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[vsnet-preprint 31] V1159 Ori preprint
V1159 Ori preprint
Dear Colleagues,
The following article is accepted for publication in PASJ (Letter).
The preprint with figures is available from:
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/V1159_Ori-cycle/
Regards,
Taichi Kato
===
\documentclass{pasj00}
\draft
\begin{document}
\SetRunningHead{T. Kato}{Changing Supercycle of V1159 Ori}
\Received{}%{yyyy/mm/dd}
\Accepted{}%{yyyy/mm/dd}
\title{Changing Supercycle of the ER UMa-Type Star V1159 Ori}
\author{Taichi \textsc{Kato}}
\affil{Department of Astronomy, Faculty of Science, Kyoto University,
Sakyou-ku, Kyoto 606-8502}
\email{tkato@kusastro.kyoto-u.ac.jp}
%%% end:list of authors
\KeyWords{stars: cataclysmic variables --- stars: dwarf novae
--- stars: individual (V1159 Ori)}
\maketitle
\begin{abstract}
We examined the VSNET light curve of the ER UMa-type star V1159 Ori.
We detected large variation of the supercycle (the interval between
successive superoutbursts) between extremes of 44.6 and 53.3 d.
The outburst activity was also found to decrease when the supercycle
was long. The observed variation of supercycle corresponds to a
variation of $\sim$40\% of mass-transfer rate from the secondary star,
totally unexpected for this class of objects. We also detected a hint
of $\sim$1800 d periodicity in the variation, whose period is close to
what has been suggested for solar-type cycles for cataclysmic variables
(CVs). If this periodicity is caused by the magnetic activity of the
secondary star, this detection makes the first clear evidence of the
continuing magnetic activity in the CV evolution even after crossing the
period gap. This activity may partly explain still poorly understood
origins of high mass-transfer rates in ER UMa-type stars.
\end{abstract}
\section{Introduction}
ER UMa stars are a subgroup of SU UMa-type dwarf novae (for a review
of dwarf novae, see \cite{osa96}), whose known members are ER UMa, V1159 Ori,
RZ LMi and DI UMa. The most striking feature of ER UMa stars is the
extremely short recurrence time (19--45 d) of superoutbursts (\cite{kat95};
\cite{nog95a}; \cite{nog95b}; \cite{rob95}; \cite{mis95}; \cite{kat96}).
Another striking feature of ER UMa stars is the stability of supercycles,
both in their lengths and outburst pattern. The best exemplification of
this stability can be seen in folded light curves and $O-C$ figures presented
in \citep{rob95}. The extremely short supercycle length and
the stability of outburst patterns are basically explained, in the
framework of the disk instability model, as a result of constant high
mass-transfer rates from the secondary \citep{osa95a}. The mass-transfer
rates in SU UMa-type dwarf novae are generally considered to be confined
in a small range determined by the angular momentum removal by the
gravitational wave radiation. The origin of high-mass transfer rates in
ER UMa stars is still an open question. Some models assume irradiation
effect from the hot white dwarf, which may be a result of a hypothetical
recent nova eruption (the possibility was originally raised by \cite{nog95b},
see also \cite{pat98}). Examination of secular changes in supercycle in
these systems would provide an essential clue in testing these hypotheses.
\section{Observation and Analysis}
We examined the observations posted to VSNET
(http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/), and found an appreciable
change in one of ER UMa stars, V1159 Ori. The object has been very
well sampled by many observers around the world since 1995 September
(Fig. \ref{fig:figure1}).
\begin{figure}
\begin{center}
\FigureFile(80mm,80mm){fig1.eps}
\end{center}
\caption{Light curve of V1159 Ori from VSNET observations. Ticks represent
the start of superoutbursts as listed in Table
\ref{tab:table1}}\label{fig:figure1}
\end{figure}
The time of the start of a superoutburst was defined as its mid-rising
branch. Occasional observational gaps introduced an uncertainty
of 1--2 d, but most of these superoutbursts were well sampled and
the times were usually determined within an uncertainty of 1 d. Table
\ref{tab:table1} lists the observed times of superoutbursts. The cycle
number ($E$) represents number of supercycles since the JD 2449982
superoutburst.
\begin{table}
\caption{Superoutbursts of V1159 Ori}\label{tab:table1}
\begin{center}
\begin{tabular}{cccc}
\hline
JD start & cycle number & JD start & cycle number \\
\hline
2449982 & 0 & 2451110 & 25 \\
2450072 & 2 & 2451157 & 26 \\
2450118 & 3 & 2451202 & 27 \\
2450161 & 4 & 2451249 & 28 \\
2450340 & 8 & 2451295 & 29 \\
2450386 & 9 & 2451399 & 31 \\
2450431 & 10 & 2451450 & 32 \\
2450475 & 11 & 2451501 & 33 \\
2450523 & 12 & 2451559 & 34 \\
2450574 & 13 & 2451614 & 35 \\
2450665 & 15 & 2451667 & 36 \\
2450714 & 16 & 2451759 & 38 \\
2450759 & 17 & 2451812 & 39 \\
2450803 & 18 & 2451853 & 40 \\
2450850 & 19 & 2451896 & 41 \\
2450892 & 20 & 2451942 & 42 \\
2451030 & 23 & 2451987 & 43 \\
2451070 & 24 & & \\
\hline
\end{tabular}
\end{center}
\end{table}
\vskip 3mm
A regression to these times has yielded a linear ephemeris
$2449962.9 + 46.82 E$. The derived supercycle length of 46.82 d is
slightly longer than the 44.5 d by \cite{rob95}.
Fig. \ref{fig:figure2} shows the $O-C$ diagram against this ephemeris.
The most remarkable feature is the presence of large $O-C$ changes compared
to \cite{rob95}. This large change is mainly caused by
the increase of supercycle length between $E=29$ and $E=36$, corresponding
to the period between 1999 May and 2000 May. The supercycle during
interval is 53.3 d, which is 14\% longer than the long-term average.
Such a large change in supercycle has not been seen in ER UMa.
\begin{figure}
\begin{center}
\FigureFile(60mm,60mm){fig2.eps}
\end{center}
\caption{$O-C$ diagram of V1159 Ori superoutbursts}\label{fig:figure2}
\end{figure}
\section{Discussion}
The long-term average of supercycle lengths in V1159 Ori being close
to the minimum value predicted by \citep{osa95a}, the supercycle length
near this period is expected to be insensitive to the mass-transfer rate
from the secondary. If the observed change in V1159 Ori was caused by the
variable mass-transfer rate, a relatively large change is necessary
to reproduce the observation. Using the $\dot{M}-supercycle$ diagram
in \citep{osa95a}, the supercycle of 53.3 d corresponds to a reduction
of $\sim$40\% of mass-transfer rates from what is expected for a 44.5-d
supercycle. The marked reduction of the superoutburst duty cycle during
this period (Fig. \ref{fig:figure3}) also supports this interpretation.
\begin{figure}
\begin{center}
\FigureFile(80mm,80mm){fig3.eps}
\end{center}
\caption{Folded light curves. The upper panel shows the epoch with
a short supercycle length (44.59 d). The lower panel shows the epoch
with a long supercycle (53.30 d) and decreased outburst activity.
The duty cycle of a superoutburst (phase 0 -- 0.45 in the upper panel,
phase 0 - 0.35 in the lower panel) is markedly decreased in the latter
epoch.}\label{fig:figure3}
\end{figure}
Another observational evidence of a large period change in ER UMa stars
has been reported in DI UMa \citep{fri99}. However, the extreme shortness of
supercycles in DI UMa and RZ LMi requires an additional (still poorly
identified) mechanism \citep{osa95b}, and its change may be of different
nature. Another noteworthy feature in the observed $O-C$ diagram of
V1159 Ori is the possible periodicity with a period of $\sim$38 cycles,
corresponding to $\sim$1800 d, rather than a monotonous change originally
proposed by \citep{rob95}, and this is contrary to the expected effect
by the decreasing heating from a hypothetical recent nova eruption on
a white dwarf. The observed possible long-term period is close to those
observed as possible solar-type cycles in cataclysmic variables
(e.g. \cite{bia88}; \cite{ak01}). If such a ``solar-type" cycle is
responsible for the change in the supercycle of V1159 Ori, this may provide
a promising evidence for the presence of magnetic activity in dwarf novae
below the period gap, which has been usually considered to cease or to be
markedly reduced when the secondary becomes fully convective after crossing
the period gap. Furthermore, the continuing magnetic activity may be one
of mechanisms for effectively removing the angular momentum from the binary
system, with which the required high mass-transfer in ER UMa-type systems
may be partly explained.
\vskip 3mm
The author is grateful to VSNET members, especially to Rod Stubbings,
Gene Hanson, Gary Poyner, Andrew Pearce, Seiichiro Kiyota, Eddy Muyllaert,
Tsutomu Watanabe and numerous observes for providing vital observations.
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\end{document}
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