Non-equilibrium effects on stability of hybrid stars with first-order phase transitions

Constraining the dense matter equation of state (EOS), including the nature of its phase transitions, is a fundamental goal of nuclear physics and neutron star astrophysics. To this end, we consider the effects of out-of-chemical-equilibrium physics at first-order phase transitions on the radial modes and hence stability of compact stars. For barotropic EOS, this is done by allowing the adiabatic sound speed to differ from the equilibrium sound speed. We show that doing so extends the stable branches of stellar models, allowing stars with rapid phase transitions to support stable higher-order stellar multiplets similarly to stars with multiple slow phase transitions. For non-barotropic EOS, we derive a new junction condition to impose on the oscillation modes at the phase transitions. This "reactive condition" is consistent with the generalized junction conditions between two phases and has the common rapid and slow conditions as limiting cases. We apply this condition to hybrid stellar models and show that like in the slow limiting case, some stars that are unstable according to the standard stability criterion are stabilized by a finite chemical reaction speed.