arXiv subjects

Julian Leszek Zdunik

Publications and source records attributed to Julian Leszek Zdunik.

4 recordsLinked to original sources

arXiv2023

Crustal failure as a tool to probe hybrid stars

It is currently unknown if neutron stars (NSs) are composed of nucleons only or are hybrid stars, i.e., in addition to nucleonic crusts and outer cores, they also possess quark cores. Quantum chromodynamics allows for such a possibility, but accurate calculations relevant for compact stars are still elusive. Here we investigate some crust-breaking aspects of hybrid stars. We show that the crust-breaking frequency and maximum fiducial ellipticity are sensitive to the quark-hadron density jump and equation of state stiffness. Remarkably, the crust-breaking frequency related to static tides scales linearly with the mass of the star (for a given companion's mass), and its slope encompasses information about the microphysics of the star. However, for precise crust-breaking frequency predictions, relativistic corrections to Kepler's third law and the Newtonian tidal field should not be ignored. When a liquid quark core touches an elastic hadronic phase (the result of a significant energy-density jump), the maximum ellipticity can increase around an order of magnitude when compared to a liquid quark core touching a liquid hadronic phase. That is relevant because it would increase the odds of detecting continuous gravitational waves from NSs. Our order-of-magnitude analysis also suggests that a given upper limit to the ellipticity(crust-breaking frequency) could have representatives in stars with either small or intermediate(large) energy-density jumps. Therefore, when upper limits to the ellipticity for isolated stars are better constrained or electromagnetic radiation (e.g., gamma-ray precursors) is detected along with gravitational waves in inspiraling binary systems, they may help constrain some aspects of phase transitions in NSs.

astro-ph.HE↗

arXiv2021

Influence of the crust on the neutron star macrophysical quantities and universal relations

Measurements of neutron-star macrophysical properties thanks to multimessenger observations offer the possibility to constrain the properties of nuclear matter. Indeed cold and dense matter as found inside neutron stars, in particular in their core, is not accessible to terrestrial laboratories.We investigate the consequences of using equations of state that employ models for the core and the crust that are not calculated consistently on the neutron-star macrophysical properties, on some of the so-called universal relations and on the constraints obtained from gravitational-wave observations. We use various treatments found in the literature to connect together nonconsistent core and crust equations of state. We then compute the mass, the radius, the tidal deformability, and the moment of inertia for each model. Finally, we assess the discrepancies in the neutron-star macrophysical properties obtained when consistent models for the whole star and nonconsistent ones are employed. The use of crust models nonconsistent with the core introduces an error on the macrophysical parameters which can be as large as the estimated accuracy of current and next generation telescopes. The precision of some of the universal relations reported in the literature is found to be overestimated. We confirm that the equation of the crust has limited influence on the macrophysical properties. The discrepancy between results obtained for a fully consistent equation of state and a nonconsistent one can be reduced if one connects the core and the crust models at baryon densities around 0.08-0.1 fm$^{-3}$. The equation of the crust cannot be probed with current multimessenger observations and near-future ones.

astro-ph.HE↗

arXiv2021

Probing elastic quark phases in hybrid stars with radius measurements

The internal composition of neutron stars is currently largely unknown. Due to the possibility of phase transitions in quantum chromodynamics, stars could be hybrid and have quark cores. We investigate some imprints of elastic quark phases (only when perturbed) on the dynamical stability of hybrid stars. We show that they increase the dynamical stability window of hybrid stars in the sense that the onset of instabilities happen at larger central densities than the ones for maximum masses. In particular, when the shear modulus of a crystalline quark phase is taken at face value, the relative radius differences between elastic and perfect-fluid hybrid stars with null radial frequencies (onset of instability) would be up to $1-2\%$. Roughly, this would imply a maximum relative radius dispersion (on top of the perfect-fluid predictions) of $2-4\%$ for stars in a given mass range exclusively due to the elasticity of the quark phase. In the more agnostic approach where the estimates for the quark shear modulus only suggest its possible order of magnitude (due to the many approximations taken in its calculation), the relative radius dispersion uniquely due to a quark phase elasticity might be as large as $5-10\%$. Finally, we discuss possible implications of the above dispersion of radii for the constraint of the elasticity of a quark phase with electromagnetic missions such as NICER, eXTP and ATHENA.

astro-ph.HE↗

arXiv2019

Tidal deformability and other global parameters of compact stars with strong phase transitions

Using parametric equations of state (relativistic polytropes and a simple quark bag model) to model dense-matter phase transitions, we study global, measurable astrophysical parameters of compact stars such as their allowed radii and tidal deformabilities. We also investigate the influence of stiffness of matter before the onset of the phase transitions on the parameters of the possible exotic dense phase. The aim of our study is to compare the parameter space of the dense matter equation of state permitting phase transitions to a sub-space compatible with current observational constraints such as the maximum observable mass, tidal deformabilities of neutron star mergers, radii of configurations before the onset of the phase transition, and to give predictions for future observations. We studied solutions of the Tolman-Oppenheimer-Volkoff equations for a flexible set of parametric equations of state. We compare tidal deformabilities of stars with weak and strong phase transitions with the results of the GW170817 neutron star merger. Specifically, we study characteristic phase transition features in the $Λ_1-Λ_2$ relation, and estimate the deviations of our results from the approximate formulæ for $\tildeΛ-R(M_1)$ and $Λ$-compactness proposed in the literature. We find constraints on the hybrid equations of state to produce stable neutron stars on the twin branch. For the exemplary equations of state most of the high-mass twins occur for the minimum values of the density jump $λ= 1.33-1.54$; corresponding values of the square of the speed of sound are $α= 0.7-0.37$. We compare results with observations of gravitational waves and with the theoretical causal limit and find that the minimum radius of a twin branch is 9.5 - 10.5 km. For these solutions, the phase transition occurs below 0.56 [fm$^{-3}$].

astro-ph.HE↗