Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996newa....1...97m&link_type=abstract
New Astronomy, vol. 1, no. 1, p. 97-119.
Astronomy and Astrophysics
Astronomy
17
Accretion, Accretion Disks, Line: Profiles, Binaries: Close, Novae, Cataclysmic Variables, Stars: Magnetic Fields, Stars: Individual: Fo Aqr
Scientific paper
FO Aqr is a close binary star in which a magnetic white dwarf accretes from a cool companion. Light curves and spectra show variations on the orbital frequency, the white dwarf's spin frequency and combinations of the two. It is not clear from observation or theory whether accretion in FO Aqr is mediated by stream, a disc or a combination of both. We have taken 938 spectra of FO Aqr with the 3.9 m AAT. The high signal-to-noise ratio of our spectra enable us to see variations in the lines directly without recourse to phase folding. HeII 4686 shows an `S'-wave on the orbital period but on top of this sharp features run across from red-to-blue and blue-to-red, the direction reversing with orbital phase. We apply the method of Doppler tomography in an effort to understand these features. To cope with the variations that occur as the white dwarf rotates we compute Doppler images as a function of the orientation of the white dwarf relative to the binary system. The process is analogous to the use of a stroboscope in freezing the motion of rotating objects. In animated form a spot of emission is seen in the region of the gas stream and secondary star. The spot changes brightness and position as the white dwarf rotates. The movement in position is consistent with an azimuthally extended structure extending 120° around the white dwarf. It is this movement that corresponds to the sharp features which run across the `S'-wave. We show that these features are the cause of variations in the ratio of blue to red-shifted emission that were formerly interpreted as signs of an eclipse in FO Aqr. There is no evidence for a disc in FO Aqr from our data. Models of the secondary star and gas stream go some way to explaining the variations in spot position and flux, but the observed structure appears more extended than the models can fit. We speculate that magnetospheric interaction may help solve this problem.
Duck S. R.
Marsh Thomas R.
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