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Are Disks in Dwarf Novae during their Superoutbursts Really Eccentric?

Smak J.

Acta Astronomica

2009

Vol. 59, No. 1

89--101

The evidence presented earlier by several authors for the substantial disk eccentricity in dwarf novae during their superoutbursts is shown to result either from errors, or from arbitrary, incorrect assumptions. (1) The evidence for Z Cha and WZ Sge based on radial velocities measured from absorption components, was an artifact, resulting from miscalculated beat phases. (2) The evidence for OY Car and IY UMa based on the observed dependence of eclipse parameters on the beat phase, involved an implicit assumption that the observed eclipses are pure disk eclipses, which is not true. In particular, the observed variations of eclipse parameters are likely due to the contributions from the hot spot and from the superhump source, which depend strongly on the beat phase. (3) The evidence for OY Car and WZ Sge resulting from the analysis of hot spot eclipses, was based on the assumption that the spot distances are identical with the radius of the disk, which is not always correct. In particular, in the case of eclipses of "peculiar" spots (involving the stream overflow), observed at beat phases away from φb≈0.5, the resulting spot distances are smaller than the radius of the disk. New determination of disk eccentricity in Z Cha, using Vogt's radial velocities measured from emission components, gives e=0.05±0.05.