BILAYERED STARS. “The observation of 2-solar mass neutron stars suggests that the EoS for compact stars needs to be sufficiently stiffer than the normal nuclear matter to accommodate the large mass. This enables one to think of a stable mass configurations with “exotic” matter in their interiors” (EoS = equations of state).
https://www.sciencedirect.com/science/article/pii/S221268642030039X