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[vsnet-preprint 24] V803 Cen preprint
V803 Cen preprint
Dear Colleagues,
The following article is accepted for publication as IBVS No. 5091.
The figures are placed under:
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/V803_Cen-2/
Regards,
Taichi Kato
===
\documentstyle[twoside,epsf]{article}
\input{ibvs2.sty}
\begin{document}
\IBVShead{xxxx}{xx May 2001}
\IBVStitletl{Standstill of the helium ER UMa star, V803 Cen}
\IBVSauth{Taichi~Kato$^1$, Rod~Stubbings$^2$, Berto~Monard$^3$,
Andrew~Pearce$^4$, Peter~Nelson$^5$}
\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 11426, Tiegerpoort 0056, South Africa,
e-mail: LAGMonar@csir.co.za}
\IBVSinst{32 Monash Ave, Nedlands, WA 6009, Australia,
e-mail: Andrew.Pearce@worley.com.au}
\IBVSinst{RMB 2493, Ellinbank 3820,Australia,
e-mail: pnelson@dcsi.net.au}
\IBVSobj{V803 Cen}
\IBVStyp{*}
\IBVSkey{dwarf novae -- photometry}
\begintext
A 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). Some of AM CVn stars show large-amplitude
variations, and have been considered to be analogous to (hydrogen-rich)
dwarf novae. The authors have revealed that two of these systems (CR Boo
and V803 Cen) show regular alternations of bright and faint states
(Kato et al. 2000a, 2000b), which bear characteristics of ER UMa stars,
hydrogen-rich SU UMa-type dwarf novae with very short supercycles
(for a review of ER UMa stars, see Kato et al. 1999). The overall and
phased light curves of V803 Cen in Kato et al. (2000b) clearly showed
these features. However, the behavior of V803 Cen suddenly changed
after 2000 June outburst. The star did not return to its faint states
as observed in Kato et al. (2000b), but varied mostly between 13.3
and 14.0, with some short occasional excursions to brighter maxima.
Figure 1 show the overall light curve by the VSNET Collaboration
(http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/), as an extension of Kato et al.
(2000b).
\IBVSfig{8cm}{fig1.ps}{Overall light curve of V803 Cen}
\IBVSfig{8cm}{fig2.ps}{Phase-averaged light curve at a period of 77 d. The
77-d supercycle has completely disappeared.}
\vskip 3mm
In order to check the persistence of the 77-d supercycle as reported
by Kato et al. (2000b), the data of the best-sampled period of
JD 2451873--2452040 were analyzed. The observations were done as
described in Kato et al. (2000b) and a 32-cm reflector by (P.N.).
The total number of observations in this period was 206. Neither Fourier
nor Phase Dispersion Minimization (PDM) method (Stellingwerf 1978) gave
no clear periodicity between 10 and 100 d, indicating that the 77-d
supercycle has completely disappeared (see Figure 2). This change is
very reminiscent of a standstill of Z Cam-type dwarf nova. The possibility
of these helium ER UMa stars spend some time in their ``standstill" state
has been proposed by Kato et al. (2000a) from past observations of
CR Boo, and is also consistent with the locations of these object
close to the thermal stability border predicted by the disk instability
theory (Tsugawa and Osaki 1997). The present observation first clearly
demonstrate such a standstill actually appears in helium ER UMa stars.
Such standstills have not been observed in hydrogen-rich ER UMa stars
(ER UMa, V1159 Ori, RZ LMi and DI UMa). This may suggest that mass-transfer
rates are more variable in helium ER UMa stars than hydrogen-rich ER UMa
stars.
\IBVSfig{8cm}{fig3.ps}{Power spectrum between frequencies 0.1 and 3.0 d$^{-1}$.
Each point represents power and error averaged over 0.04 d$^{-1}$ frequency
bin. The signal at frequency close to 0.15 d$^{-1}$ is the one-day alias
of the main signal, which is likely to be reject as a true signal, as this
disappears in better-sampled data (Figure 4).}
\IBVSfig{8cm}{fig4.ps}{Period analysis (left) and phase-averaged light
curve (right) at P=0.851 d (frequency=1.175 d$^{-1}$) for
JD 2451971--2452040}
\vskip 3mm
During this standstill, low-amplitude ($\sim$0.5--1 mag) variations
were present throughout the period. This may be equivalent to the
22$\pm$1 hr period reported by Patterson et al. (2000). Fourier analysis
of the data shows a significant increase of the power near this period,
but has a maximum power at a shorter period of $\sim$20 hr (Figure 3).
Detailed period analysis of the data, by dividing them into several shorter
segments, has yielded the strongest power in the period of JD
2451971--2452040, during which the period of 0.851 d is dominantly
seen (Figure 4). However, this period was less significant between
JD 2451873--2451970. In the period of JD 2451873--2451930, the variation
has possibly had a longer period of 0.96 d. These findings indicate
that the $\sim$20 hr variation is one of major causes of variation in
this standstill, but the nature of this variation was highly variable
both in amplitude and period.
\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., 2000a,
MNRAS, 315, 140
Kato, T., Stubbings, R., Monard, B., Pearce, A., 2000b, IBVS No. 4915
Patterson, J., Walker, S., Kemp, J., O'Donoghue, D. B., Stubbings, R.,
2000, PASP, 112, 625
Stellingwerf, R. F., 1978, ApJ, 224, 953
Tsugawa, M., Osaki, Y., 1997, PASJ, 49, 75
Warner, B., 1995, ApSS, 225, 249
\end{document}
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