Abstract : In dense stellar environments, nuclei may become unstable against electron captures and/or neutron
emissions. These processes are of particular importance for determining the internal constitution of
white-dwarf cores and neutron-star crusts. In this paper, the role of electron exchange and polarization
effects is studied. In particular, the instability condition for the onset of electron captures and neutron
emissions is extended so as to account for electron exchange and polarization. Moreover, general
analytical expressions for the corresponding density and pressure are derived. The corrections to the
electron-capture threshold in white-dwarf cores are found to be very small. Likewise, the neutrondrip
density and pressure in the crusts of accreting and nonaccreting neutron stars are only slightly
shifted. Depending on the nuclear mass model employed, electron polarization may change the
composition of the crust of nonaccreting neutron stars. On the other hand, the current uncertainties
in the masses of neutron-rich Kr and Sr isotopes are found to be more important than electron exchange
and polarization effects.
