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[vsnet-preprint 17] DK Cas preprint
DK Cas preprint
The following paper is accepted for publication in IBVS No. 5070.
The figures are placed under
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/DK_Cas/
Regards,
Taichi Kato
==========================================================================
\documentstyle[twoside,epsf]{article}
\input{ibvs2.sty}
\begin{document}
\IBVShead{xxxx}{xx May 2001}
\IBVStitletl{Outburst Photometry of DK Cas}
\IBVSauth{Taichi~Kato$^1$, Makoto~Uemura$^1$}
\vskip 5mm
\IBVSinst{Dept. of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: tkato@kusastro.kyoto-u.ac.jp,\\
uemura@kusastro.kyoto-u.ac.jp}
\IBVSobj{DK Cas}
\IBVStyp{UGSS}
\IBVSkey{dwarf nova}
\begintext
DK Cas is a poorly studied dwarf nova which was discovered by Hoffmeister
(1943). The fourth edition of the General Catalogue of Variable Stars
gives a range of 15.3 -- 19.5p. Little has been known regarding its nature
and outburst characteristics. However, there have been a few evidences
that outbursts of this object are relatively rare. Bruch et al. (1987)
observed this object on 17 nights and found no outbursts. A vigorous
search for outbursts since 1995 by visual and CCD observers (mainly by
T. Vanmunster and G. Poyner), contributed to VSNET
(http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/) had yielded no positive
detections until the detection by P. Schmeer on 1999 November 27
(Schmeer 1999). Since the low frequency of outbursts as well as
a relatively large outburst amplitude makes DK Cas a good candidate
for an SU UMa-type dwarf nova, we started time-resolved CCD photometry.
\vskip 3mm
The CCD observations were done using an unfiltered ST-7 camera attached to
the Meade 25-cm Schmidt-Cassegrain telescope. The exposure time was 30 s.
The images were dark-subtracted, flat-fielded, and
analyzed using the Java$^{\rm TM}$-based PSF photometry package
developed by one of the authors (TK). The magnitudes were determined
relative to BD +56$^{\circ}$ 35 = GSC 3661.1642, whose Tycho-2 magnitude
is $V=10.02\pm0.03$ and $B-V=+0.28\pm0.04$. The constancy of comparison
star during the run was confirmed by comparison with GSC 3661.1306.
The log of observations together with nightly average magnitudes is
given in Table 1. The light curve drawn from these data is presented
in Figure 1.
\begin{table}
\begin{center}
Table 1. Nightly averaged magnitudes of DK Cas \\
\vspace{10pt}
\begin{tabular}{ccccc}
\hline
JD start$^a$ & JD end$^a$ & mean mag$^b$ & error$^c$ & N$^d$ \\
\hline
51509.992 & 51510.024 & 5.075 & 0.049 & 37 \\
51511.002 & 51511.185 & 4.978 & 0.004 & 415 \\
51512.056 & 51512.176 & 4.986 & 0.005 & 302 \\
51516.187 & 51516.292 & 5.243 & 0.015 & 262 \\
51520.162 & 51520.165 & 5.803 & 0.061 & 10 \\
51521.147 & 51521.150 & 5.995 & 0.140 & 10 \\
51521.981 & 51521.984 & 6.309 & 0.090 & 10 \\
51522.975 & 51522.979 & 6.603 & 0.122 & 10 \\
51523.965 & 51523.969 & 6.923 & 0.098 & 8 \\
\hline
\end{tabular}
\end{center}
\hskip 35mm $^a$ JD$-$2400000
\hskip 35mm $^b$ Magnitude relative to BD +56$^{\circ}$ 35
\hskip 35mm $^c$ Standard error of nightly average
\hskip 35mm $^d$ Number of frames
\end{table}
\IBVSfig{10cm}{fig1.ps}{Overall light curve of DK Cas}
\vskip 3mm
DK Cas reached a maximum (unfiltered CCD magnitude 14.86, roughly
corresponding to an $R_c$ magnitude assuming the usual color close to
$B-V=0.0$ for outbursting dwarf novae) within two days of the outburst
detection. The object stayed at maximum for three days and started
fading slowly. Time-resolved CCD photometry during the outburst plateau
showed only slow variation with small random variations, and no clear
indication of periodic modulations (Figure 2). In addition to
the presence of a short plateau at maximum, followed by a slow fade,
the absence of clear superhump modulations is sufficient to rule out
the object as being an SU UMa-type dwarf nova. The suggested classification
of an SS Cyg-type star (UGSS) is supported by this observation.
\IBVSfig{10cm}{fig2.ps}{Time-series photometry of DK Cas. Each point
represents an average of adjacent five frames. The magnitudes are
arbitrarily shifted for clarity.}
\vskip 3mm
The later part of the fading from outburst is characterized by a linear
fade at a rate of 0.30$\pm$0.01 mag d$^{-1}$, which is relatively slow
among SS Cyg-type dwarf novae. Based on the calibration by Szkody and
Mattei (1984) of Bailey's relation, the orbital period of DK Cas is
expected to be longer than 5 hours, and is most likely longer than
that of a long-period system DX And (10.6 hours, Drew et al. 1993).
Altough recent spectroscopic observation by Liu and Hu (2000) was not
able to detect a feature of the secondary, the above data suggest that
DK Cas belongs to a rare class of long-period dwarf novae with a low
outburst frequency (i.e. low mass-transfer rate), a further spectroscopic
search for the secondary and accurate determination of the orbital period
are highly encouraged.
\vskip 3mm
The authors are grateful to VSNET members for providing dense observations
covering years, and P. Schmeer for promptly and publicly notifying the
outburst.
Part of this work is supported by a Research Fellowship of the
Japan Society for the Promotion of Science for Young Scientists (MU).
\references
Bruch, A., Fischer, F.-J., Wilmsen, U., 1987, A\&AS, 70, 481
Drew, J. E., Jones, D. H. P., Woods, J. A., 1993, MNRAS, 260, 803
Hoffmeister, C., 1943, {\it Publ. Berlin-Babelsburg Univ. Obs.}, 28
Liu, Wu, Hu, J. Y., 2000, ApJS, 128, 387
Schmeer, P., 1999, {\it VSNET alert circulation}, No. 3745 \\
(available from
http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Mail/alert3000/msg00745.html).
Szkody, P., Mattei, J. A., 1984, PASP, 96, 988
\end{document}
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