  We consider the relativistic ion-acoustic mode in a plasma composed by cold
ions and an ultra-degenerate electron gas, described the relativistic
Vlasov-Poisson system. A critical examination of popular fluid models for
relativistic ion-acoustic waves is provided, comparing kinetic and hydrodynamic
results. The kinetic linear dispersion relation is shown to be reproduced by
the rigorous relativistic hydrodynamic equations with Chandrasekhar's equation
of state.
