[vsnet-preprint 77] DM Dra preprint

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

DM Dra preprint

Dear Colleagues,

   The following paper is accepted for publication as IBVS No. 5284.

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

Regards,
Taichi Kato

===

\documentstyle[twoside,natbib,epsf]{article}

\input{ibvs2.sty}

\def\cite#1{\citealt{#1}}
\def\ibvs{Inf. Bull. Var. Stars}
\def\aj{AJ}
\def\an{Astron. Nachr.}
\def\Ap{Astrophysics}
\def\apj{ApJ}
\def\apjl{ApJ}
\def\apjs{ApJS}
\def\apss{Ap\&SS}
\def\aap{A\&A}
\def\aaps{A&AS}
\def\mnras{MNRAS}
\def\pasp{PASP}
\def\pasj{PASJ}
\def\PZ{Perem. Zvezdy}

\begin{document}

\IBVShead{xxxx}{xx April 2002}

\IBVStitletl{New SU UM\lowercase{a}-Type Dwarf Nova DM D\lowercase{ra}}

\IBVSauth{Kato,~Taichi$^1$, Uemura,~Makoto$^1$, Ishioka,~Ryoko$^1$, Matsumoto,~Katsura$^{2,1}$, \\
Tanabe,~Kenji$^3$
}

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

\IBVSinst{Graduate School of Natural Science and Technology,
          Okayama University, Okayama 700-8530, Japan,
          e-mail: katsura@cc.okayama-u.ac.jp}

\IBVSinst{Department of Biosphere-Geosphere Systems, Faculty of Informatics,
          Okayama University of Science, Ridaicho 1-1,Okayama 700-0005, Japan,
          e-mail: tanabe@big.ous.ac.jp}

\IBVSobj{DM Dra}
\IBVStyp{UGSU}
\IBVSkey{dwarf nova, photometry. classification}

\begintext

    DM Dra (=SVS~2426) was originally discovered by \citet{ste82dmdra},
who detected an outburst at 15.5 m$_{pg}$ on JD 2443965.  The object
was reported to be $\sim$19.5 m$_{pg}$ in quiescence.  \citet{ste82dmdra}
also obtained a low-resolution spectrum during this outburst, and reported
that the spectrum resembled an O-B star.  The object was also included
in the Second Byurakan Sky Survey as a blue stellar object SBS~1533+599
(\cite{bal97SBS2spec}).  The object was also detected by the ROSAT X-ray
satellite (\cite{ver97ROSAT}).

\IBVSfig{6cm}{fig1.eps}{
  Overall light curve of the 2001 November outburst of DM Dra.
}

    During the survey of faint cataclysmic variables (CVs) at high
galactic latitudes (\cite{how90highgalCV}), \citet{how90faintCV3}
reported a short CCD time-series photometry of DM Dra in quiescence.
\citet{how90faintCV3} suggested the possible existence of a period
of $\sim$125 min, and a possible fading by 0.4 mag for a duration of
5 min.  From this observation, DM Dra has been considered as a candidate
SU UMa-type dwarf nova (see \citet{war95suuma} and \citet{osa96review} for
recent review of SU UMa-type dwarf novae).  The star has therefore been
monitored since 1995 by the members of the VSNET Collaboration
(http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/).  Two secure long outbursts
(initial positive detections on JD 2451818 and 2452227) and a possible
outburst on JD 2451082 have been reported.  We conducted CCD time-series
observations during the two recent long outbursts.

\begin{table}
\begin{center}
Table 1. Log of observations \\
\vspace{10pt}
\begin{tabular}{ccccccc}
\hline
JD start$^a$ & JD end$^a$ & mean mag$^b$ & error$^c$ & N$^d$ & exp$^e$ &
        Inst.$^f$ \\
\hline
51844.916 & 51845.008 &  4.195 &  0.021 & 192 & 30 & 1 \\
52228.327 & 52228.367 &  3.197 &  0.013 &  96 & 30 & 2 \\
52230.343 & 52230.369 &  3.506 &  0.030 &  39 & 30 & 2 \\
52230.896 & 52230.900 &  3.818 &  0.042 &  10 & 30 & 3 \\
52230.901 & 52231.001 &  3.484 &  0.043 & 138 & 45 & 3 \\
52231.338 & 52231.370 &  3.512 &  0.019 &  58 & 30 & 2 \\
52231.898 & 52231.997 &  3.851 &  0.005 & 220 & 30 & 3 \\
52232.329 & 52232.369 &  3.655 &  0.015 &  82 & 30 & 2 \\
52233.309 & 52233.366 &  3.747 &  0.011 & 136 & 30 & 2 \\
52234.305 & 52234.361 &  3.860 &  0.008 & 103 & 30 & 2 \\
52235.325 & 52235.372 &  4.056 &  0.008 & 111 & 30 & 2 \\
52236.326 & 52236.373 &  4.244 &  0.026 & 110 & 30 & 2 \\
52237.305 & 52237.368 &  4.765 &  0.038 & 151 & 30 & 2 \\
52240.355 & 52240.372 &  7.491 &  0.303 &  41 & 30 & 2 \\
\hline
 \multicolumn{7}{l}{$^a$ JD$-$2400000.} \\
 \multicolumn{7}{l}{$^b$ Relative magnitude to GSC 3875.203.} \\
 \multicolumn{7}{l}{$^c$ Standard error of nightly average.} \\
 \multicolumn{7}{l}{$^d$ Number of frames.} \\
 \multicolumn{7}{l}{$^e$ Exposure time (s).} \\
 \multicolumn{7}{l}{$^f$ 1: Kyoto (25cm + ST-7), 2: Kyoto (30cm + ST-7E),} \\
 \multicolumn{7}{l}{\phantom{$^f$} 3: OUS (30cm + ST-9E)} \\
\end{tabular}
\end{center}
\end{table}

    The observations were mainly done using an unfiltered ST-7E camera
attached to the Meade 30-cm Schmidt-Cassegrain telescope at Kyoto University.
Some Kyoto observations were made using an unfiltered ST-7 camera attached
to the Meade 25-cm Schmidt-Cassegrain telescope.  The observations at
Okayama University of Science (OUS) were done using an unfiltered ST-9E
camera attached to a 30-cm Cassegrain telescope.  All systems give magnitudes
close to R$_{\rm c}$.  The exposure times were mostly 30 s; the OUS
observations on JD 2452230 mostly used 45 s exposure times.  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 3875.203 (Tycho-2 $V$-magnitude 11.24,
$B-V$=0.44), whose constancy during the run was confirmed by comparison
with GSC 3875.555 (Tycho-2 $V$-magnitude 10.75, $B-V$=0.36).
The log of observations is summarized in table 1.  Barycentric corrections
were applied before the period analysis.

    Figure 1 shows the overall light curve of the best observed outburst
(2001 November).  The initial observation started within 1 d of the
outburst detection by Timo Kinnunen.  The outburst lasted at least 10 d.
The overall light curve resembles the plateau portion of an SU UMa-type
superoutburst (cf. \cite{war85suuma}).  The mean rate of decline between
JD 2451844 and 2452236 was 0.12 mag d$^{-1}$, which is a quite typical
value for an SU UMa-type superoutburst.  The object then experienced
a sudden drop by 2.7$\pm$0.3 mag in 3.0 d.

    Figure 2 shows a representative enlarged light curve of DM Dra during
the plateau stage of the outburst.  The light curve clearly shows the
presence of superhumps with amplitudes of 0.2--0.3 mag.  The observation
established the SU UMa-type nature of DM Dra, which is consistent with
the above findings of the general behavior of the outburst.

\IBVSfig{8cm}{fig2.eps}{
  Enlarged light curve of the 2001 November outburst of DM Dra.
}

    In order to more precisely determine the superhump period, we performed
period analysis using Phase Dispersion Minimization (\cite{PDM}), after
removing the systematic trend of decline and a small systematic difference
between Kyoto and OUS observations.  The $\theta$-diagram and
the phase averaged profile of superhumps is shown in Figure 3.
The best-determined superhump period is 0.07561$\pm$0.00003 d, which is
consistent with the period (0.0734$\pm$0.0030 d) based on single-night
independent superhump observation by T. Vanmunster, vsnet-alert 6886).
The superhump period is far shorter than the possible 125 min periodicity
(\cite{how90faintCV3}) in quiescence, making the 125 min unlikely for
the orbital period.  No eclipse-like fading was observed (Figure 2).
The slight difference between superhump profiles between Figure 2 and 3
suggests the presence of time-evolution of superhumps, which is commonly
seen in SU UMa-type dwarf novae (e.g. \cite{bab00v1028cyg}).  The
evolution was not, however, followed in detail owing to the unavoidable
shortness of nightly runs.

\refstepcounter{figure}
\begin{figure}[hpt]
\normalsize
\vskip 3mm plus 1mm minus 1mm
\centerline{\epsfysize=6cm \epsffile{fig3a.eps} \hskip 1mm
            \epsfysize=6cm \epsffile{fig3b.eps}}
\vskip 2mm plus 1mm minus 1mm
\begin{center}
{\figfon Figure \thefigure .} 
\figcap $\theta$-diagram and the phase averaged profile of superhumps. \large
\end{center}
\end{figure}
\vskip 4mm plus 1mm minus 1mm

    The long duration of the 2000 October outburst (at least 11 d;
possibly preceded by a precursor brightening) as inferred from the
VSNET observation qualifies the superoutburst nature of this
outburst.\footnote{
  Although the 2000 Oct. 27 observation covered 0.091 d, the upper
  limit of superhump-type variation was 0.1 mag.  This suggests that
  superhumps had already decayed (cf. \cite{bab00v1028cyg}
  for a similar example) at the time of the observation.
}
The interval between these two superoutbursts (supercycle) is 393 d,
which is a relatively long one among known SU UMa-type dwarf novae
(see compilations in \cite{war95suuma} and \cite{nog97sxlmi}).

\vskip 3mm

The authors are grateful to the observers who reported vital observations
to VSNET.  We are particularly grateful to Timo Kinnunen and Patrick
Schmeer for notifying us of the outbursts.
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).

\begin{thebibliography}{}

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  2000, \pasj, 52, 429

\bibitem[Balayan(1997)]{bal97SBS2spec}
  Balayan, S.~K.\ 1997, \Ap, 40, 211

\bibitem[Howell, Szkody(1990)]{how90highgalCV}
  Howell, S.~B. \& Szkody, P.\ 1990, \apj, 356, 623

\bibitem[Howell et~al.(1990)]{how90faintCV3}
  Howell, S.~B., Szkody, P., Kreidl, T.~J., Mason, K.~O., \& Puchnarewicz,
  E.~M.\ 1990, \pasp, 102, 758

\bibitem[Nogami et~al.(1997)]{nog97sxlmi}
  Nogami, D., Masuda, S., \& Kato, T.\ 1997, \pasp, 109, 1114

\bibitem[Osaki(1996)]{osa96review}
  Osaki, Y.\ 1996, \pasp, 108, 39

\bibitem[Stellingwerf(1978)]{PDM}
  Stellingwerf, R.~F.\ 1978, \apj, 224, 953

\bibitem[Stepanian(1982)]{ste82dmdra}
  Stepanian, J.~A.\ 1982, \PZ, 21, 691

\bibitem[Verbunt et~al.(1997)]{ver97ROSAT}
  Verbunt, F., Bunk, W.~H., Ritter, H., \& Pfeffermann, E.\ 1997, \aap, 327,
  602

\bibitem[Warner(1985)]{war85suuma}
  Warner, B.\ 1985, in Interacting Binaries, ed. P.~P. Eggelton, \& J.~E.
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\bibitem[Warner(1995)]{war95suuma}
  Warner, B.\ 1995, \apss, 226, 187

\end{thebibliography}

\end{document}

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