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[vsnet-preprint 3] V803 Cen preprint
V803 Cen preprint
Dear Colleagues,
The following article is accepted for publication as IBVS No. 4915.
Please note this IBVS article is purely based on visual observations!
The PostScript figures are available from
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/V803_Cen/
and GIF figures (possibly more convenient for many browsers):
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/V803_Cen/fig1.gif
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/V803_Cen/fig2.gif
Regards,
Taichi Kato
===========
\documentstyle[twoside,epsf]{article}
\input{ibvs2.sty}
\begin{document}
\IBVShead{xxxx}{xx June 2000}
\IBVStitletl{V803 Cen -- the second ``helium ER UMa star"}
\IBVSauth{T.~Kato$^1$,R.~Stubbings$^2$,B.~Monard$^3$,A.~Pearce$^4$}
\vskip 5mm
\IBVSinst{Dept. of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: tkato@kusastro.kyoto-u.ac.jp}
\IBVSinst{19 Greenland Drive, Drouin 3818, Victoria, Australia,
e-mail: stubbo@qedsystems.com.au}
\IBVSinst{PO Box 70284, Die Wilgers 0041, Pretoria, South Africa,
e-mail: LAGMonar@csir.co.za}
\IBVSinst{32 Monash Ave, Nedlands, WA 6009, Australia
e-mail: andrew.pearce@clough.com.au}
\IBVSobj{V803 Cen}
\IBVStyp{*}
\IBVSkey{dwarf novae -- photometry}
\begintext
Small group of helium-rich variable stars, known as AM CVn stars,
are considered as ultra-short period interacting binary white dwarfs
(for a review, see Warner 1995). Among AM CVn stars, CR Boo, V803 Cen and
CP Eri are known to show large-amplitude variations up to five magnitudes,
with a time-scale of less than a day to several months. The origin of
such large-amplitude variation is still poorly understood. Warner (1995)
proposed the similarity to VY Scl-type cataclysmic variables,
whose ``low states" are generally believed to result from reduced
mass-transfer. Tsugawa and Osaki (1997) applied the dwarf nova-type
thermal and tidal instability model to the helium disk systems, including
CR Boo and V803 Cen. They succeeded in understanding the behavior of
these systems by considering the stability of the accretion disk depending
on the mass-transfer rate. Tsugawa and Osaki (1997) expected that
intermediate mass-transfer systems, such as CR Boo and V803 Cen, will
undergo dwarf nova-type disk instability, analogous to SU UMa-type
dwarf novae in hydrogen-rich systems. They suggested higher mass-transfer
systems would resemble ER UMa stars, which are a subgroup of SU UMa-type
dwarf novae having extremely short supercycle length (for a review, see
Kato et al. 1999). Subsequent observation indeed confirmed the presence
of 46.3-d supercycle in CR Boo (Kato et al. 2000), whose behavior is
extremely analogous to ER UMa stars. The application to the next candidate,
V803 Cen, has been naturally sought as a part of VSNET Collaboration
(http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/).
\IBVSfig{8cm}{fig1.ps}{Overall light curve of V803 Cen}
Visual observations were performed using 32-cm (R.S.), 32-cm (B.M.)
and 40-cm (A.P.) reflectors. All observations were done using
photoelectrically calibrated $V$-magnitude comparison stars. The typical
error of visual estimates was less than 0.2 mag, which does not affect
the following discussion. The total number of observations between 1998
November 17 and 2000 June 12 was 464.
The overall light variation is presented in Figure 1. Each filled
square represents single estimates and ``v" sign represents upper limits.
The quasi-periodic occurrence of bright states and faint states associated
with brief brightenings is clearly demonstrated. The behavior is
very reminiscent of that of CR Boo (Kato et al. 2000). The light maxima
are separated by $\sim$77 d in each observing seasons of 1998--1999 and
1999--2000. We consider this period as the representative supercycle.
Individual observations are folded by this period, using the maximum epoch
of JD 2451271 for the 1998--1999 season and JD 2451545 for the 1999--2000
season. The interval was 111 d between the last maximum of the 1998--1999
season and the first maximum of the 1999--2000 season. This may suggest
some change occurred around the solar conjunction. However, by treating
the two seasons separatedly, the quasi-periodic outburst pattern was
found to be highly stable within each seasons.
\IBVSfig{8cm}{fig2.ps}{The 77-d supercycle of V803 Cen}
Figure 2 represents the folded light curve of V803 Cen, which shows
a pattern very similar to that of CR Boo, ``the helium ER UMa star"
(Kato et al. 2000). The bright phase (superoutburst) comprises a duty
cycle of $\sim$0.4 supercycle, which is close to the value $\sim$0.5
in CR Boo. Large-amplitude damping oscillations were observed during
the decay from the superoutburst maximum, which may correspond to
short-term modulations with a time-scale of a day (Patterson et al. 2000),
and the feature suspected as ``dips" in CR Boo (Kato et al. 2000).
During the rest of supercycle phase (phase 0.6--1.0), the object is
mostly faint with short brightenings, which are likely to correspond
to normal outbursts in SU UMa-type dwarf novae. The overall behavior
of V803 Cen can be understood as a natural extension of the CR Boo
activity toward the lower mass-transfer rate, which is perfectly
what is expected from its supposed orbital period (Tsugawa and Osaki 1997).
The parameters of these two ``twin" systems are summarized in Table 1:
outburst parameters from this work and Kato et al. (2000) and main
photometric period from Warner (1995).
\begin{table}
\begin{center}
Table 1. Comparison of V803 Cen and CR Boo \\
\vspace{10pt}
\begin{tabular}{ccc}
\hline
& V803 Cen & CR Boo \\
\hline
$V$-magnitude range & 12.2--16.8 & 13.0--17.5 \\
main photometric period (s) & 1611 & 1490 \\
supercycle length (d) & 77 & 46.3 \\
superoutburst duty cycle & 0.4 & 0.5 \\
\hline
\end{tabular}
\end{center}
\end{table}
\references
Kato, T., Nogami, D., Baba, H., Masuda, S., Matsumoto, K., Kunjaya, C.,
1999, {\it Disk Instabilities in Close Binary Systems}, p45, eds.
S. Mineshige, J. C. Wheeler (Universal Academy Press, Tokyo)
Kato, T., Nogami, D., Baba, H., Hanson, G., Poyner, G., 2000,
MNRAS, in press
Patterson, J., Walker, S., Kemp, J., O'Donoghue, D. B., Stubbings, R.,
2000, PASP, 112, 625
Tsugawa, M., Osaki, Y. 1997, PASJ, 49, 75
Warner, B. 1995, ApSS, 225, 249
\end{document}
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