Magnetically controlled accretion in cataclysmic variable star systems

Bradford W. Greeley

Through observations with the Hopkins Ultraviolet Telescope and the Berkeley Extreme Ultraviolet Spectrometer, we extend the ultraviolet wavelength coverage of six magnetic cataclysmic variable stars and two novae to the Lyman limit. The FUV continuum of the polar AM Her is not well fit by a white dwarf (WD) spectrum. Instead, simultaneous observations with HUT and EUVE suggest that the FUV continuum is dominated by the accreting hotspot on the WD. We discover an eclipse of the He II narrow line emission at inferior conjunction. Our observations are more consistent with steady nuclear burning on the WD surface than the popular clumpy accretion theory of Kuijpers & Pringle (1982) as an explanation for the soft X-ray excess in polars. Simultaneous observations with HUT, ASCA, and EUVE have allowed us to study the reprocessing of ionizing photons in AM Her. We have constructed a model of the irradiated face of the secondary star which produces good agreement with the observations for the low to moderate ionization narrow lines, but suggests that the secondary cannot account for the observed narrow line flux for higher ionization lines. We find that models of reprocessing in the accretion column are able to reproduce the observations without resorting to clumpy accretion. The HUT EX Hya spectrum suggests that the source of the broad UV emission lines is the accretion curtain. The lack of variation of the line ratios suggests a modulation caused by geometric projection effects of an optically thick line emission region. The short timescale variability of the continuum flux suggests absorption of emission from the heated inner accretion curtain or WD surface may account for the modulated continuum. We find anomalous abundances for AE Aqr, BY Cam, and Nova Cir 1995. EUVS observations of AE Aqr show a complete lack of C III emission, suggesting that carbon is underabundant relative to nitrogen and oxygen. An EUVS observation of BY Cam shows not only a lack of C III,but O VI as well, pointing to a significant enhancement of nitrogen relative to both carbon and oxygen. A HUT observation reveals a similar overabundance of nitrogen in Nova Cir 1995.

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