Relativistic hybrid stars with sequential first-order phase transitions and heavy-baryon envelopes

TL;DR

This paper models hybrid stars with phase transitions to quark matter, analyzing their mass, radius, and tidal deformability, and compares results with gravitational wave data to explore signatures of such transitions.

Contribution

It introduces models of hybrid stars with sequential first-order phase transitions and diverse hadronic envelopes, assessing their compatibility with gravitational wave observations.

Findings

Hybrid star configurations can match GW170817 mass limits.

Non-nucleonic phases are favored by GW data if the nuclear EOS is stiff.

Multiple star types can merge, with up to six combinations in triplet configurations.

Abstract

We compute the mass, radius and tidal deformability of stars containing phase transitions from hadronic to quark phase(s). These quantities are computed for three types of hadronic envelopes: purely nuclear, hyperonic, and $\Delta$-resonance--hyperon admixed matter. We consider either a single first-order phase transition to a quark phase with a maximally stiff equation of state (EOS) or two sequential first-order phase transitions mimicking a transition from hadronic to a quark matter phase followed by a second phase transition to another quark phase. We explore the parameter space which produces low-mass twin and triplet configurations where equal-mass stars have substantially different radii and tidal deformabilities. We demonstrate that while for purely hadronic stiff EOS the obtained maximum mass is inconsistent with the upper limit on this quantity placed by GW170817, the inclusion of the hyperonic and $\Delta$-resonance degrees of freedom, as well as the deconfinement phase transition at sufficiently low density, produces a configuration of stars consistent with this limit. The obtained hybrid star configurations are in the mass range relevant for the interpretation of the GW170817 event. We compare our results for the tidal deformability with the limits inferred from GW170817 showing that the onset of non-nucleonic phases, such as $\Delta$-resonance--hyperon admixed phase and/or the quark phase(s), are favored by this data if the nuclear EOS is stiff. Also, we show that low-mass twins and especially triplets proliferate the number of combinations of possible types of stars that can undergo a merger event, the maximal number being six in the case of triplets. The prospects for uncovering the first-order phase transition(s) to and in quark matter via measurements of tidal deformabilities in merger events are discussed.