[vsnet-preprint 36] AQ Eri preprint

[Taichi Kato <tkato>]

Dear Colleagues,

   The following article is accepted for publication as IBVS No. 5107.

   The figures are placed at:
   http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/AQ_Eri-2/

Regards,
Taichi Kato

===

\documentstyle[twoside,epsf]{article}

\input{ibvs2.sty}

\begin{document}

\IBVShead{xxxx}{xx May 2001}

\IBVStitletl{Superoutburst observation of AQ Eri:}{evidence for an anomalous superhump excess?}

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

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

\IBVSobj{AQ Eri}
\IBVStyp{UGSU}
\IBVSkey{dwarf nova, photometry}

\begintext

   AQ Eri is a one of relatively bright SU UMa-type dwarf novae.
Thorstensen et al. (1996) reported a spectroscopic orbital period
($P_{\rm orb}$) of 0.06093 d, which makes AQ Eri a member of SU UMa-type
dwarf novae with the shortest orbital periods.  Intermediate nature
between usual SU UMa-type dwarf novae and extreme WZ Sge-type systems
has been proposed for dwarf novae with such periods (cf. Nogami et al.
1996).  However, only little is known about superhumps of AQ Eri.
No observations of its superhumps have been reported since Kato (1991),
who reported a superhump period ($P_{\rm SH}$)of 0.06225 d.
Thorstensen et al. (1996) reported this superhump period gives a
fractional superhump excess ($P_{\rm SH}/P_{\rm orb}-1$) acceptable
for a dwarf nova of this orbital period.  During the superoutburst
in 1992 January, the author succeeded in taking another time-resolved
CCD photometry, which is far superior in quality than in Kato (1991).

\vskip 3mm

   The observations were done on 1992 January 4 using a CCD camera
(Thomson TH~7882, 576 $\times$ 384 pixels, on-chip 3 $\times$ 3 binning
adopted) attached to the Cassegrain focus of the 60 cm reflector (focal
length=4.8 m) at Ouda Station, Kyoto University (Ohtani et al. 1992).
An interference filter was used which had been designed to reproduce
the Johnson {\it V} band.  The exposure time was 30 s.  The frames were
first corrected for standard de-biasing and flat fielding, and were then
processed by a microcomputer-based aperture photometry package developed
by the author.  A total of 430 high-quality images were obtained.
The magnitudes of the object were measured relative to GSC 4758.334
($V=10.93, B-V=+1.24$), whose constancy during the run was confirmed
using GSC 4758.622.  The observation on following nights was unfortunately
hindered by bad weather.  Barycentric corrections to observed times were
applied before the following analysis.

\IBVSfig{10cm}{fig1.ps}{Light curve of AQ Eri on 1992 January 4}
\vskip 3mm

   Figure 1 shows the resultant light curve.  Three superhumps are clearly
visible with a full amplitude of 0.24 mag.  This observation confirms
the SU UMa-type nature of AQ Eri.  Short-period oscillations (quasi-periodic
oscillation; QPOs) became stronger around superhump minima.  This feature
was also observed in AK Cnc (Mennickent et al. 1996), another SU UMa-type
star with a short $P_{\rm orb}$.  This feature may be common to superhumps
of short-period systems.

\IBVSfig{10cm}{fig2.ps}{Period analysis of AQ Eri superhumps}
\vskip 3mm

   Period analysis using the Phase Dispersion Minimization (PDM) method
(Stellingwerf 1978) has yield a superhump period of 0.0642$\pm$0.0004 d,
which is remarkably longer than the previously reported value of 0.06225 d.
The fractional superhump excess is 5.4$\pm$0.7\%, which is remarkably
larger than the typical superhumps excesses (1--3\%) of short-period
systems.  The reason of this discrepancy is not well understood.
The author has checked the stability of the computer clock and recording
system, and found no abnormalities.  The seemingly abnormal period is
thus most likely attributed to the superhump period itself.  The relation
between the observed $P_{\rm SH}$, previously observed $P_{\rm SH}$ and
$P_{\rm orb}$ is shown on Figure 2.  Although it may be still possible
AQ Eri has an intrinsically abnormally high fractional superhump excess,
such a high superhump excess may have been a transient one.  Future
more extensive observations during superoutbursts are thus strongly
encouraged.

\references

Kato, T., 1991, IBVS, No. 3671

Mennickent, R. E., Nogami, D., Kato, T., Worraker, W., 1996, A\&A, 315, 493

Nogami, D., Kato, T., Hirata, R., 1996, PASJ, 48, 607

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

Stellingwerf, R. F., 1978, ApJ, 224, 953

Thorstensen, J. R., Patterson, J., Shambrook, A., Thomas, G., 1996,
     PASP, 108, 73

\end{document}

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