strictly fixed parameters of the NJL model, namely the coupling strengths in the vector meson and diquark interaction channels. We perform this investigation for two different parameterizations characterized by a different scalar coupling constant. The analysis of flow data obtained in heavy-ion collisions resulted in a further constraint which we account for in our discussion. Massive hybrid stars with extended quark matter cores can be obtained in accordance with all of the considered constraints.

Strange star candidates revised within a quark model with chiral mass scaling

We calculate the properties of static strange stars using a quark model with chiral mass scaling. The results are characterized by a large maximum mass (∼ 1.6 M⊙) and radius (∼10km). Together with a broad collection of modern neutron star models, we discuss some recent astrophysical observational data that could shed new light on the possible presence of strange quark matter in compact stars. We conclude that none of the present astrophysical observations can prove or confute the existence of strange stars.

Equation of state at high densities and modern compact star observations

Recently, observations of compact stars have provided new data of high accuracy which put strong constraints on the high-density behaviour of the equation of state of strongly interacting matter otherwise not accessible in terrestrial laboratories. The evidence for neutron stars with high mass (M = 2.1 ± 0.2 M⊙ for PSR J0751 + 1807) and large radii (R > 12 km for RX J1856-3754) rules out soft equations of state and has provoked a debate whether the occurrence of quark matter in compact stars can be excluded as well. In this contribution, it is shown that modern quantum field theoretical approaches to quark matter including colour superconductivity, and a vector meanfield allow a microscopic description of hybrid stars which fulfil the new, strong constraints. The deconfinement transition in the resulting stiff hybrid equation of state is weakly first order so that its signals have to be expected due to specific changes in transport properties governing the rotational and cooling evolution caused by the colour superconductivity of quark matter. A similar conclusion holds for the investigation of quark deconfinement in future generations of nucleus–nucleus collision experiments at low temperatures and high baryon densities such as CBM @ FAIR.

General relativistic effects on the conversion of nuclear to two-flavor quark matter in compact stars

We investigate the general relativistic (GR) effects on the conversion from nuclear to two-flavor quark matter in compact stars, both static as well as rotating. We find that GR effects lead to qualitative differences in rotating stars, indicating the inadequacy of nonrelativistic or even special relativistic treatments for these cases.

Quark matter in compact stars?

In a theoretical interpretation of observational data from the neutron star EXO 0748-676, Ozel concludes that quark matter probably does not exist in the centre of neutron stars. However, this conclusion is based on a limited set of possible equations of state for quark matter. Here we compare Ozel's observational limits with predictions based on a more comprehensive set of proposed quark-matter equations of state from the literature, and conclude that the presence of quark matter in EXO 0748-676 is not ruled out.

Quark matter in compact stars? (Reply)

Replying to: M. Alford et al. Nature 445, doi: 10.1038/nature05582 (2007) In their comment1 on my theoretical interpretation2 of the observations of EXO 0748–676, Alford . suggest variants of quark-matter equations of state that produce stars consistent with my results. They do not challenge either the method that I propose or my conclusion that the data require a stiff equation of state2.

Gamma Ray Bursts and the transition to Quark Matter in Compact Stars

We discuss a model for long Gamma-Ray-Bursts in which the central engine is associated with the conversion process of a metastable hadronic star into a star containing quark matter. We analyze also the observational signatures of the model, i.e. the Supernova-GRB temporal connection and the existence of long quiescent times in the temporal structure of Gamma-Ray-Bursts.

Equation of state of strongly interacting matter in compact stars

We study the equation of state of strongly interacting matter at large densities and vanishing temperature. The hadronic matter equation of state is computed in a relativistic mean-field model and the quark matter equation of state is computed using a NJL-type model which takes into account the possibility of formation of the gapless color flavor locked phase. We focus in particular on the possible phase transition from hadronic matter to quark matter using both Maxwell and Gibbs constructions. We finally discuss the relevance of the equation of state in the context of compact stars and we propose some astrophysical signatures of the presence of quark matter in compact stars.

Neutrino emission from ungapped quark matter

We study neutrino emission from a normal, ungapped, quark phase in the core of a compact star. Neutrino emission from noninteracting quark matter leads to an emissivity that scales as $ϵ\ensuremath{\sim}{T}^{7}$. We show that the emissivity is enhanced by a combination of Fermi liquid and non-Fermi liquid effects. Fermi liquid effects lead to an emissivity that scales as $ϵ\ensuremath{\sim}{\ensuremath{\alpha}}_{s}{T}^{6}$, as originally shown by Iwamoto. We demonstrate that non-Fermi liquid effects further enhance the rate, leading to $ϵ\ensuremath{\sim}{\ensuremath{\alpha}}_{s}^{3}{T}^{6}\mathrm{log}(m/T{)}^{2}$, where $m$ is the electric screening scale and $m\ensuremath{\gg}T$ under the conditions found in compact stars. We show, however, that combined with non-Fermi liquid effects in the specific heat the enhancement in the emissivity only leads to a modest reduction in the temperature of the star at late times. Our results confirm existing bounds on the presence of ungapped quark matter in compact stars. We also discuss neutrino emission from superconducting phases with ungapped fermionic excitations.