Other
Scientific paper
Sep 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990mnras.246...29o&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 246, NO. 1/SEP1, P. 29, 1990
Other
31
Scientific paper
Using published white-light eclipses of the SU UMa dwarf nova Z Cha during several superoutbursts, the eclipse of the superhump light source is separated from that of the non-varying background. The shape of this eclipse is distinguished by the presence of two local minima (near orbital phases -0.05 and 0.05) and a local maximum centred on orbital phase 0.0.
By modelling the brightness of the superhump during eclipse with observations from adjacent days, maximum-entropy eclipse deconvolution is applied to the eclipse of the superhump light source to derive its surface-brightness distribution. The results show that, in accordance with previous studies, the superhump light source is located on the rim of the disc, but with strong departures from axisymmetry. The superhump light is distributed among three distinct features on the leading and trailing lunes of the disc and at the disc edge nearest to the secondary, along the line-of-centres of the stars. This threefold structure is shown to be independent of uncertainties in the modelling procedure used to derive it and is instead a consequence of the local minima and maximum in the eclipse of the superhump light source. These features were present during at least three superoutbursts of Z Cha and there is evidence for the same morphology in OY Car. However, an anomalous eclipse of the superhump, probably at a critical phase of another superoutburst in Z Cha, shows an entirely different morphology, with most of the light confined to a compact region of the disc near the bright spot.
From consideration of models of the secondary's tidal influence on accretion discs, general arguments are presented that the non-axisymmetric nature of the surface- brightness distribution of the superhump light source indicates that superhumps are a tidal phenomenon. This agrees with Whitehurst's tidally stressed disc model for superhumps, but a comparison between the dissipation patterns in his model and the derived superhump surface-brightness distribution was not possible. The non-axisymmetric nature of the surface-brightness distribution of the superhump light source explains much of the phenomenology of superhumps.
Theoretical work shows that the secondary's tidal influence should be manifested at the outer edges of discs, especially in systems with small mass ratios. Observational evidence which may indicate such effects is discussed for various sub-classes of cataclysmic variables including the helium-double degenerates, old novae and nova-like variables and some ultra-short-period dwarf novae.
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