The existence of neutron stars with masses of $$\sim${}2{M}_{$\bigodot${}}$ requires a stiff equation of state at high densities. On the other hand, the necessary appearance also at high densities of new degrees of freedom, such as hyperons and $\mathrm{$\Delta${}}$ resonances, can lead to a strong softening of the equation of state with resulting maximum masses of $$\sim${}1.5{M}_{$\bigodot${}}$ and radii smaller than $$\sim${}10\text{ }\text{ }\mathrm{km}$. Hints for the existence of compact stellar objects with very small radii have been found in recent statistical analyses of quiescent low-mass X-ray binaries in globular clusters. We propose an interpretation of these two apparently contradicting measurements, large masses and small radii, in terms of two separate families of compact stars: hadronic stars, whose equation of state is soft, can be very compact, while quark stars, whose equation of state is stiff, can be very massive. In this respect an early appearance of $\mathrm{$\Delta${}}$ resonances is crucial to guarantee the stability of the branch of hadronic stars. Our proposal could be tested by measurements of radii with an error of $$\sim${}1\text{ }\text{ }\mathrm{km}$, which is within reach of the planned Large Observatory for X-ray Timing satellite, and it would be further strengthened by the discovery of compact stars heavier than $$\sim${}2{M}_{$\bigodot${}}$.

We investigate the possibility that the low mass companion of the black hole in the source of GW190814 was a strange quark star. This possibility is viable within the so-called two-families scenario in which neutron stars and strange quark stars coexist. Strange quark stars can reach the mass range indicated by GW190814, $M\sim (2.5–2.67)M_{\odot }$ due to a large value of the adiabatic index, without the need for a velocity of sound close to the causal limit. Neutron stars (actually hyperonic stars in the two-families scenario) can instead fulfill the presently available astrophysical and nuclear physics constraints which require a softer equation of state. In this scheme it is possible to satisfy both the request of very large stellar masses and of small radii while using totally realistic and physically motivated equations of state. Moreover it is possible to get a radius for a $1.4M_{\odot }$ star of the order or less than 11 km, which is impossible if only one family of compact stars exists.

The analysis of the central compact object within the supernova (SN) remnant HESS J1731-347 suggests that it has a small radius and, even more interestingly, a mass of the order or smaller than 1 M _⊙. This raises the question of which astrophysical process could lead to such a small mass, since the analysis of various types of SN explosions indicate that is it not possible to produce a neutron star with a mass smaller than about 1.17 M _⊙. Here we show that masses of the order or smaller than 1 M _⊙ can be obtained in the case of strange quark stars and that it is possible to build a coherent model explaining not only the mass and the radius of that object, but also its slow cooling suggested in various analyses. We also show that an astrophysical path exists, which leads to the formation of such an object, and we discuss the role played in that scenario by strangelets assumed to constitute the dark matter.

Baryon-to-meson Transition Distribution Amplitudes (TDAs) encoding valuable new information on hadron structure appear as building blocks in the collinear factorized description for several types of...

We propose a model for short duration gamma-ray bursts (sGRBs) based on the formation of a quark star after the merger of two neutron stars. We assume that the sGRB central engine is a proto-magnetar, which has been previously invoked to explain the plateaulike X-ray emission observed following both long and short GRBs. Here, we show that: (i) a few milliseconds after the merger it is possible to form a stable and massive star made in part of quarks; (ii) during the early cooling phase of the incompletely formed quark star, the flux of baryons ablated from the surface by neutrinos is large and it does not allow the outflow to achieve a bulk Lorentz factor high enough to produce a GRB; (iii) after the quark burning front reaches the stellar surface, baryon ablation ceases and the jet becomes too baryon poor to produce a GRB; (iv) however, between these two phases a GRB can be produced over the finite time scale required for the baryon pollution to cease; a characteristic time scale of the order of $\sim 0.1\mathrm{s}$ naturally results from the time the conversion front needs to cover the distance between the rotational pole and the latitude of the last closed magnetic field line; (v) we predict a correlation between the luminosity of the sGRB and its duration, consistent with the data; (vi) our model also predicts a delay of the order of ten seconds between the time of the merger event and the sGRB, allowing for the possibility of precursor emission and implying that the jet will encounter the dense cocoon formed immediately after the merger.

The total binding energy of compact stars is the sum of the gravitational binding energy $(BE{)}_g$ and the nuclear binding energy $(BE{)}_n$, the last being related to the microphysics of the interactions. While the first is positive (binding) both for hadronic stars and for strange quark stars, the second is large and negative for hadronic stars (antibinding) and either small and negative (antibinding) or positive (binding) for strange quark stars. A hadronic star can convert into a strange quark star with a larger radius because the consequent reduction of $(BE{)}_g$ is overcompensated by the large increase in $(BE{)}_n$. Thus, the total binding energy increases due to the conversion and the process is exothermic. Depending on the equations of state of hadronic matter and quark matter and on the baryonic mass of the star, the contrary is obviously also possible, namely, the conversion of hadronic stars into strange quark stars having smaller radii, a situation more often discussed in the literature. We provide a condition that is sufficient and in most of the phenomenologically relevant cases also necessary in order to form strange quark stars with larger radii while satisfying the exothermicity request. Finally, we compare the two schemes in which quark stars are produced (one having large quark stars and the other having small quark stars) among themselves and with the third-family scenario and we discuss how present and future data can discriminate among them.