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Astron. Astrophys. 350, 517-528 (1999)

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9. Conclusions

The present HST/FOS and IUE spectroscopy has allowed us for the first time to infer the characteristics of multiple periodicities in the UV emission of FO Aqr. Also, coordinated optical photometry has provided further constraints summarized as follows:

  1. The rotational pulsations in the UV and optical are consistent with the current accretion curtain scenario. They reveal the presence of strong temperature gradients within the curtain moving from the footprints onto the white dwarf surface to several white dwarf radii. A spin S-wave is observed in the UV emission lines which maps the innermost regions of the accretion curtain.

  2. Orbital sideband variabilities indicate that reprocessing is occurring at fixed regions within the binary frame. Different behaviour is observed in the emission lines and continuum beat pulses indicating a complex reprocessing scenario.

  3. The orbital UV variability is confirmed to arise from the illuminated side of the inflated disc (bulge) while the optical modulation is produced by heating effects at the secondary star.

  4. A change in the relative proportion of rotational and orbital modulation amplitudes is found on a timescale of five years. These are interpreted as a reduction in the dimensions of the accretion curtain accompained by an increase in the bulge size.

These observations indicate a long term change in the accretion mode where FO Aqr has switched from a disc-fed to a disk-overflow state triggered by changes in the mass accretion rate.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999