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\IBVShead{4281}{20 December 1995}

\centerline{\ft OPTICAL OBSERVATIONS OF THE ACTIVE STAR RE J2220+493} 
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\begintext


The sky was surveyed in the extreme ultraviolet (EUV) region of the
spectrum by the EUVE satellite (Malina et al., 1994) and the ROSAT
satellite (Pounds et al.  1993) and catalogs of the sources included RE
J2220+493 = EUVE J2220+49.5 = SAO\,51891 = BD\,+48\degr 3686.  The star was one
of the subjects of an investigation by Jeffries (1995), who concluded
it was a ``single, rapidly rotating K0V-IV star with a v\,sin\,i of
15 km/sec''.

The automated 0.5-m.  telescope, Johnson V filter and CCD camera of the
Climenhaga Observatory of the University of Victoria (Robb and Honkanen,
1992) were used to make photometric observations of RE J2220+493.  The
frames were bias subtracted and flat fielded in the usual manner using
IRAF \footnote{IRAF is distributed by National Optical Astronomy
Observatories, which is operated by the Association of Universities for
Research in Astronomy, Inc., under contract to the National Science
Foundation}.  The magnitudes were found from aperture photometry using
the PHOT package.  The x y pixel coordinates of each star for photometry
were found from inspection of a few frames and these positions were used
as starting points for the Gaussian centering option which precisely
centered the 5 arc second aperture on each star for each frame. 

From the Hubble Space Telescope Guide Star Catalog (Jenkner et al.,
1990) the coordinates and magnitudes of the comparison star are RA=22\fh
19\fmm 38\fs, Dec=49\degr 32\farcmm 20\farcss, V=9.6 and of the check
star are RA=22\fh 19\fmm 38\fs, Dec=49\degr 28\farcmm 07\farcss, V=8.9
(J2000).  The standard deviation of the difference between the check and
the comparison star during a night ranged from 0\fm 003 to 0\fm 007. 
The mean and standard deviation of the eleven nightly mean differential
V magnitudes are  0\fm429$\pm$0\fm005 ensuring the constancy of both
comparison and check stars at this level.  The precision of the
differential variable star minus comparison star measurements are
expected to be at this level.  Due to the small field of view first
order differential extinction effects were negligible and no corrections
have been made for them.  No corrections have been made for the colour
difference between the stars to transform the V magnitude to a standard
system. 

Photometric observations were made 22 to 25 September and 29 October to
4 November 1995 UT.  Brightness variations were evident both during a
night and from night to night.  A ``Phase Dispersion Minimisation''
routine modelled after that of Jurkevich (1971) reveals a minimum
average sigma at a period of 2.43$\pm$0.02 days.  A least squares
fit of a single sine wave to the data also shows a deep minimum in chi
squared at a period of 2\fd 43 as seen in Figure 1.  The other deep
minima correspond to periods of 2\fd58 and 2\fd29 and inspection of the
light curves at these periods show rather unlikely discontinuities. 

So the best ephemeris from our data is: 

\centerline{HJD of Maxima = 2449981.94 + 2.43 $\times$ E}
\hspace*{8.2cm}$\pm$.20\ \ \,$\pm$0.01
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Figure 1. Chi squared of a single sine curve fit for various periods for 
RE J2220+493 for 1995
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Figure 2. Light curve of 1995 differential V data of RE J2220+493
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A plot of the 2112 differential V magnitudes phased at this period is
shown in Figure 2 with different symbols for each of the different
nights.  Close inspection of the ``$\odot$'' points at phase 0.75 (HJD
= 2450022.68) shows
a small flare, which was ignored during the period finding runs.  The
flare had an amplitude of approximately 0\fm1 and a duration of an hour. 
The light curve also shows shifts of a few hundredths of a magnitude in
mean level from night to night. 

From our period, the $v$sin$i$ and assuming a radius (Allen 1973),
appropriate for the spectral type K0V we find the inclination of the
axis of rotation to be $58^\circ\pm7^\circ$.  A luminosity class of IV
is unlikely since it implies an inclination of approximately $20^\circ$,
so the area on the star passing in and out of view is small compared to
the amplitude of the light curve.  From the shape of the light curve,
the changes in the shape of the light curve, the flare, the spectral
type, Hydrogen $\alpha$ emission (Mulliss and Bopp, 1994), and the
brightness in the EUV, we conclude that this star has large active
regions on it causing the brightness variations.  The changes in the
light curve shape are likely due to differential rotation or active
region evolution and could be studied by further photometric
observations. 

 
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\beginauthors
R. M. ROBB
E. STEINBRING
M. BALOGH
B. ANSELL
M. GLADDERS
J. KARR
D. GILBERT
W. HUGHES

Climenhaga Observatory 
Dept. of Physics and Astronomy
University of Victoria
Victoria, BC, CANADA,  V8W 3P6
Internet: robb@uvphys.phys.uvic.ca
\endauthors

\references

Allen, C.W., 1973, Astrophysical Quantities, 3rd ed. Athlone Press, Univ. 
London

Jeffries, R.D., 1995, {\it MNRAS}, {\bf 273}, 559

Jenkner, H., Lasker, B., Sturch, C. McLean, B., Shara, M., Russell, J., 1990, 
{\it AJ}, {\bf 99}, 2082

Jurkevich, I., 1971, {\it Ap.\&Sp. Sci.}, {\bf 13}, 154
  
Malina, R.F., et al., 1994, {\it AJ}, {\bf 107}, 751

Mulliss, C. L. and Bopp, B. W., 1994, PASP, 106, 822

Pounds, K.  A., et al., 1993, {\it MNRAS}, {\bf 260}, 77
 
Robb, R. M.  and Honkanen, N.  N., 1992, {\it A.S.P. Conf. Ser.}, {\bf 28}, 105,
Automated Telescopes for Photometry and Imaging, ed.  Adelman, Dukes and
Adelman
%Original: {\it A.S.P. Conf. Ser.}, {\bf 38}, 
%Corrected: Mon Sep 26 12:50:10 MEST 2005
   
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