Fluid-dynamical approximation for finite Fermi systems

To use static Thomas-Fermi functionals for the local energy density in dynamical problems is to implicity assume locally equilibrated momentum distributions. This assumption, however, is highly questionable for small degenerate Fermi systems like nuclei, an indication being the drastic failure of the standard hydrodynamical model for the simple case of the isoscalar quadrupole resonance. A fluid-dynamical description for degenerate Fermi systems is derived without assuming local equilbrium. The essential feature is the dynamical distortion of the local Fermi surface which leads to additional restoring forces for the elementary modes. Application to giant resonances resolves previous discrepancies and brings the results into agreement with microscopic RPA sum rules. In addition to the usual normal parity models the new formalism leads to non-normal parity modes which correspond to the convectional part of magnetic-type resonances and cannot be obtained from standard hydrodynamics. (Auth.)