Exploring thermal effects of the hadron-quark matter transition in neutron star mergers

Abstract

We study the importance of the thermal behavior of the hadron-quark phase transition in neutron star (NS) mergers. To this end, we devise a new scheme approximating thermal effects to supplement cold, barotropic equation of state (EOS) models, which is particularly designed for hybrid EOSs, i.e. two-phase EOS constructions with a hadronic regime and a phase of deconfined quark matter. As in a previous, commonly adopted, approximate thermal treatment, we employ an ideal-gas component to model thermal pressure but, additionally, we include an improved description for the coexistence phase of hybrid EOSs. In contrast to the older scheme, our method considers the temperature dependence of the phase boundaries. This turns out to be critical for a quantitative description of quark matter effects in NS mergers, since the coexistence phase can introduce a strong softening of the EOS at finite temperature, which is even more significant than the change of the EOS by the phase transition at $T=0$. We validate our approach by comparing to existing fully temperature-dependent EOS models and find a very good quantitative agreement of postmerger gravitational-wave (GW) features as a figure of merit sensitively tracking the dynamics and thus the impact of quark matter in merger remnants. Simulations with the original thermal ideal-gas approach exhibit sizable differences compared to full hybrid models implying that its use in NS merger simulations with quark matter is problematic. Importantly, our new scheme provides the means to isolate thermal effects of quark matter from the properties of the cold hybrid EOS and thus allows an assessment of the thermal behavior alone. Generally, we find that the thermal properties in hybrid models are more important compared to the thermal behavior of purely baryonic matter. We show that different shapes of the phase boundaries at finite temperature can have a large impact on the postmerger dynamics and GW signal for the same cold hybrid model. This finding demonstrates that postmerger GW emission contains important complementary information compared to properties extracted from cold stars in isolation or during a binary inspiral. We also show by concrete examples that it is even possible for quark matter to only occur and thus be detectable in finite-temperature systems like merger remnants but not in cold NSs. All these findings also illustrate that heavy-ion collision experiments as a probe of the phase diagram at finite temperature bear relevant information for the astrophysics of NS mergers and core-collapse supernovae. Furthermore, our new thermal treatment features the flexibility to be combined with any cold, barotropic hybrid model including effective models of phase transitions, where a large number of models is available. This allows to conduct large parameter studies to comprehensively understand the effects of quark matter in NS mergers.

Topics & Concepts