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IW And preprint
Dear Colleagues,
The following paper will appear as IBVS No. 5376.
The PostScript files are available at:
http://vsnet.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/IW_And/
Regards,
Taichi Kato
===
\documentstyle[twoside,natbib,epsf]{article}
\input{ibvs2.sty}
\begin{document}
\IBVShead{xxxx}{xx xx 2003}
\IBVStitletl{IW A\lowercase{nd} is a Z C\lowercase{am}-Type Dwarf Nova}
\IBVSauth{Kato,~Taichi$^1$, Ishioka,~Ryoko$^1$, Uemura,~Makoto$^1$}
\vskip 5mm
\IBVSinst{Dept. of Astronomy, Kyoto University, Kyoto 606-8502, Japan, \\
e-mail: (tkato,ishioka,uemura)@kusastro.kyoto-u.ac.jp}
\IBVSobj{IW And}
\IBVStyp{UGZ}
\IBVSkey{dwarf nova, classification}
\begintext
IW And (S~10792) is a blue variable discovered by Meinunger (1975).
Meinunger (1980) studied the object spectroscopically
and described that the object seems to be a unique object: in spite of
broad absorption lines of H$\beta$, H$\gamma$ and H$\delta$ resembling
those of an O or early B dwarf or subdwarf, the spectrum was found to be
featureless around H$\alpha$. Meinunger (1980) further
stated that a couple of doubtful emission lines at the limit of detectability
seem to be present. From 330 observations for the period JD 2440802--46706,
Meinunger and Andronov (1987) found that 72\% of the observations are in the
inactive state (15.1--15.3) of the star.
The object infrequently showed maximum brightness (18\% of time,
13.7--15.0 mag) and minimum brightness (10\% of time, 15.4--17.3 mag).
Meinunger and Andronov (1987) stated that such behavior is significantly
different from those of dwarf novae or polars. More recently,
Liu et al. (1999) obtained a higher quality spectrum, and detected
H$\alpha$ emission line with broad absorption troughs. Although
Liu et al. (1999) classified the object as a confirmed cataclysmic
variable, the exact nature of the object has not been evident owing to
the lack of dense photometric observations.
We observed IW And on 55 nights between 2001 December 6 and 2002
March 25. The observations were done using an unfiltered ST-7E camera
(system close to R$_{\rm c}$) 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 on of the authors (TK). The differential
magnitudes of the variable were measured against GSC 2811.1573
(Tycho-2 $V$-magnitude 12.05, $B-V$=0.11), whose long-term constancy was
confirmed to 0.10 mag by comparison with GSC 2811.2117 (Tycho-2
$V$-magnitude 11.57, $B-V$=0.69). The log of observations is summarized
in table 1.
\begin{table}
\begin{center}
Table 1. Nightly averaged magnitudes of IW And \\
\vspace{10pt}
\begin{tabular}{cccccccc}
\hline
Mid-JD$^a$ & Mean mag$^b$ & Error$^c$ & $N$$^d$ &
Mid-JD$^a$ & Mean mag$^b$ & Error$^c$ & $N$$^d$ \\
\hline
52250.1090 & 2.224 & 0.005 & 31 & 52305.8840 & 2.794 & 0.014 & 31 \\
52250.9993 & 1.973 & 0.006 & 31 & 52306.8847 & 2.721 & 0.017 & 31 \\
52252.1208 & 1.967 & 0.007 & 31 & 52309.9958 & 2.675 & 0.037 & 10 \\
52255.1118 & 2.215 & 0.017 & 31 & 52311.8993 & 2.846 & 0.024 & 31 \\
52255.9875 & 2.325 & 0.043 & 12 & 52312.8972 & 2.786 & 0.030 & 31 \\
52257.0632 & 2.688 & 0.044 & 31 & 52316.9007 & 2.675 & 0.010 & 31 \\
52257.9840 & 3.293 & 0.013 & 31 & 52317.8951 & 2.713 & 0.019 & 31 \\
52259.9757 & 4.398 & 0.048 & 31 & 52318.8986 & 2.639 & 0.017 & 31 \\
52260.9667 & 4.686 & 0.067 & 31 & 52319.8931 & 2.591 & 0.031 & 31 \\
52261.9993 & 4.908 & 0.094 & 31 & 52320.9243 & 2.647 & 0.012 & 31 \\
52262.9660 & 4.415 & 0.034 & 31 & 52323.9382 & 2.887 & 0.179 & 5 \\
52266.9660 & 3.366 & 0.022 & 31 & 52325.9062 & 2.738 & 0.014 & 31 \\
52267.9562 & 4.186 & 0.043 & 31 & 52327.9569 & 2.659 & 0.039 & 31 \\
52270.9486 & 5.017 & 0.119 & 31 & 52329.9188 & 2.866 & 0.013 & 31 \\
52276.9639 & 2.753 & 0.021 & 29 & 52330.9021 & 2.852 & 0.032 & 31 \\
52277.9076 & 2.985 & 0.010 & 31 & 52336.9167 & 2.802 & 0.028 & 31 \\
52279.0493 & 3.347 & 0.159 & 6 & 52337.9062 & 2.732 & 0.044 & 26 \\
52282.9604 & 2.589 & 0.011 & 31 & 52341.9042 & 2.718 & 0.036 & 18 \\
52286.9056 & 2.741 & 0.012 & 31 & 52342.9590 & 2.817 & 0.041 & 12 \\
52291.9104 & 2.852 & 0.042 & 31 & 52344.9076 & 2.875 & 0.046 & 31 \\
52293.8875 & 2.753 & 0.012 & 31 & 52345.9132 & 2.770 & 0.041 & 31 \\
52296.8764 & 2.641 & 0.013 & 31 & 52346.9090 & 2.672 & 0.036 & 31 \\
52297.8875 & 2.704 & 0.027 & 31 & 52348.9111 & 2.800 & 0.087 & 19 \\
52298.8861 & 2.748 & 0.017 & 31 & 52351.9181 & 2.728 & 0.031 & 31 \\
52301.9292 & 2.767 & 0.026 & 31 & 52352.9146 & 2.619 & 0.051 & 31 \\
52302.9847 & 2.769 & 0.038 & 31 & 52353.9153 & 2.829 & 0.079 & 20 \\
52303.8979 & 2.813 & 0.021 & 31 & 52358.9181 & 2.121 & 0.030 & 22 \\
52304.9639 & 2.858 & 0.024 & 31 & & & & \\
\hline
\multicolumn{8}{l}{$^a$ JD$-$2400000.} \\
\multicolumn{8}{l}{$^b$ Relative magnitude to GSC 2811.1573.} \\
\multicolumn{8}{l}{$^c$ Standard error of nightly average.} \\
\multicolumn{8}{l}{$^d$ Number of frames.} \\
\end{tabular}
\end{center}
\end{table}
The resultant light curve is shown in Fig. 1. The light curve clearly
shows oscillations at the beginning of the observation. After
that, the object entered a standstill. The behavior is quite characteristic
to a Z Cam-type dwarf nova entering a standstill (Szkody and Mattei 1984;
Honeycutt et al. 1998; Oppenheimer et al. 1998; Kato 2001).
There was even a small hint of
small-amplitude oscillations during the early part of the standstill
which are quite analogous to those of Z Cam (Kato 2001) and
HX Peg (Honeycutt et al. 1998). The last observation may indicate that the
object was caught during an outburst from the standstill.
The present observations establish that IW And is a previously
unrecognized Z Cam-type dwarf nova. The ``inactive" state described
in Meinunger and Andronov (1987) must have been standstills.
\IBVSfig{10cm}{fig1.eps}{Light curve of IW And}
Although determination of the actual duty cycle of standstills would
require further continuous observations, we regard the 72\% of
observations being inactive state mentioned in Meinunger and
Andronov (1987) to mean the percentage of the duty cycle of
standstills in this star. Among Z Cam stars, the duty cycle (nearly
72\%) of standstills is exceptionally high. Although Z Cam stars have
long been understood as intermediate systems between dwarf novae and
novalike (NL) systems (e.g. Meyer, Meyer-Hofmeister (1983)) in the
framework of the disk-instability model (see Osaki (1996) for a
review), there has been a wide gap between Z Cam stars and NL
systems. IW And may be the first object to fill this gap. Since such
an object is expected to provide strong observational constraints to
the mechanism of Z Cam stars (Meyer and Meyer-Hofmeister 1983;
Honeycutt et al. 1998; Buat-M\'{e}nard et al. 2001), further
continuous observations to precisely determine the pattern of outbursts
and standstills, and spectroscopic observations to determine system
parameters (orbital period, component masses etc.) are strongly encouraged.
\vskip 3mm
This work is partly supported by a grant-in aid (13640239) from the
Japanese Ministry of Education, Culture, Sports, Science and Technology.
Part of this work is supported by a Research Fellowship of the
Japan Society for the Promotion of Science for Young Scientists (MU).
\references
Buat-M\'{e}nard, V., Hameury, J.-M., \& Lasota, J.-P.\ 2001, A\&A, 369, 925
Honeycutt, R.~K., Robertson, J.~W., Turner, G.~W., \& Mattei, J.~A.\ 1998,
PASP, 110, 676
Kato, T.\ 2001, IBVS, 5093
Liu, W., Hu, J.~Y., Zhu, X.~H., \& Li, Z.~Y.\ 1999, ApJS, 122, 243
Meinunger, L.\ 1975, MVS, 7, 1
Meinunger, L.\ 1980, IBVS, 1795
Meinunger, L. \& Andronov, I.~L.\ 1987, IBVS, 3081
Meyer, F. \& Meyer-Hofmeister, E.\ 1983, A\&A, 121, 29
Oppenheimer, B.~D., Kenyon, S.~J., \& Mattei, J.~A.\ 1998, AJ, 115, 1175
Osaki, Y.\ 1996, PASP, 108, 39
Szkody, P. \& Mattei, J.~A.\ 1984, PASP, 96, 988
\endreferences
\end{document}
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