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1

Z Cha and its Superhumps

Smak J.

Acta Astronomica

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2009

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Vol. 59, No. 1

109--120

EN

The superhump eclipse light curves are re-determined for five eclipses of Z Cha (E54036, E54037, E67877, E74693, E74694) observed by Warner and O'Donoghue (1988) during its superoutbursts. Qualitatively they are similar to those obtained by O'Donoghue (1990), showing two local minima at φ≈-0.05 and 0.04. Arguments are then presented which imply that the first minimum is not due to an occultation but is produced by absorption effects in the overflowing parts of the stream. The location of the superhump light source (SLS) determined from the analysis of the second minimum coincides with the trajectory of the overflowing parts of the stream. The light curve of the sixth eclipse (E77878) could be simply decomposed into its disk and superhump components. The location of SLS, obtained from the analysis of the SLS eclipse light curve, coincides in this case with the position of the standard hot spot. This implies that superhumps are due to modulated mass transfer rate resulting in periodically enhanced dissipation of the kinetic energy of the stream.

2

Are Disks in Dwarf Novae during their Superoutbursts Really Eccentric?

Smak J.

Acta Astronomica

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2009

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Vol. 59, No. 1

89--101

EN

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.

3

Superoutbursts of Z Cha and their Interpretation

Smak J.

Acta Astronomica

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2008

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Vol. 58, No. 1

55--64

EN

Disk eclipses of Z Cha during its superoutbursts are analyzed. The resulting flux/temperature distributions in the outer parts of the disk show peculiarities which are interpreted as being due to absorption effects in the overflowing parts of the stream. The temperatures of the inner parts of the disk are used to determine the accretion rates as a function of time since superoutburst maximum. They are practically identical with mass transfer rates determined from the hot spot. Results are summarized in the form of the following, purely observational scenario: Superoutbursts are due and begin with a major enhancement in the mass transfer rate. During the "flat-top" part of the superoutburst the mass transfer rate decreases slowly, causing the observed luminosity to decline. The superoutburst ends when the mass transfer rate decreases below its critical value, resulting in a transition to the quiescent state of the dwarf nova cycle.

4

Superoutbursts of Z Chameleontis. Detection of the Hot Spot

Smak J.

Acta Astronomica

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2007

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Vol. 57, No. 1

87--101

EN

Eclipse light curves of Z Cha collected during its superoutbursts are decomposed into the disk eclipse and spot eclipse components. It is found that the presence (or absence) of the hot spot and all its parameters depend on the phase φb of the beat period (related to the orbital and superhump periods). At beat phases 0.40<φb<0.60 the "standard" hot spot is present, the analysis of its parameters shows that (a) the mass transfer rate is strongly enhanced and (b) the radius of the disk is roughly equal to the tidal radius. No hot spot can be detected during eclipses located close to the superhump (beat phases 0.80<φb<0.35). The spot observed at intermediate beat phases (φb≈0.35 and φb≈0.75) shows peculiarities which are interpreted as being due to the stream overflow. Rediscussion of system parameters, based on the mass ratio obtained from the analysis of the spot eclipses during quiescence (Smak 1996) and K2 from Wade and Horne (1988), gives: M1=0.93±0.10 Msolar, M2=0.186±0.030 Msolar, q=0.20±0.01 and i=80°.2±0°.3.