Exploring pathways to forming twin stars

A viable model for the dense matter equation of state above the nuclear saturation density includes a hadron-to-quark phase transition at densities relevant to compact objects. In this case, stable hybrid hadron-quark stars can arise. An even more interesting scenario is one where the hadron-to-quark phase transition results in the emergence of a third branch of stable compact objects (in addition to white dwarfs and neutron stars). Inherent to the presence of a third family of compact stars is the existence of twin stars – hybrid stars with the same mass as the corresponding neutron stars, but with smaller radii. If twin stars exist in nature, it raises a question about the mechanism that leads to their formation. Here, we explore gravitational collapse as a pathway to the formation of low-mass twin stars. We perform fully general relativistic simulations of the collapse of a stellar iron core, modeled as a cold degenerate gas, to investigate whether the end product is a neutron star or a twin star. Our simulations show that even with unrealistically large initial perturbations, the core fails to form a twin star. We find that twin stars can potentially form due to stellar mass loss from a slightly more massive hybrid star that was initially produced in the collapse of a more massive core. The challenge in producing twin stars in gravitational collapse suggests the rarity of twin stars in nature.