Astrophysics in the XXI Century with Compact Stars

The following sections are included:

• Dedication
• Foreword
• Preface
• Contents
• List of Figures
• List of Tables

Birth events of neutron stars are tightly related with their observed masses, which in turn are also linked to the state of matter that constitutes them. Using a sample of 76 measured neutron star masses, we show that its distribution requires more than one mass scale, a result that is probably related with the existence of different formation and/or evolutionary channels. The highest value allowed for the mass of a neutron star, inferred from the sample, is compatible with 2.5 M_⨀, suggesting that very heavy neutron stars may be present in Nature. The case of some “spider” systems and the enigmatic high mass object in the GW190408 event, as well as the highest measured values from other binaries may support a high M_max, that gets closer to the Rhoades–Ruffini limit. Now we have entered the multi-messenger astronomy era, gravitational waves observations can substantially contribute to all these open questions. In the following chapter, we review all these issues and suggest future directions.

The Bodmer-Terazawa-Witten hypothesis of absolutely stable quark matter made of up, down, and strange quarks ((u,d,s) matter) is of much astrophysical and fundamental interest. One consequence is a large binding energy release associated with the conversion of a neutron star to a quark star. A quantitative understanding of the dynamic aspect of the conversion is necessary in order to find out whether this energy is released quietly or in an explosive manner. We address numerically (i.e. solving the reaction-diffusion-advection equations for (u,d) to (u,d,s) combustion) the dynamic processes through which the conversion appears. We find fundamentally very different results from semi-analytic calculations, with front speeds that are several orders of magnitude higher for the former. Resolving the hadronic-quark-matter interface is necessary, since approximations like Coll’s condition may quench the burning, while properly resolving the flame can make combustion always thermodynamically favourable if the hypothesis of absolutely stable strange quark matter is true. We find that lepton physics, including weak decays, electron equation-of-state (EoS), neutrino EoS, and neutrino transport are at the very least as important to the physics of the burning front as the EoS of highly dense matter. In concert, these effects induce novel wrinkling instabilities (such as the deleptonization instability) possibly leading to a deflagration-detonation and/or a quark-core collapse Quark-Nova.

What if normal baryonic matter is squeezed so that atomic nuclei come in close contact? This is a problem implying a broad interest in astrophysics, but is not solved because of non-perturbative issues of the strong interaction. The weak interaction also plays an important role there: although it is in the mainstream that neutronization occurs to make a state with asymmetric isospin, we are addressing that quark flavour-symmetry from 2 (u and d quarks) to 3 (u, d and s quarks) could be restored during squeezing. The latter is called strangeonization. Strangeon is conjectured to be the constituent of bulk strong matter, as an analogy of nucleon for an atomic nucleus. We will explain strangeon and strangeon matter in this chapter, as well as discuss their observational manifestations in astrophysics.

As is well known, the existence of fast and ultra-massive white dwarfs (WDs) has been discovered in recent years. We estimate the mass density thresholds for the onset of electron capture reactions and pycnonuclear reactions in the cores of fast, massive and highly magnetized white dwarfs (HMWDs) and discuss the impact of microscopic stability and rapid rotation on the structure and stability of different WD constitutions. We confirm that fast rotation increases the mass of a WD by up to 10%, while the central density may drop by one to two orders of magnitude, depending on stellar mass and rate of rotation. We also note that the central densities of the rotating WDs are smaller than those of the non-rotating stars, since less pressure is to be provided by the equation of state (EoS) in the rotating case, and that the maximum-mass limit slightly decreases when lattice contributions are taken into account, which soften the EoS mildly. The softening leads to WDs with somewhat smaller radii and therefore smaller Kepler periods. Overall, one sees that very massive and magnetic ¹²C + ¹⁶O WDs have rotational Kepler periods on the order of 0.5 s. Pycnonuclear reactions are triggered in these WDs at masses that are markedly smaller than the maximum WD masses. The corresponding rotational periods turn out to be in the 5 s (∼0.2 Hz) range.

We review the covariant density functional approach to the equation-of-state (EoS) of the dense nuclear matter in compact stars. The main emphasis is on the hyperonization of the dense matter, and the role played by the Δ-resonances. The implications of hyperonization for the astrophysics of compact stars, including the EoS, composition, and stellar parameters are examined. The mass–radius relation and tidal deformabilities of static and rapidly rotating (Keplerian) configurations are discussed in some detail. We briefly touch upon some other recent developments involving hyperonization in hot hypernuclear matter at high- and low-densities.

Spinning neutron stars can emit long-lived gravitational waves. There are several mechanisms that can produce such continuous wave emission. These mechanisms relate to the strains in the elastic crust, the star’s magnetic field, superfluidity of the neutron fluid, and bulk oscillations of the entire star. In this chapter, we describe how the frequency content of the gravitational wave signal, and its relation to any electromagnetically observed spin frequency, can be used to constrain the mechanism producing the gravitational waves. These ideas will be of use in the event of the first detections of such signals, and help convert a detection into useful physical insight.

The presence of superfluid phases in the interior of a neutron star affects its dynamics, as it allows for neutrons to flow relative to the non-superfluid (normal) components of the star with little or no viscosity. A probe of superfluidity comes from pulsar glitches, sudden jumps in the observed rotational period of radio pulsars. Most models of glitches build on the idea that a superfluid component of the star is decoupled from the spin-down of the normal component, and its sudden re-coupling leads to a glitch. This transition in the strength of the hydrodynamic coupling is explained in terms of quantum vortices (long-lived vortices that are naturally present in the neutron superfluid at the microscopic scale). After introducing some basic ideas, we derive (as a pedagogical exercise) the formal scheme shared by many glitch studies. Then, we apply these notions to present some progress in understanding and modelling pulsar glitches, notably the possibility to constrain the nuclear physics input and the extraction of the coupling strength from observations of the elusive spin-up phase of a glitch.

We outline a theoretical approach supporting strong phase transitions from normal nuclear matter to the deconfined quark–gluon plasma, in the equation of state (EOS) for compact star matter. Implications of this hypothesis are discussed for astrophysical applications. Special emphasis is devoted to potentially detectable signatures, which can be directly related with the occurrence of a sufficiently strong phase transition. Therefore, simulations of core-collapse supernovae and binary compact star mergers are considered, including the subsequent emission of gravitational waves and, in the case of supernova, in addition the neutrinos play the role of messengers.