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[vsnet-preprint 29] AT Cnc preprint
AT Cnc preprint
Dear Colleagues,
The following article is accepted for publication as IBVS No. 5099.
The figures are placed at:
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/AT_Cnc-2/
Regards,
Taichi Kato
===
\documentstyle[twoside,epsf]{article}
\input{ibvs2.sty}
\begin{document}
\IBVShead{xxxx}{xx May 2001}
\IBVStitletl{Unusual Slow Fading of Standstill in AT Cnc}
\IBVSauth{Taichi~Kato$^1$, Rod~Stubbings$^2$, Maciej Reszelski$^3$, Eddy~Muyllaert$^4$, \\
Mike~Simonsen$^5$, Gary~Poyner$^6$, Pavol~A.~Dubovsky$^7$, Andrew~Pearce$^8$, \\
Timo~Kinnunen$^9$, Hiroyuki~Maehara$^{10}$}
\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} % Stu
\IBVSinst{Al. 1-go Maja 29/4, 64500 Szamotuly, Poland
e-mail: macres@pro.onet.pl} % RMQ
\IBVSinst{VVS Belgium - Werkgroep Veranderlijke Sterren,
Eksterstraat 6, 8400 Oostende, Belgium
e-mail: eddy.muyllaert@ping.be} % MUY
\IBVSinst{46394 Roanne Drive Macomb, MI USA 48044
e-mail: mikesimonsen@mindspring.com} % SXN
\IBVSinst{BAA Variable Star Section,
67 Ellerton Road, Kingstanding, Birmingham B44 0QE, England
e-mail: gp@star.sr.bham.ac.uk} % POY
\IBVSinst{MEDUZA group, Vedecko-kulturne centrum na Orave,
027 42 Podbiel 194, Slovakia
e-mail: vkco@isternet.sk} % DPV
\IBVSinst{32 Monash Ave, Nedlands, WA 6009, Australia,
e-mail: Andrew.Pearce@worley.com.au} % Pea
\IBVSinst{Sinirinnantie 16, SF-02660 Espoo, Finland,
e-mail: stars@personal.eunet.fi} % KNN
\IBVSinst{Variable Star Observers League in Japan (VSOLJ),
Namiki 1-13-4, Kawaguchi, Saitama 332-0034, Japan,
e-mail: mira@cablenet.ne.jp} % Mhh
\IBVSobj{AT Cnc}
\IBVStyp{UGZ}
\IBVSkey{dwarf novae -- photometry}
\begintext
AT Cnc is a well-established Z Cam-type dwarf nova with clear alternations
of dwarf nova-type phase and standstills (see Nogami et al. 1999 for the
extensive review on this object). The presence of standstills, in addition
to outbursts, is the defining feature of Z Cam stars, and standstills
are considered to be equivalent to non-outbursting novalike (NL) variables
with thermally stable accretion disks (cf. Osaki 1996 for a review of
disk instability in dwarf novae). Standstills in Z Cam stars usually
terminate with a sudden fading to quiescence, whose rate of decline is
roughly equal to the rate of decline from outbursts.
\IBVSfig{10cm}{fig1.ps}{Light curve of AT Cnc. The solid line shows the
decline at the rate of 0.0054 mag d$^{-1}$.}
\IBVSfig{10cm}{fig2.ps}{Light curve of AT Cnc in 1998, showing normal
outburst cycles.}
\vskip 3mm
The recent standstill of AT Cnc was entered in 2000 November, and has
shown peculiar phenomena until now, 2001 May. [N.B. The portion prior to
the standstill was not well covered by observations, since the object
was only visible in the morning twilight. Several negative estimates
with upper limits below 14-th magnitudes indicate that the object had not
entered this standstill before October.]
The object has been monitored as a part of VSNET Collaboration
(http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/).
Visual observations have been made by a number of observers, using
the comparison star sequences calibrated in the $V$ band. The typical
error of visual estimates is 0.2 mag, which will not affect the following
discussion. The light curve drawn from these observations is presented
in Figure 1. A portion of the long-term light curve of AT Cnc is presented
for comparison in Figure 2, which clearly shows the normally outbursting
state of this dwarf nova. The remarkable difference between them
demonstrates how unusual the present behavior is. The long-term light
curve covering the 1995--2001 period is also available at
http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/LCs/CNCAT.html.
The first noticeable event during this standstill occurred
in 2000 December (around JD 2451900), an outburst starting from the
standstill. Since dwarf nova outbursts are believed to be a consequence
of thermal disk instability, which occurs when the mass-transfer rate is
below a certain limit, $\dot{M}_{crit}$ (cf. Osaki 1996), such an outburst
from the standstill is unexpected if it is triggered by the thermal
disk instability. Another small one occurred in 2001 April (around
JD 2452026), but this may be of different nature, as it was on the
slow fading branch on the standstill, as described below.
\vskip 3mm
The other very peculiar feature of this standstill is the slow
fading throughout the standstill (except two ``outbursts" described
above). Such a trend is extremely peculiar among all known Z Cam stars.
The rate of decline, measured by linear fitting to the light curve,
after removing two ``outbursts", is 0.0054 mag d$^{-1}$, corresponding
to the $e$-folding time of $sim$200 d, which is totally inconsistent
with the usual fading rate (0.3--0.4 mag d$^{-1}$) of this dwarf nova.
This fading rate is more characteristic to slow fadings in VY Scl-type
novalike variables (cf. Table 1 of Honeycutt et al. 1994). Together with
the similarity of standstills with novalike variables, this similarity
of fading rate and pattern suggests that we may be witnessing a
VY Scl-type phenomenon (temporary reduction of mass-transfer rate) in
a Z Cam star. However, as discussed in Honeycutt et al. (1994) and also
in King et al. (1998), the standard disk-instability theory predicts
that the system should undergo dwarf nova outbursts as the mass-transfer
rate decreases. What has been observed in AT Cnc is the contrary:
the ``outbursts" are much more infrequent and smaller than in its normally
outbursting state.
\vskip 3mm
In VY Scl-type stars, an idea has been proposed to solve the same
dilemma (Leach et al. 1999). Leach et al. (1999) could reproduce the
VY Scl-type fading, without causing major outbursts, by taking the
irradiation by the hot white dwarf into account. One may speculate
that the same process may be taking place in the present peculiar
standstill of AT Cnc. This is not the only similarity of AT Cnc with
VY Scl-type stars. Nogami et al. (1999) discovered an intermittent
P Cyg-type absorption feature which they interpreted as winds.
Such intermittent winds are more commonly seen in VY Scl-type stars,
and are rare in dwarf novae. The best example is BZ Cam (originally
discovered by J. R. Thorstensen and presented in Patterson et al. 1996;
see also Ringwald and Naylor 1998). The spectroscopic evidence of
similarity of AT Cnc to BZ Cam was already addressed by Nogami et al.
(1999), and the present observation of standstill may be an additional
support to the relation between these seemingly difference classes
of objects. There may be a common underlying mechanism to produce
the observed VY Scl-type or VY Scl-like features, as well as high-speed
winds, in AT Cnc and BZ Cam. Since some of VY Scl-type stars are
suspected to be steadily burning hydrogen on the surface of their white
dwarfs (Greiner et al. 1999), the present anomalous state of AT Cnc
would be an attractive target to search for similar phenomenon
in a Z Cam-type dwarf nova.
\references
Greiner, J., Tovmassian, G. H., di Stefano, R., Prestwich, A.,
Gonzalez-Riestra, R., Szentasko, L., Chavarra, C., 1999,
A\&A, 343, 183
Honeycutt, R. K., Cannizzo, J. K., Robertson, J. W., 1994, ApJ, 425, 835
King, A. R., Cannizzo, J. K., 1998, ApJ, 499, 348
Leach, R., Hessman, F. V., King, A. R., Stehle, R., Mattei, J.,
1999, MNRAS, 305, 225
Nogami, D., Masuda, S., Kato, T., Hirata, R., 1999, PASJ, 51, 115
Osaki, Y. 1996, PASP, 108, 39
Patterson, J., Patino, R., Thorstensen, J. R., Harvey, D., Skillman, D. R.,
Ringwald, F. A., 1996, AJ, 111, 2422
Ringwald, F. A., Naylor, T., 1998, AJ, 115, 286
\end{document}
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