THE STABILITY PROPERTIES OF 2-TEMPERATURE WHITE-DWARF RADIATIVE SHOCK-WAVES

The stability properties of two-temperature radiative shocks with powe r-law cooling functions and cooling functions appropriate to accretion onto nonmagnetic white dwarfs are studied. Two-temperature effects ar e important whenever the cooling is fast compared to the electron-ion, e-i, energy coupling because, in most astrophysical environments, the electrons dominate the cooling while the ions act simply as stores of the thermal energy. Two-temperature effects are usually important for the shocks encountered in compact X-ray binary systems. We find that: (1) two-temperature effects enhance the oscillatory instability of ra diative shock waves. The weaker the e-i coupling through the shock tra nsition and in the postshock cooling region, the stronger the destabil ization due to two-temperature effects. For example, for radial oscill ations, the fundamental mode (F), the lowest frequency mode, is stable in one-temperature hows dominated by bremsstrahlung. In two-temperatu re flows, the F mode is unstable if tau(br)/tau(ei) less than similar to 1, where tau(br) and tau(ei) are the postshock bremsstrahlung and e -i energy equilibration timescales, if the e-i energy coupling through the shock transition is weak. By weak, we mean shocks for which the e lectrons emerge from the shock transition with less than similar to 40 % of the thermal energy. When e-i coupling through the shock transitio n is strong, the radial F mode is stable regardless of the strength of the e-i coupling in the cooling region. This property of two-temperat ure flows means that observations of radial F mode oscillations in bre msstrahlung-dominated shocks could place constraints on the dissipatio n process at work in the strong shock waves encountered in astrophysic al situations. (2) Radial oscillation modes behave as expected for rea listic white dwarf functions (bremsstrahlung plus Compton cooling) in that Compton cooling strongly damps radial oscillations. However, for nonradial oscillations, Compton cooling actually enhances instability. This suggests that for weakly magnetic accretion flows where both bre msstrahlung and Compton cooling are important, shock oscillations coul d arise, contrary to expectations based on the effect Compton cooling has on the radial oscillations.