[vsnet-preprint 34] YZ Cnc preprint

[Taichi Kato <tkato>]

Dear Colleagues,

   The following article on YZ Cnc is accepted for publication as IBVS 5104.

   The figures are available at:
   http://ftp.kusastro.kyoto-u.ac.jp/vsnet/preprints/YZ_Cnc/

Regards,
Taichi Kato

===

\documentstyle[twoside,epsf]{article}

\input{ibvs2.sty}

\begin{document}

\IBVShead{xxxx}{xx May 2001}

\IBVStitletl{Development of late superhumps in YZ Cnc}

\IBVSauth{Taichi~Kato$^1$}
\vskip 5mm

\IBVSinst{Dept. of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
          e-mail: tkato@kusastro.kyoto-u.ac.jp}

\IBVSobj{YZ Cnc}
\IBVStyp{UGSU}
\IBVSkey{dwarf nova, photometry}

\begintext

   YZ Cnc is a well-known dwarf nova and is a prototypical object
representing for a population of SU UMa-type systems with short outburst
recurrence times and long orbital periods.  However, little photometric
observation of superhumps had been done since its identification as
an SU UMa-type star (Patterson 1979).  We undertook time-resolved CCD
photometry during its superoutburst in 1994 January.

\vskip 10mm

   The observations were done on three successive nights between 1994
January 1 and 3, using a CCD camera (Thomson TH~7882, 576 $\times$ 384
pixels, on-chip 2 $\times$ 2 binning adopted) attached to the Cassegrain
focus of the 60 cm reflector (focal length=4.8 m) at Ouda Station,
Kyoto University (Ohtani et al. 1992).  An interference filter was used
which had been designed to reproduce the Johnson {\it V} band.
The exposure time was 60--120 s depending on the brightness of the object.
The frames were first corrected for standard de-biasing and flat fielding,
and were then processed by a microcomputer-based aperture photometry
package developed by the author.  The magnitudes of the object were
determined relative to GSC 1939.1130 (GSC magnitude 13.4), but its
constancy was not confirmed because of the lack of suitable check stars
in the same field.  Barycentric corrections to observed times were applied
before the following analysis.  Table 1 lists the log of observations,
together with nightly averaged magnitudes.

\IBVSfig{10cm}{fig1.ps}{Light curve of the 1994 January superoutburst
of YZ Cnc}

\vskip 3mm

\begin{table}
\begin{center}
Table 1. Log of observations \\
\vspace{10pt}
\begin{tabular}{ccccc}
\hline
start$^a$ & end$^a$ & mean mag$^b$ & error$^c$ & N$^d$ \\
\hline
49354.211 & 49354.385 & -0.800 & 0.005 & 162 \\
49355.178 & 49355.340 &  0.285 & 0.014 &  94 \\
49356.213 & 49356.317 &  1.119 & 0.035 &  30 \\
\hline
\end{tabular}
\end{center}
\hskip 35mm $^a$ BJD$-$2400000

\hskip 35mm $^b$ Magnitude relative to GSC 1939.1130

\hskip 35mm $^c$ Standard error of nightly average

\hskip 35mm $^d$ Number of frames

\end{table}

   The light curve drawn from these data is presented in Figure 1.
The light curve shows the declining portion from a superoutburst.
Superhumps were evident on the first night, but became more complex
on the next night, when the system entered the rapidly declining phase.
The period analysis over the entire, or selected, data set does not
yield a coherent signal, because of the development of late superhumps
as described below.  So we used the primary superhump period of
$P=0.09204$ d (Patterson 1979) for the following analysis.

\IBVSfig{10cm}{fig2.ps}{Phase-averaged light curve of YZ Cnc, assuming
the superhump period of 0.09204 d.  The origin of the phase is arbitrarily
taken as BJD 2449350.
}
\vskip 3mm

   Figure 2 shows the phase-averaged light curves of 1994 Jaunary 1
(upper panel) and January 2 (lower panel).  The January 1 light curve
clearly shows typical superhumps, with a shoulder (secondary superhumps)
on its declining branch.  However, the phase of the maximum dramatically
changed by $\phi\sim0.3-0.4$ on the next night (lower panel).  The newly
appeared humps correspond to what are called ``late superhumps"
(Haefner et al. 1979), which is considered to reflect the modulation
of the precessing accretion disk properties at the stream impact point
(Hessman et al. 1992).  The clear appearance of late superhumps in YZ Cnc
may be consistent with its high mass-transfer rate.

\references

Haefner, R., Schoembs, R., Vogt, R., 1979, A\&A, 77, 7

Hessman, F. V., Mantel, K.-H., Barwig, H., Schoembs, R., 1992, A\&A,
     263, 147

Ohtani, H., Uesugi, A., Tomita, Y., Yoshida,
     M., Kosugi, G., Noumaru, J., Araya, S., Ohta, K.
     1992, Memoirs of the Faculty of Science, Kyoto University,
     Series A of Physics, Astrophysics, Geophysics and Chemistry,
     38, 167

Patterson, J., 1979, AJ, 84, 804

\end{document}

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