Computer Science
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........10r&link_type=abstract
Ph.D. Thesis Oregon Univ., Eugene.
Computer Science
Cyclotron Radiation, Light Emission, Stellar Mass Accretion, Stellar Radiation, White Dwarf Stars, Oscillations, Shock Waves
Scientific paper
Quasi-periodic oscillations (QPO's) of period of 1-3 seconds, with root mean square amplitude 1-3 percent have been discovered in the optical emission from four AM Her objects, AN UMa, V834 Cen, VV Pup, and EF Eri. Middleditch (1982) suggested that the QPO's were due to the oscillatory instability of radiative shocks discovered by Langer, Chanmugam, and Shaviv (1981). If the QPO's are due to shock oscillations then the QPO's can be used to constrain several fundamental parameters of the AM Her QPO systems such as the mass and magnetic field strength of the white dwarf. Unfortunately, it is not clear if the QPO's are due to the oscillatory instability. The QPO's are found only in the optical portion of the spectrum and, thus, presumably, are due to variations in the cyclotron luminosity of the shock. Prior to this work, no detailed calculations of the cyclotron (optical) emission from accreting white dwarfs have been performed. Chanmugam et al. (1985) presented calculations for the cyclotron emission of white dwarf shocks, but they did not self-consistently solve the radiative transfer and hydrodynamic equations. This work provides theoretical results for the stability properties and the optical spectra of white dwarf radiative accretion shocks for the weak cyclotron case thought to apply to the AM Her objects. Comparison of these results with observations of the AM Her QPO sources suggests that the properties of the shock oscillation model, in its current form, are not compatible with observations. Namely, radiative shocks with parameters typical of the AM Her QPO sources probably cannot sustain limit cycle oscillations due to strong cyclotron emission and/or electron thermal conduction. However, because several aspects of the observations are naturally explained as shock oscillations, it may turn-out that shock oscillations, in some guise, are responsible for the QPO's.
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