[vsnet-preprint 45] IR Gem preprint

[Taichi Kato <tkato>]

IR Gem preprint

   The following article is accepted for publication as IBVS No. 5122.

   The figures are available at:
   http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/IR_Gem/

Regards,
Taichi Kato

===

\documentstyle[twoside,epsf]{article}

\input{ibvs2.sty}

\begin{document}

\IBVShead{xxxx}{xx May 2001}

\IBVStitletl{Observation of superhumps in IR Gem}

\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{IR Gem}
\IBVStyp{UGSU}
\IBVSkey{dwarf nova, photometry}

\begintext

   IR Gem is a well-known SU UMa-type dwarf nova.  However, little
observation of superhumps has been reported since the identification
as an SU UMa-type dwarf nova (Szkody et al. 1984).  We observed this
star during its 1991 March superoutburst.

\vskip 10mm

   The observations were done on two successive nights, 1991 March 18
and 19, 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 $I_{\rm c}$ band.
The exposure time was 10 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.
The magnitudes of the object were determined relative to GSC 1905.753
(GSC magnitude 11.07), whose constancy during the run was confirmed
using the check star USNO A1.0-1125.04589035.  Barycentric corrections to
observed times were applied before the following analysis.  Table 1 lists
the log of observations, together with nightly averaged magnitudes.

\vskip 3mm

\IBVSfig{10cm}{fig1.ps}{Light curve of the 1991 March superoutburst
of IR Gem}

   Figure 1 shows the resultant light curve.  Superhumps are prominently
seen.  After removing the trend of decline, we applied Phase Dispersion
Minimization (PDM) method (Stellingwerf 1978).  The resultant theta
diagram is shown in Figure 2.  The result generally confirms the superhump
period of 0.07076 d reported by Szkody et al. (1984).  The best period
determined from our data is 0.07094$\pm$0.00006 d, which is slightly
longer than that by Szkody et al. (1984).  By taking the orbital period
of 0.0684 d (Feinswog et al. 1988), the fractional superhump excess
is 3.7\%.  The most remarkable difference of superhumps from those
observed by Szkody et al. (1984) is the clear presence of secondary
superhumps, i.e. bump-like feature on the fading branch of superhumps.
The feature was markedly seen on 1991 Mar 18, but became less clear
on the subsequent night.  This feature was discussed by Udalski (1990)
on SU UMa itself.  Udalski (1990) proposed that this feature may arise
from a cooler component of the disk, but the nature is not still well
understood.  The appearance of secondary superhumps in $I_{\rm c}$ band
light curve may be consistent with Udalski's (1990) hypothesis.

\IBVSfig{10cm}{fig2.ps}{Period analysis of IR Gem}

\begin{table}
\begin{center}
Table 1. Log of observations \\
\vspace{10pt}
\begin{tabular}{ccccc}
\hline
start$^a$ & end$^a$ & mean mag$^b$ & error$^c$ & N$^d$ \\
\hline
48333.932 & 48334.084 & 2.234 & 0.002 & 760 \\
48334.936 & 48335.115 & 2.420 & 0.002 & 816 \\
\hline
\end{tabular}
\end{center}
\hskip 35mm $^a$ BJD$-$2400000

\hskip 35mm $^b$ Magnitude relative to GSC 1905.753

\hskip 35mm $^c$ Standard error of nightly average

\hskip 35mm $^d$ Number of frames

\end{table}

\references

Feinswog, L., Szkody, P., Garnavich, P., 1988, AJ, 96, 1702

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

Szkody, P., Shafter, A. W., Cowley, A. P., 1984, ApJ, 282, 236

Udalski, A., 1990, AJ, 100, 226

\end{document}

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