The search for the true ground state of the dense matter remains open since Bomer, Terasawa and other raised the possibility of stable quarks, boosted by Witten's strange matter hypothesis in 1984. Within this proposal, the strange matter is assumed to be composed of strange quarks in addition to the usual ups and downs, having an energy per baryon lower than the strangeless counterpart, and even lower than that of nuclear matter. In this sense, neutron stars should actually be strange stars. Later work showed that a paired, symmetric in flavor, color-flavor locked (CFL) state would be preferred to the one without any pairing for a wide range of the parameters (gap ∆, strange quark mass m s , and bag constant B). We use an approximate, yet very accurate, CFL equation of state (EoS) that generalizes the MIT bag model to obtain two families of exact solutions for the static Einstein field equations constructing families anisotropic compact relativistic objects. In this fashion, we provide exact useful solutions directly connected with microphysics.

From the beginning of current century to the present, the sample of observed neutron star (NS) with measured masses grew owing to the great effort in the observational field, and the so-called "canonical" unique value formerly attributed to NS at birth was put into question. Different groups pointed out the existence of different channels for a NS formation, possibly leading to a multimodal distribution. We employ in the present work tools from frequentist and Bayesian analysis to test some inferences about the mass distribution of neutron stars, in order to clarify if the observed objects belong to the same populations or show differences at the birth/evolutionary history.

The search for the true ground state of dense matter has remained open since Edward Witten proposed the strange matter hypothesis in 1984. In this hypothesis, the strange matter is assumed to be composed of u, d, and s quarks, having an energy per baryon lower than that of quark matter (only u and d), and even lower than that of the nuclear matter. In this sense, neutron stars would be strange stars. Later work showed that a color-flavor-locked (CFL) state would be preferred to the one without any pairing for a wide range of the parameters (gap Δ, strange quark mass m s , and bag constant B). We use an approximate, yet very accurate, CFL equation of state (EoS) to obtain exact solutions for the static Einstein field equations that describe a compact relativistic object, providing the first solution directly connected with microphysics. KEYWORDSCFL phase -exact solution -neutron star -strange star 1

The lack of objects between 2 and 5 M ⊙ in the joint mass distribution of compact objects has been termed the “mass gap,” and attributed mainly to the characteristics of the supernova mechanism precluding their birth. However, recent observations show that a number of candidates reported to lie inside the “gap” may fill it, suggesting instead a paucity that may be real or largely a result of small number statistics. We quantify in this work the individual candidates and evaluate the joint probability of a mass gap. Our results show that an absolute mass gap is not present, to a very high confidence level. It remains to be seen if a relative paucity of objects stands in the future, and how this population can be related to the formation processes, which may include neutron star mergers, the collapse of a neutron star to a black hole, and others.

This research develops a well-established analytical solution of the Einstein-Maxwell field equations. We analyze the behavior of a spherically symmetric and static interior driven by a charged anisotropic matter distribution. The class I methodology is used to close the system of equations and a suitable relation between the anisotropy factor and the electric field is imposed. The inner geometry of this toy model is described using an ansatz for the radial metric potential corresponding to the well-known isotropic Buchdahl space-time. The main properties are explored in order to determine if the obtained model is appropriate to represent a real compact body such as neutron or quark star. We have fixed the mass and radii using the data of the compact objects SMC X-1 and LMC X-4. It was found that the electric field and electric charge have magnitudes of the order of ∼ 10 21 [V /cm] and ∼ 10 20 [C], respectively. The magnitude of the electric field and electric charge depends on the dimensionless parameter χ. To observe these effects on the total mass, mass-radius ratio and surface gravitational red-shift, we computed numerical data for different values of χ.

We present in this talk an overview of the neutron star origin events and masses. Using the existing sample of ∼80 measured neutron star masses, we show that the distribution requires more than one mass-scale and that its highest value inferred from the sample is compatible with 2.5M ⊙ , 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. Finally, we review the connection of supernovae compact remnants with the observed masses and discuss some important features still unclear from simulations related to the former problem of the mass origins.

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