Radial oscillations of protoneutron stars

We investigate radial oscillations of protoneutron stars (PNSs), employing equations of state described by the Brueckner-Hartree-Fock theory or the relativistic mean field model, and assuming isentropy and fixed lepton fractions for the internal structure. We calculate the eigenfrequencies and corresponding oscillation functions, which show different characteristics in different mass regions. In the low-mass region around $1.4{M}_{\ensuremath{\bigodot}}$, the radial oscillation frequencies are lowered by large entropy and neutrino trapping, along with a reduction of the average adiabatic index. Therefore, during the PNS evolution to a cold neutron star, the frequency increases substantially. In the region close to the maximum mass, the fundamental oscillation frequency drops rapidly and vanishes at the maximum mass, in accordance with the critical stability criterion $\ensuremath{\partial}M/\ensuremath{\partial}{\ensuremath{\rho}}_{c}=0$, as for cold neutron stars.