Other
Scientific paper
Oct 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt........13s&link_type=abstract
PhD thesis, Univ. Sydney, Australia , (1999)
Other
6
Scientific paper
This thesis presents and applies a formulation for perturbative stability analyses of radiative shocks with multiple cooling processes, longitudinal and transverse perturbations, and unequal electron and ion temperatures. The stand-off accretion shock above a stationary wall (e.g. white dwarf surface) may experience global thermal instability, depending on the functional forms of radiative cooling processes present. This analysis assumes a composite cooling function that explicitly sums bremsstrahlung cooling (which promotes instability), with a stabilizing cooling process which is a power-law in density and temperature. One illustrative case approximates the effects of cyclotron cooling in magnetic cataclysmic variables (mCVs), some of which exhibit ~1Hz optical quasi-periodic oscillations.
Complex eigenfrequencies describe stability properties and frequencies of oscillatory modes. Details of the corresponding eigenfunctions portray local physics of the shock instabilities. Oscillations of post-shock structure modulate the X-ray and optical emissions. Profiles and integrated luminosity responses are calculated for modes of mCV accretion shocks, and the relative amplitudes and phasings are considered.
Oscillatory properties depend on efficiency of the secondary cooling process (ɛs), and other parameters. Stability differs for each mode, and is functionally more complicated than timescale comparisons. Instability of a mode n does not imply the instability of mode n+1. For one-temperature shocks, increasing ɛs stabilizes each mode monotonically. Higher-order temperature dependence tends to reduce instability. Cyclotron cooling generally suppresses instabilities, which is consistent with numerical simulations by other investigators. In two-temperature shocks the inefficient e-i energy exchange influences the oscillations, e.g. destabilizing the shock when cyclotron cooling is very efficient, (causing stability to be non-monotonic in ɛs).
Frequencies are approximately quantised. In many conditions the modes resemble those of a pipe open at one end; at two-temperature extremes they resemble a doubly-open pipe. The influence of the lower boundary condition is investigated further by calculating eigenfrequencies and eigenfunctions subject to alternatives to the fixed-wall condition.
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