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\input ibvs2.sty

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\IBVShead{5449}{18 September 2003}

\IBVStitle{Brightness Variations of SAO 84309}

\IBVSauth{Robb, R. M.$^{1,2}$; Vincent J.$^2$; Thanjavur, K.$^2$}

\IBVSinst{Guest User, Canadian Astronomy Data Centre, which is operated by the Herzberg Institute of Astrophysics, National Research Council of Canada}

\IBVSinst{Dept.\ of Physics and Astronomy, University of Victoria, Victoria, BC, Canada, V8W 3P6; e-mail: robb@uvic.ca}

\IBVSabs{SAO 84309 varies in brightness with an amplitude of 0.1} 
\IBVSabs{magnitude and a period of 19 days.}

\SIMBADobj{SAO 84309}

\IBVStyp{ FK }
\IBVStyp{ BY }
\IBVStyp{ DCEPS }

\IBVSkey{ stars: activity}
\IBVSkey{ stars: pulsating}

\begintext

The study of stars with active regions may further our
understanding of the solar dynamo.  These stars can be discovered
by searching for X-ray sources with periodic photometric
variations. The star SAO 84309 is catalogued as the X-ray source
1RXSJ162013.2+243606 by the ROSAT satellite (Voges et al., 1999).
It is listed in the TYCHO Catalogue (ESA, 1997) as TYC 2047-320-1,
which gives its parallax as $7\pm14$ mas, its V as $9.60\pm.02$,
its B as $10.91\pm.04$ and its ($B-V$) as $1.11\pm.04$.  2MASS
measurements of SAO 84309 reveal that J=7.55, H=6.99 and K=6.83
all with an uncertainty of $\pm0.02$. All these colour
measurements are consistent with a spectral class of K5V$\pm1$ or
K2III$\pm2$.  A K5V star would have a parallax of about 36 mas so
the giant classification is much more likely.

\IBVSfig{6cm}{5449-f1.eps}{Finder chart labelled with the GSC
identification numbers from region 2047. 320 is SAO~84309.}
%%\IBVSfigKey{5449-f1.eps}{SAO 84309}{finding chart}

The observations were made with the University of Victoria automated 0.5m
telescope, VRI filters and Star I CCD, and reduced in a fashion similar to
that described in Robb and Greimel (1999).  The field of stars observed is
shown in Figure 1.

The Julian Dates of observation (--2452800) are 16-19R, 23R, 24R, 26R, 27R,
32R, 37-39VRI, 41-45VRI, 47VRI, 48VRI, and 50-56VRI.  Table 1 lists the
stars' identification numbers, positions and magnitudes from the Hubble
Space Telescope Guide Star Catalogue (GSC) (Jenkner et al., 1990)  All
observations were made using a filters identical to the Johnson V and the
Cousins RI, but were not transformed to the standard system.

Our differential $\Delta R$ and $\Delta (V-I)$ magnitudes are calculated
in the sense of the star minus GSC 2047-262.  Brightness variations during
a night were measured by the standard deviation of the differential
magnitudes and are listed for the most photometric night in the last
column as ``Std Dev Within".  A``Std Dev Within" one night of 0.004 sets
an upper limit on variations of an hourly timescale. For each star the
mean of the nightly means is shown as $\Delta R$ in Table 1.  The standard
deviation of the nightly means is a measure of the night to night
variations and is called ``Std Dev Between" in Table 1. The smallest ``Std
Dev Between" is 0.005 magnitudes. This excellent photometry shows that
night to night variations in GSC 2047-262 must be less than a few
millimagnitudes.

\begin{table} 
\caption{Stars observed in the field of SAO 84309=GSC 2047-320}
\begin{center}
\begin{tabular}{ccccccccc}
\hline
Star & R.A. & Dec. & GSC & $\Delta R$ & Std Dev & Std Dev&$\Delta$&Std Dev\\
GSC Id & J2000&J2000 & Mag.& Mag.      & Between & Within &(V-I)&Between\\

\hline

0320&$16\hr20\mm14\sec$&$24\deg$36\arcm11\arcs&9.3&-1.536&0.045&0.004&0.513&0.063\\
0262&$16\hr20\mm19\sec$&$24\deg$36\arcm31\arcs&10.8&--&--&--&--&--\\
0994&$16\hr20\mm08\sec$&$24\deg$35\arcm53\arcs&14.3&3.891&0.012&0.046&1.462&0.043\\
1177&$16\hr20\mm14\sec$&$24\deg$39\arcm25\arcs&14.7&4.285&0.022&0.078&1.121&0.082\\
0049&$16\hr20\mm20\sec$&$24\deg$38\arcm19\arcs&12.7&2.376&0.005&0.013&0.341&0.007\\
0090&$16\hr20\mm25\sec$&$24\deg$39\arcm08\arcs&13.5&2.648&0.009&0.013&0.433&0.010\\

\hline
\end{tabular}
\end{center}
\end{table}


 
The star SAO 84309 did not vary significantly in brightness during
any night but had obvious variations from night to night.  The
data were fit with sine curves using Period98 (Sperl, 1998), and
two periods were found to be significant; 19.07 days and the first
overtone (9.54 days) with amplitudes of 0.0588 and 0.0175
magnitudes respectively.  Our best estimate of the ephemeris is:

\vspace {3mm}
\centerline{HJD of Maximum Brightness=
2452819\fday8(9)+19\fday1(2)$\times$E.} 
\vspace {3mm}

\noindent where the uncertainty in each final digit is given in
brackets. 
In Figure 2 the differential $\Delta R_C$ magnitudes are
plotted as filled circles with the Period98 fit plotted as a dark line.
The differential V and I light curves are also plotted in Figure 2;
shifted by an arbitrary amount and with line segments connecting the
observations.  Also in Figure 2 we plot the $\Delta$(V-I) showing a
small but significant variation, which is indicative of an effective 
temperature change during the last cycle.
 

SAO 84309 has a light curve which brightens more rapidly than it
fades, so a possibility is that the star is a pulsating star
similar to the small amplitude cepheid variables. The light curve
is almost identical in period, shape and amplitude to V1658 Aql =
HD191616 (Robb et al., 2001). However both stars also have a K
spectral type and are X-ray sources, which is not consistent with
Cepheid variability.

\IBVSfig{10cm}{5449-f2.eps}{Differential VRI and ($V-I$) filtered light curves of SAO 84309.}
%%\IBVSfigKey{5449-f2.eps}{SAO 84309}{light curve}

\vspace*{-0.4cm}
Stars with active regions and/or spots such as BY Dra or FK Comae
stars can have light curves of almost any shape.  Light curve
differences between the two rotations are probably due to the
active regions' area and/or position changes. 

SAO 84309 seems to be a rotating, late type giant star with active
regions covering a significant part of its surface and energizing
a hot corona producing X-rays.  Further spectral observations will
be of interest to see if there is H$\alpha$ emission as found in
V1658~Aql. Photometric observations will be important to tell if
differential rotation will modify the period and/or shape of the
light curve.

\bigskip

{\bf Acknowledgements}

This research has made use of the NASA/ IPAC Infrared Science
Archive, which is operated by the Jet Propulsion Laboratory,
California Institute of Technology, under contract with the
National Aeronautics and Space Administration.


\vskip 5mm
\references

ESA, 1997, {\it The Hipparcos and Tycho Catalogues}, ESA SP-1200

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

Robb, R. M. and Greimel, R., 1999, {\it ASP Conf. Ser.}, {\bf 189}, 198

Robb, R. M., Lindner, J. V., Stern, L. A., Lindner, T. H., Balam, D. D., Clem, 
J. L., 2001, {\it IBVS}, 5211

Sperl, M., 1998, Master's Thesis, Institut f\"ur Astronomie, Universit\"at
Wien

Voges, W., Aschenbach, B., Boller, Th.,
Br\"auninger, H., Briel, U., Burkert, W.,
Dennerl, K., Englhauser, J., Gruber, R.,
Haberl, F., Hartner, G., Hasinger, G.,
K\"urster, M., Pfeffermann, E., Pietsch, W.,
Predehl, P., Rosso, C., Schmitt, J. H. M. M.,
Tr\"umper, J., Zimmermann, H. U.,
1999, {\it A\&A}, {\bf 349}, 389

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