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[vsnet-preprint 73] V893 Sco paper
V893 Sco paper
The following paper is accepted for publication as IBVS No. 5262.
The figures are aveilable at:
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/V893_Sco-cycle/
---
\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\apj{ApJ}
\def\apjl{ApJ}
\def\apjs{ApJS}
\def\apss{Ap\&SS}
\def\aap{A\&A}
\def\aapr{A\&A\ Rev.}
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\def\pasj{PASJ}
\def\PublisherUAP{Tokyo: Universal Academy Press}
\begin{document}
%\IBVShead{xxxx}{xx February 2002}
\IBVShead{5262}{xx April 2002}
\IBVStitle{V893 Sco Is Not an ER UMa-Type Star}
\IBVSauth{Kato,~Taichi$^1$, Matsumoto,~Katsura$^{2,1}$, Uemura,~Makoto$^1$,}
\vskip 5mm
\IBVSinst{Dept. of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: (tkato,uemura)@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}
\IBVSobj{V893 Sco}
\IBVStyp{UG+E}
\IBVSkey{dwarf nova, classification}
\begintext
V893 Sco is a recently rediscovered bright dwarf nova \citep{kat98v893sco}.
The star was subsequently found to be an eclipsing dwarf nova below the
period gap (\cite{tho99v893sco}; \cite{mat00v893sco}; \cite{bru00v893sco}).
Most recently, \citet{mas01v893sco} proposed an idea that V893 Sco
is an ER UMa-type dwarf nova. From their analysis of the evolutionary
state of V893 Sco, \citet{mas01v893sco} proposed that all ER UMa stars
may be newly formed cataclysmic variables (CVs).
ER UMa stars are a class of SU UMa-type dwarf novae (for a recent
review of SU UMa-type dwarf novae, see \cite{war95suuma}), whose known
members are ER UMa, V1159 Ori, RZ LMi, DI UMa and IX Dra
(\cite{kat95eruma}; \cite{rob95eruma}; \cite{nog95rzlmi};
\cite{kat96diuma}; \cite{ish01ixdra}).
ER UMa stars are known to show extremely short (19--50 d) supercycles
(intervals between successive superoutbursts), short intervals (3--5 d)
between normal outbursts, low amplitudes of superoutbursts ($<$3 mag),
and extremely large (0.30--0.45) duty cycles of superoutbursts
(see folded figures in \cite{rob95eruma}; \cite{kat01v1159ori}).
A comparison of basic parameters of ER UMa stars can be found in
Table 1 of \citet{kat99erumareview}. ER UMa stars are also known to
show large-amplitude superhumps during the earliest or rising
stage of an superoutburst \citep{kat96erumaSH}.
\citet{mas01v893sco} analyzed the light curve from VSNET
(http://vsnet.kusastro.kyoto-u.ac.jp/vsnet), and identified outburst
intervals of $\sim$30 d as being a supercycle and normal outburst
with amplitudes of $<$1 mag every few days.
Here we report an argument against this interpretation.
First, a supercycle of ER UMa-type dwarf novae is largely occupied
by a long-lasting superoutburst \citep{kat99erumareview}. In contrast
to the usual duty cycle (0.30--0.45) of superoutbursts in ER UMa stars,
the outbursts of V893 Sco, which occur every $\sim$30 days, last
only less than a few days (see Fig. 1), and amount to a duty cycle of only
$\sim$0.1. Furthermore, no superhumps,
which are always seen during ER UMa-type superoutbursts, have yet been
observed during these outbursts (S. Kiyota, private communication).
The observations on 1999 May 12 and 13 by \citet{mat00v893sco} were
done during a rise of such an outburst, and no signature of superhumps
was observed, in spite of the fact that all known ER UMa stars
exhibit strong superhumps even during the rise to superoutburst
\citep{kat96erumaSH}. These outbursts bear all characteristics of normal
outbursts rather than those of superoutbursts.
\IBVSfig{8cm}{fig1.eps}{Representative long-term light curve of V893 Sco
from visual observations reported to VSNET.
}
\IBVSfig{15cm}{fig2.eps}{Comparison of light curves between V893 Sco
(upper) and ER UMa (lower).
Filled squares and open circles represent visual observations by
VSNET observers and CCD observations by the authors.
CCD observations were carried out using a 25-cm Schmidt-Cassegrain
telescope and an ST-7 CCD. The zero-point adjustment was made
using Kiyota's CCD observation (private communication).
The typical errors of observations are 0.2 mag (visual), 0.1 mag
(CCD). The light curve of V893 Sco is strikingly different from
that of ER UMa in that it completely lacks
long-lasting superoutbursts (ER UMa, marked with ticks) and
frequent short outbursts between superoutbursts.
}
Secondly, what were referred to as possible minioutbursts ($<$1 mag)
every few days in \citet{mas01v893sco} are not evident as shown in
Figures 1 and 2. Both visual and CCD observations show only {\it irregular}
variations which are frequently met in CVs. Furthermore, normal outbursts
of ER UMa have amplitudes of $\sim$2 mag (Figure 2, lower panel), which
are significantly larger than the variation seen in V893 Sco.
The presence of quiescent variations in V893 Sco had also been
independently discovered and discussed by \citet{bru00v893sco}.
\citet{bru00v893sco} found a variation up to 0.5 mag from one orbit
to next. The amplitude of this variation is quite comparable to the
one described in \citet{mas01v893sco}.
Furthermore, the time scale of this variation is an order of an orbit
($\sim$0.076 d), which is far shorter than the time scales of dwarf-nova
outbursts. As pointed out by \citet{bru00v893sco}, the short-term
variations of V893 Sco in some aspects bear similarity to those of OY Car
and Z Cha (cf. \cite{coo84zcha}; \cite{woo89oycar}).
These variations may therefore be better understood
as an enhanced activity sometimes observed in high-inclination systems
(\citet{kat01v2051ophiyuma} and references therein).
From these findings, we conclude that V893 Sco bears no similar
characters with ER UMa star, and that the argument in \citet{mas01v893sco}
according to which V893 Sco provides evidence that all ER UMa stars may be
newly formed CVs needs to be reconsidered.
\vskip 3mm
We are grateful to Rod Stubbings, Berto Monard, and Andrew Pearce,
who reported vital observations to VSNET.
Part of this work is supported by a Research Fellowship of the
Japan Society for the Promotion of Science for Young Scientists (KM, MU).
This work is partly supported by a grant-in aid (13640239) from the
Japanese Ministry of Education, Culture, Sports, Science and Technology.
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\end{thebibliography}
\end{document}
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