[vsnet-preprint 55] UV Gem, FS And, AS Psc preprint

[Taichi Kato <tkato>]

UV Gem, FS And, AS Psc preprint

Dear Colleagues,

    The following article is accepted for publication as IBVS No. 5158.
The figures are available at
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/UV_Gem/

Regards,
Taichi Kato

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\documentstyle[twoside,epsf]{article}

\input{ibvs2.sty}

\begin{document}

\IBVShead{xxxx}{xx August 2001}

\IBVStitletl{Outburst characteristics of three likely SU UMa-type}\
            {dwarf novae: UV G\lowercase{em},
             FS A\lowercase{nd} and AS P\lowercase{sc}}

\IBVSauth{Kato,~Taichi$^1$, Uemura,~Makoto$^1$}
\vskip 5mm

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

\IBVSobj{UV Gem}
\IBVStyp{UGSU:}
\IBVSobj{FS And}
\IBVStyp{UGSU:}
\IBVSobj{AS Psc}
\IBVStyp{UGSU:}
\IBVSkey{dwarf nova, photometry}

\begintext

   UV Gem is a dwarf nova which is given a UGSS subtype (SS Cyg-type)
in the 4-th Edition of General Catalogue of Variable Stars (GCVS).
The GCVS lists its range of variability of 14.7--18.5p and a mean
cycle length of outbursts of 54 d.  Very little information is found
in the literature.  Zwitter and Munari (1994) obtained a spectrum
and confirmed the presence of a weak H$\alpha$ emission line, suggesting
that the mass-transfer rate is rather high.  We observed this star
in order to clarify its outburst pattern.

\IBVSfig{7cm}{fig1.eps}{Overall light curve of UV Gem}

\vskip 3mm

   The CCD observations were done using an unfiltered ST-7 camera attached
to the Meade 25-cm Schmidt-Cassegrain telescope.  The exposure time was
30 s.  The images were dark-subtracted, flat-fielded, and
analyzed using the Java$^{\rm TM}$-based PSF photometry package
developed by one of the authors (TK).  The magnitudes were determined
relative to GSC 1333.247, whose Tycho-2 magnitude is $V=10.78\pm0.10$ and
$B-V=+0.57\pm0.14$.  The constancy of comparison star during the run was
confirmed by comparison with GSC 1333.543 and GSC 1333.680.
The light curve drawn from these observations is presented in Figure 1.

\vskip 3mm

   Three distinct outbursts were observed during this period:
short outbursts on JD 2451481 and 2451493, and a long outburst starting
on JD 2451522.  The interval of the first two outbursts is only 12 d.
The interval between the second and third being 29 d, it is likely
one outburst was missed between the second and third outbursts.
The true cycle length is thus likely 1/4 of the GCVS period.
The first and second outburst decayed very quickly, with a rate of
decline exceeding 1 mag d$^{-1}$, which is characteristic of normal
outbursts of SU UMa-type dwarf novae.  The third outburst lasted
more than 11 d, which is very characteristic of a superoutburst.
The observed outburst pattern suggests that UV Gem is an SU UMa-type
dwarf nova with a short cycle length.  This seems to be consistent
with the high mass-transfer rate inferred from spectroscopy.

   FS And is a dwarf nova discovered by Hoffmeister (1967).  Hoffmeister
(1967) reported relatively frequent detections of outbursts.
The object was studied by Meinunger (1986), who reported an approximate
outburst cycle length of $\sim$10 d, and the presence of a possible
standstill.  The object has been classified as a possible Z Cam star based
on this observation.  However, the lack of detailed published photometry has
made the detailed classification slightly ambiguous.  Bruch (1989) obtained
spectroscopy and confirmed the dwarf nova classification.

\vskip 3mm

\begin{table}
\begin{center}
Table 1. Outbursts of FS And \\
\vspace{10pt}
\begin{tabular}{cccccc}
\hline
JD at max & peak mag & d$^a$ (d) &
  JD at max & peak mag & d$^a$ (d) \\
\hline
2449659 & 17.0 & -$^b$ & 2450040 & 15.8 & $<$3  \\
2449681 & 16.7 & -     & 2450333 & 15.4 & -     \\
2449694 & 16.5 & -     & 2450345 & 15.7 & -     \\
2449997 & 16.6 & -     & 2450361 & 15.0 & -     \\
2450005 & 15.5 & 3     & 2450711 & 15.5 & -     \\
2450016 & 15.6 & 2     & 2450744 & 15.7 & -     \\
2450026 & 15.8 & $<$6  &         &      &       \\
\hline
 \multicolumn{6}{l}{$^{a}$ Outburst duration.} \\
 \multicolumn{6}{l}{$^{b}$ Not determined (too few data).} \\
\end{tabular}
\end{center}
\end{table}

   We observed FS And in order to study its outburst behavior.  We took
three $V$-band data at Ouda Station, Kyoto University (Ohtani et al. 1992),
between 1996 September 10 and 17.  We further studied unfiltered CCD
observations reported to the public database of the VSOLJ (Variable Star
Observers League in Japan), and VSNET
(http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/).
The former contains observations by M. Iida, and the latter those by
L. T. Jensen.  Although zero-point calibrations were rather uncertain
for unfiltered CCD observations, the zero-point error seems to be smaller
than $\sim$0.3 mag by comparison with the Ouda data.  This degree of
uncertainty will not affect the analysis of the overall outburst behavior.
Table 1 lists the observed maxima of outbursts.

\vskip 3mm

   The shortest observed interval between successive outbursts was 8 d,
which generally confirmed the cycle length reported by Meinunger (1986).
All observed outbursts faded quickly.  Figure 2 represents the best
observed portion of the light curve.  Frequent short outbursts are
clearly seen on Figure 2.  The mean interval between these outbursts
was 10 d.  The quick fade from the outburst maxima is not characteristic
of a Z Cam star (or a short period SS Cyg star) having this cycle length.
The characteristics of outbursts more resemble those of a frequently
outbursting SU UMa-type dwarf nova, best exemplified by HS Vir (Kato 1995;
Kato et al. 1998), which showed similar frequent, short outbursts recurring
with a period of 8 d.  From these similarities, we propose that FS And is
a good candidate for an SU UMa-type dwarf nova.  The possible ``standstill"
reported by Meinunger (1986) may have been a superoutburst.  Further
monitoring for outbursts is strongly recommended.

\IBVSfig{7cm}{fig2.eps}{Light curve of FS And.  Frequent, short outbursts
with a recurrence time of $\sim$10 d are seen.}

   AS Psc (=S 10828) was originally discovered as an eruptive variable
reaching $B$=16.5 in 1963 in the vicinity of the galaxy M33 (Richter 1979).
Since no other outbursts were detected between 1963 and 1980 (Richter 1979),
the star was suspected to be a nova in M33.  The second outburst was
detected in 1980 (Sharov 1982), which made a long-period dwarf nova more
likely.  However, the possibility of a recurrent nova in M33 remained
(Richter 1983).  Since then, three more outbursts were detected, at least
one of which faded very quickly (Sharov 1988).  Together with the shortest
interval of 293 d between outbursts, the object is now considered to be
a dwarf nova with rather infrequent outbursts (Richter 1989).
Richter (1989) listed all the observations of five known outbursts.
From the lack of the visible counterpart on POSS and other deep exposures,
the quiescent magnitude is considered to be fainter than $B$=21.7.
Combined with the brightest observed maximum, reaching $B$=15.3,
the total amplitude of outburst is larger than 6.4, which makes AS Psc
a good candidate of an SU UMa-type dwarf nova.

\vskip 3mm

   Further evidence for an SU UMa-type dwarf nova can be found in the
extremely rapid decline (1.6 mag d$^{-1}$), observed on the occasion
of the 1984 outburst.  This rate of decline corresponds to that of a
normal outburst of an SU UMa-type dwarf nova with a short orbital
period.  If AS Psc is an indeed SU UMa-type dwarf nova, the bimodal
distribution of outbursts (normal outbursts and superoutbursts) would
make the simple statistical analysis of outbursts by Richter (1989)
misleading.  No further outburst has been reported both in the literature
and to VSNET.

\vskip 3mm

   While surveying exposures taken by the members of Kyoto University
Astronomy Lovers' Association, the authors found a new outburst of AS Psc
occurring in 1989 October.  The exposure was taken by Mr. Nishida with
a hypersensitized TP 2415 film and a 13-cm reflector on JD 2447801.231.
The exposure clearly showed AS Psc in outburst.  Using $V$-magnitude
comparison stars for TX Tri, we estimated the magnitude of the variable
as 16.3.  The outburst occurred 1028 d after the last known outburst in
1986.

   Looking at available materials (summarized in Richter 1989), the existence
of two types outbursts is evident: short or faint outbursts, as in
JD 2444461 and 2445964, and long or bright outbursts.  Such a bimodal
distribution is consistent with the supposed classification of an
SU UMa-type dwarf nova.  By assuming that outbursts reaching 16.5 mag
are long, bright outbursts (likely superoutbursts), there is a clear
indication of regular intervals between them.  The interval between
the 1983 and 1986 outbursts is 1102 d, which is close to interval
of 1028 d between the 1986 and 1989 outbursts.  The interval of 7384 d
between the 1963 and 1983 outbursts may be 7 times of this fundamental
period.  The available material thus suggests that the supercycle of
AS Psc is 1000--1100 d, which is an intermediate value between WZ Sge-type
dwarf novae and usual SU UMa-type dwarf novae (c.f. Nogami et al. 1997).
Although there still remains a possibility that the true supercycle
could be $N$-th of this value, the large outburst amplitude seems to
be consistent with a long supercycle.  Further observations to search
for outbursts, and time-resolved photometry to search for superhumps
are strongly encouraged.

\vskip 3mm

   We are grateful to M. Iida for providing observations, and the manager
of the VSOLJ for making them publicly available.  We are also grateful
to L. T. Jensen for providing observations of FS And.
The author is grateful for Kyoto University Astronomy Lovers' Association
and Mr. Nishida for providing photographs for the author's examination.
Part of this work is supported by a Research Fellowship of the
Japan Society for the Promotion of Science for Young Scientists (MU).

\references

Bruch, A., 1989, A\&AS, 78, 145

Hoffmeister, C., 1967, AN, 289, 205

Kato, T., Nogami, D., Masuda, S., Hirata, R., 1995, IBVS No. 4193

Kato, T., Nogami, D., Masuda, S., Baba, H., 1998, PASP, 110, 1400

Meinunger, L., 1986, MVS, 11, 1

Nogami, D., Masuda, S., Kato, T., 1997, PASP, 109, 1114

Ohtani, H., Uesugi, A., Tomita, Y., Yoshida,
     M., Kosugi, G., Noumaru, J., Araya, S., Ohta, K.
     1992, Memoirs of the Faculty of Science, Kyoto University,
     Series A of Physics, Astrophysics, Geophysics and Chemistry,
     38, 167

Richter, G. A., 1979, MVS, 8, 119

Richter, G. A., 1983, Astron. Tsirk., No. 1262, 7

Richter, G. A., 1989, MVS, 12, 1

Sharov, A. S., 1982, Astron. Tsirk., No. 1229, 8

Sharov, A. S., 1988, Pis'ma Astron. Zh., 13, 427

Zwitter, T., Munari, U., 1994, A\&AS, 107, 503

\end{document}

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