\documentstyle[twoside,epsf]{article}

\input{ibvs2.sty}

\begin{document}

\IBVShead{5093}{25 May 2001}

\IBVStitle{Oscillation During a Standstill of Z C\lowercase{am}}

\IBVSauth{Kato, Taichi}

\IBVSinsto{Dept.\ of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: tkato@kusastro.kyoto-u.ac.jp}

\SIMBADobj{Z Cam}
\IBVStyp{ UGZ }
\IBVSkey{dwarf nova, photometry}

\begintext

The standstill phenomenon in Z Cam stars has been still poorly
understood, even in the present successful era of the disk instability
model (for a review, see Osaki 1996).  This phenomenon is usually regarded
as a state of enhanced mass-transfer rate ($\dot{M}$) in dwarf novae
(cf.\ Warner 1995), which mimics novalike (NL) systems having thermally
stable disks.  However, it has been an old and new problem for
theoreticians to reproduce standstills by numerical simulations.
Meyer and Meyer-Hofmeister (1983) proposed that a normal outburst
below the critical surface density can trigger a standstill,
which is maintained by an enhanced mass-transfer caused by increased
irradiation.  Even the most recent detailed modeling (Buat-M\'{e}nard
et al.\ 2001), by taking irradiation and enhanced mass-transfer into
account, is far from satisfactory reproduction of observed properties
of standstills and Z Cam stars.  The most striking departure from
observations can be seen when the system enters a standstill.
Theories involving enhanced mass-transfer are accompanied by the increased
system luminosity, and the disk is thermally most stable at the beginning
of standstills.  Honeycutt et al.\ (1998) systematically studied
standstills of Z Cam stars, and concluded that some of them showed
damping oscillations when entering a standstill, on the contrary
to theoretical predictions.  The same feature in RX And was reported
by Szkody and Mattei (1984).  However, the conclusion by Honeycutt et al.\
(1998) was largely based on their observation of HX Peg, which differs
from other ``classical'' Z Cam stars in that it shows relatively frequent
and short standstills and rather anomalous behavior in its excursions
between standstills and outbursting states (Honeycutt et al.\ 1998).
Whether such damping oscillations when entering standstills are a
common feature of Z Cam stars, is therefore left as an open question.

\IBVSfig{10cm}{5093-f1.ps}{Light curve of Z Cam. Ticks show small outbursts
in the early stage of standstill}

\vskip-11pt
The author has examined visual observations of Z Cam reported to VSNET \\
(http://www.kusastro.kyoto-u.ac.jp/vsnet/) and found small-scale outbursts
occurring in the early part of a standstill (Figure 1).  These visual
observations used $V$-band comparison stars and have typical errors
of $\sim 0\fmm2$, which will not affect the discussion.
The entrance to this standstill was not associated with a gradual brightening
of preceding minima, as observed in Szkody and Mattei (1984) and Honeycutt
et al.\ (1998).  Hence the observed phenomenon does not have a feature
of damping oscillations.  The mean recurrence time of these small outbursts
is 12 day, which is about the half of intervals (20--25 d) of preceding
normal outbursts, which is different from the phenomenon in Szkody and
Mattei (1984), who reported small outbursts during standstill having a
similar recurrent time to those of usual outbursts.  The present phenomenon
strongly suggests the presence of weak disk instability occurring
in the early stage of a standstill, when the accretion disk is thought to
be most stable.  The present discovery of departure from theories
in the prototypical, and most typical, Z Cam star also suggests that
such departures are a common features of Z Cam stars, which need
to be explained by future theories.

\bigskip

The authors are grateful to many VSNET members for providing vital
observations.

\references

Buat-M\'{e}nard, V., Hameury, J.-M., Lasota, J.-P., 2001, {\it A\&A}, {\bf 369}, 925

Honeycutt, R. K., Robertson, J. W., Turner, G. W., Mattei, J. A.,
1998, {\it PASP}, {\bf 110}, 676

Meyer, E., Meyer-Hofmeister, E., 1983, {\it A\&A}, {\bf 121}, 29

Osaki, Y., 1996, {\it PASP}, {\bf 108}, 39

Szkody, P., Mattei, J. A., 1984, {\it PASP}, {\bf 96}, 988

Warner, B., 1995, Cataclysmic Variable Stars (Cambridge Univ.\ Press,
Cambridge)

\end{document}


