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[vsnet-preprint 39] UW Tri preprint
Dear Colleagues,
The following article is accepted for publication as IBVS No. 5116.
The figures are available at:
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/UW_Tri/
Regards,
Taichi Kato
===
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\input{ibvs2.sty}
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\begin{document}
\IBVShead{xxxx}{xx May 2001}
\IBVStitletl{UW Tri: another likely WZ Sge-type star}
\IBVSauth{Taichi~Kato$^1$, Daisaku~Nogami$^{2,1}$, J. J.~Lockley$^{3,4}$,
M.~Somers$^4$}
\vskip 5mm
\IBVSinst{Dept. of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: tkato@kusastro.kyoto-u.ac.jp}
\IBVSinst{Hida Observatory, Kyoto University, Gifu 506-1314, Japan,
e-mail: nogami@kwasan.kyoto-u.ac.jp}
\IBVSinst{Department of Physics and Astronomy,
the University of Southampton, Highfield, Southampton SO17 1BJ,
UK, e-mail: jjl@astro.soton.ac.uk}
\IBVSinst{Physics Department, Keele University, Keele, Staffordshire
ST5 5BG, UK}
\IBVSobj{UW Tri}
\IBVStyp{UGSU}
\IBVSkey{dwarf nova, photometry}
\begintext
UW Tri was discovered as a possible nova in 1983 by Kurochkin (1984).
The discovery was communicated by Aksenov (1983). Argyle (1983) reported
accurate astrometry of this possible nova, but the lack of spectroscopic
observation made the nova identification inconclusive. Kurochkin (1984)
reported that the object reached a maximum of 14.7pg, and the outburst
lasted at least 32 days. The light curve resembled a fast nova, but
the extreme faintness and the high galatic latitude makes a normal
Galactic nova unlikely. Another possibility is a WZ Sge-type dwarf nova,
a small subgroup of SU UMa-type dwarf novae (see Osaki 1996 for a review),
which also show a fast nova-like light curve and very long (typically
$\sim$10 years) outburst recurrence time. The latter possibility
suggested that the phenomenon can be recurrent, and a search for additional
outburst was conducted by several amateur astronomers.
\vskip 3mm
Meanwhile, the second historical outburst was detected by T. Vanmunster
(1995). The detection was made on 1995 March 3.819 UT, at visual magnitude
of 14.7. The outburst was confirmed by E. Broens and G. Poyner (Vanmunster
1995). James (1995) reported accurate astrometry of 02\h 45\m 17\fs 30,
+33\deg 31\arcm 26\farcs 5 (J2000.0), which confirmed the absence of
a counterpart of POSS-I plates. Since the presence of superhumps is the
diagnostic feature of SU UMa-type dwarf novae, we started time-resolved CCD
photometry.
\vskip 3mm
The observations at Ouda Station, Kyoto University (Ouda) were done
on eight nights between 1995 March 5 and 20, using a CCD camera (Thomson
TH~7882, 576 $\times$ 384 pixels, on-chip 2 $\times$ 2 binning adopted)
attached to the Cassegrain focus of the 60 cm reflector (Ohtani et al. 1992).
An interference filter was used which had been designed to reproduce the
Johnson {\it V} band. The exposure time was 60--180 s depending on the
brightness of the object. The frames were first corrected for standard
de-biasing and flat fielding, and were then analyzed using the
Java$^{\rm TM}$-based PSF photometry package developed by one of the authors
(TK). The observations at Keele University (Keele) on two nights 1995
March 7 and 12, using an ST-6 CCD camera and a Johnson {\it V} filter,
attached to a 60 cm telescope. The exposure times were 30 s. Total
numbers of useful frames were 216 (Ouda) and 421 (Keele). Both
observatories used GSC 2329.320 (GSC magnitude 12.8) as the comparison
star, whose constancy during the run was confirmed using GSC 2329.1501
and GSC 2329.534. By interpolating Ouda light curves and comparing
Keele observations, the observations at Keele were found to be
systematically fainter than Ouda observations by 0.21 mag. This
systematic difference is probably caused by a small difference of
the natural systems between these observatories, combined with the
blue color of an outbursting dwarf nova. The difference will not
significantly affect the following analysis. We first corrected this
systematic difference. Heliocentric corrections to the observed times
were made before the following analysis.
\vskip 3mm
\IBVSfig{10cm}{fig1.ps}{(Right) Overall light curve of UW Tri. Filled and open
circles represent Ouda and Keele observations, respectively. (Left) Enlarged
Light curves of the first two nights.}
The overall light curve is shown in Figure 1 (left). Each point represent
averages and standard errors of nightly runs. The object initially
rapidly faded at a rate of 0.2 mag d$^{-1}$, and the fading later became
slower, reproducing the 1983 outburst recorded by Kurochkin (1984).
The outburst lasted at least for 17 days. Owing to the short visibility
in the evening, it is very difficult to make a firm conclusion on its
intranight variation.
\IBVSfig{8cm}{fig2.ps}{Period analysis of UW Tri}
\IBVSfig{8cm}{fig3.ps}{Phase-averaged light curve of UW Tri}
\vskip 3mm
We applied Phase Dispersion Minimization (PDM) method (Stellingwerf
1978) to the data on the first two nights which gave the best
signal-to-noise ratio. The trend of linear decline was subtracted
before the analysis. The resultant theta diagram is shown in Figure 2.
There are indications of the presence of short-period modulations,
attributable to superhumps. Together with the long recurrence time,
this finding strengthen the possibility of UW Tri being a WZ Sge-type
dwarf nova. It is virtually impossible to select the unique period among
strong aliases close to 0.051, 0.054 and 0.057 d. Since almost all
hydrogen-rich cataclysmic variables have orbital periods longer than
the period minimum of $\sim$0.055 d (cf. Ritter and Kolb 1998),
we adopted a period of 0.0569 d as the most likely period.
However, one must bear in mind that this period should be treated as the
likely one among several possibilities. By adding data points made on
later nights, the results remained basically unchanged.
\vskip 3mm
The phase-averaged light curve by the period of 0.0569 d is shown
in Figure 3. The profile is characteristic to superhumps of SU UMa-type
dwarf novae, but the amplitude of 0.07 mag is smaller than those
(0.1--0.3 mag) in usual SU UMa-type dwarf novae. Given that observations
were made during the rapidly fading, early epoch of a superoutburst,
this modulation may be better interpreted as low-amplitude
``early superhumps", characteristic to WZ Sge-type dwarf novae
(Kato et al. 1996; Matsumoto et al. 1998; Kato et al. 1998).
Phase-averaging of the late-phase observations had a severe difficulty
with their low signal-to-noise ratio and short individual runs.
By assuming the 0.0569-d period, the Keele data give $\sim$0.1 mag
amplitudes both on March 7 and 12, suggesting that the variation may have
evolved as in other WZ Sge-type stars, but the result should be treat
with caution because the detection was marginal.
The quiescence counterpart of UW Tri was discovered by Robertson et al.
(2000) at a magnitude of $B$=22.6--22.9, and astrometry end figures
of 17\fs 29, 26\farcs 31, which are in excellent agreement with
James's astrometry in outburst (1995). This makes the outburst amplitude
of $\sim$8.0 mag, which is very similar to that ($\sim$8.5 mag) of WZ Sge.
All the available observations support that UW Tri is similar to WZ Sge,
in large outburst amplitude, long recurrence time, and short superhump
period. Confirmation of these properties, as well as spectroscopic
confirmation of its classification, is strongly encouraged in future
outbursts.
\vskip 3mm
The authors are grateful to VSNET members, for providing vital observations
and information, especially to Tonny Vanmunster for promptly notifying
the outburst.
\references
Aksenov, E. P., 1983, IAUC No. 3869
Argyle, R. W., 1983, IAUC No. 3878
James, N., 1995, {\it VSNET alert circulation}, No. 61 \\
(available at
http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-alert/msg00061.html).
Kato, T., Nogami, D., Baba, H., Matsumoto, K., Arimoto, J., Tanabe K.,
Ishikawa, K., 1996, PASJ, 48, L21
Kato, T., Nogami, D., Baba, H., Matsumoto, K., 1998, in Wild Stars
in the Old West eds. S. Howell, E. Kuulkers, C. Woodward
(ASP Conference Series 137) p.9
Kurochkin, N. E., 1984, Astron. Tsirk. No. 1325
Matsumoto K., Nogami, D., Kato, T., Hirata R., 1998, PASJ, 50, 405
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
Ritter, H., Kolb, U., 1998, A\&AS, 129, 83
Robertson, J. W., Honeycutt, R. K., Hillwig, T., Jurcevic, J. S.,
Henden, A. A., 2000, AJ, 119, 1365
Osaki, Y. 1996, PASP, 108, 39
Stellingwerf, R. F., 1978, ApJ, 224, 953
Vanmunster, T., 1995, Cataclysmic Variable Circular No. 29 \\
(also available at
http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-alert/msg00057.html).
\end{document}
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