Astrophysics Constraints on the EOS

Abstract

We present two types of models for hybrid compact stars composed of a quark core and a hadronic mantle with an abrupt first order phase transition at the interface which are in accordance with the latest astrophysical measurements of two 2 M ⊙ pulsars. While the first is a schematic one, the second one is based on a QCD motivated nonlocal PNJL model with density-dependent vector coupling strength. Both models support the possibility of so called twin compact stars which have the same mass but different radius and internal structure at high mass (∼ 2 M ⊙ ), provided they exhibit a large jump ∆ε in the energy density of the first order phase transition fulfilling ∆ε/ε crit > 0.6. We conclude that the measurement of high-mass twin stars would support the existence of a first order phase transition in symmetric matter at zero temperature entailing the existence of a critical end point in the QCD phase diagram.8th International Workshop on Critical Point and Onset of Deconfinement,

Hybrid equation of state with pasta phases, and third family of compact stars

“…These values indicate that there might be tension with the mass-radius region excluded by the tidal deformability constraint from GW170817 shown as the right cross-hatched region in that figure. Such a tension could be resolved by the existence of mass twin stars in that mass interval around 1.4 M , which are a feature accompanying the existence of a third family of compact stars and thus testify for a strong phase transition with a large jump of the energy density in compact star matter [8,9]. We look forward to the further NICER analyses for this and other mil- lisecond pulsars, with smaller statistical and systematic errors.…”

“…Such a third family branch can exist if there is a strong firstorder phase transition [7], e.g., from hadrons to quarks with a sufficiently large jump in the energy density taking place inside the CS [8]. A robust observation of pulsars with similar masses and substantially different radii (CS twin configurations) would reveal the existence of the first-order phase transition at zero temperature and thus prove the existence of the QCD critical endpoint [9].…”

Third family of compact stars within a nonlocal chiral quark model equation of state

“…If these constraints could be fulfilled, the corresponding EoS could provide a viable scenario for the recently observed [8] binary neutron star merger event: at least one of the two stars could be a hybrid star from the third family branch which is sufficiently compact to make the binary system fulfill the condition on the tidal deformabilities derived from the observation of the inspiral in the LIGO gravitational wave detector, see [9,10] for a recent discussion of this scenario. Up to now, the third family case was investigated with very schematic EoS for the high-density phase, like the bag model [6], the constant-speed-of-sound (CSS) model [9,[11][12][13][14][15][16][17], the multi-polytrope model [9,18,19], but also dynamical models for interacting quark matter like the Nambu-Jona-Lasinio (NJL) model with higher order quark interactions in the Dirac vector channel [20,21], or a relativistic density-functional model [10,22]. The question arises whether the third family phenomenon could be obtained also for EoS derived from dynamical quark models which are closer to QCD in the sense that they take into account the running of the dynamical quark masses with 4-momentum and embody a (dynamical) confinement mechanism.…”

Astrophysical Aspects of General Relativistic Mass Twin Stars

“…The question arose whether a hybrid star EoS describing a third family of compact stars with a maximum mass above 2 M could also be obtained when applying the standard scheme of a two-phase approach based on a realistic nuclear matter EoS and a microscopically well-founded quark matter EoS, both joined, e.g., by a Maxwell construction. A positive answer was given already in 2013, when two examples of this kind were presented in [Blaschke et al (2013a)], where the excluded-volume corrected nuclear EoS APR and DD2 were joined with a quark matter EoS based on the nonlocal NJL model approach [Blaschke et al (2007); Benic et al (2014)], augmented with a density dependent repulsive vector meanfield that was constructed by employing a thermodynamically consistent interpolation scheme introduced in Ref. [Blaschke et al (2013b)].…”