Astron. Astrophys. 350, 517-528 (1999)
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:
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.
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.
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.
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.
© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999