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Register a new userMaximum mass of compact stars from gravitational wave events with finite-temperature equations of state
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We conjecture and verify a set of universal relations between global parameters of hot and fast-rotating compact stars, including a relation connecting the masses of the mass-shedding (Kepler) and static configurations. We apply these relations to the GW170817 event by adopting the scenario in which a hypermassive compact star remnant formed in a merger evolves into a supramassive compact star that collapses into a black hole once the stability line for such stars is crossed. We deduce an upper limit on the maximum mass of static, cold neutron stars $ 2.15^{+0.10}_{-0.07}le M^star_{mathrm{TOV}} le 2.24^{+0.12}_{-0.10} $ for the typical range of entropy per baryon $2 le S/A le 3$ and electron fraction $Y_e = 0.1$ characterizing the hot hypermassive star. Our result implies that accounting for the finite temperature of the merger remnant relaxes previously derived constraints on the value of the maximum mass of a cold, static compact star.
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We give in this Chapter an overview of the problem of neutron star mass distribution, the issue of the maximum mass as inferred from the existing sample and the new gravitational wave events, and the connection with the formation events. It is shown that at least two different mass-scales (and possibly three) are favored by Bayesian and frequentist analysis, and the resulting maximum mass on empirical grounds only is $sim 2.5 M_{odot}$, making room for an interpretation of the lighter component of the GW190408 merger as a neutron star (as suggested by the GW event population analysis). We discuss in some length the challenges for a theoretical construction of a stiff equation of state and the status of available supernova explosions (single and binary simulations) and AIC expected to provide the masses at birth.
Observations of isolated neutron stars place constraints on the equation of state (EOS) of cold, neutron-rich matter, while nuclear physics experiments probe the EOS of hot, symmetric matter. Many dynamical phenomena, such as core-collapse supernovae, the formation and cooling of proto-neutron stars, and neutron star mergers, lie between these two regimes and depend on the EOS at finite temperatures for matter with varying proton fractions. In this paper, we introduce a new framework to accurately calculate the thermal pressure of neutron-proton-electron matter at arbitrary density, temperature, and proton fraction. This framework can be expressed using a set of five physically-motivated parameters that span a narrow range of values for realistic EOS and are able to capture the leading-order effects of degenerate matter on the thermal pressure. We base two of these parameters on a new approximation of the Dirac effective mass, with which we reproduce the thermal pressure to within <~30% for a variety of realistic EOS at densities of interest. Three additional parameters, based on the behavior of the symmetry energy near the nuclear saturation density, allow for the extrapolation of any cold EOS in beta-equilibrium to arbitrary proton fractions. Our model thus allows a user to extend any cold nucleonic EOS, including piecewise-polytropes, to arbitrary temperature and proton fraction, for use in calculations and numerical simulations of astrophysical phenomena. We find that our formalism is able to reproduce realistic finite-temperature EOS with errors of <~20% and offers a 1-3 orders-of-magnitude improvement over existing ideal-fluid models.
The first detection of gravitational waves from the binary neutron star merger event GW170817 has started to provide important new constraints on the nuclear equation of state at high density. The tidal deformability bound of GW170817 combined with the observed two solar mass neutron star poses a serious challenge to theoretical formulations of realistic equations of state. We analyze a fully comprehensive set of relativistic nuclear mean-field theories by confronting them with the observational bounds and the measured neutron-skin thickness. We find that only a few models can withstand these bounds which predict a stiff overall equation of state but with a soft neutron-proton symmetry energy. Two possible indications are proposed: Circumstantial evidence of hadron-quark phase transition inside the star and new parametrizations that are consistent with ground state properties of finite nuclei and observational bounds. Based on extensive analysis of these sets, an upper limit on the radius of a $1.4M_odot$ neutron star of $R_{1.4}lesssim 12.9$ km is deduced.
The nuclear symmetry energy plays a role in determining both the nuclear properties of terrestrial matter as well as the astrophysical properties of neutron stars. The first measurement of the neutron star tidal deformability, from gravitational wave event GW170817, provides a new way of probing the symmetry energy. In this work, we report on new constraints on the symmetry energy from GW170817. We focus in particular on the low-order coefficients: namely, the value of the symmetry energy at the nuclear saturation density, S_0, and the slope of the symmetry energy, L_0. We find that the gravitational wave data are relatively insensitive to S_0, but that they depend strongly on L_0 and point to lower values of L_0 than have previously been reported, with a peak likelihood near L_0 ~ 20 MeV. Finally, we use the inferred posteriors on L_0 to derive new analytic constraints on higher-order nuclear terms.
Motivated by the recent discoveries of compact objects from LIGO/Virgo observations, we study the possibility of identifying some of these objects as compact stars made of dark matter called dark stars, or the mix of dark and nuclear matters called hybrid stars. In particular, in GW190814, a new compact object with 2.6 $M_{odot}$ is reported. This could be the lightest black hole, the heaviest neutron star, and a dark or hybrid star. In this work, we extend the discussion on the interpretations of the recent LIGO/Virgo events as hybrid stars made of various self-interacting dark matter (SIDM) in the isotropic limit. We pay particular attention to the saddle instability of the hybrid stars which will constrain the possible SIDM models.
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