\documentstyle[twoside, ibvs2, epsf]{article}

\begin{document}
\IBVShead{4964}{19 October 2000}

\IBVStitle{Three Delta Scuti Stars in the Old Open Cluster NGC\,2506}

\IBVSauth{Kim, Seung-Lee; Chun, Moo-Young}

\IBVSinsto{Korea Astronomy Observatory, Taejon, 305-348, Korea (e-mail: slkim@kao.re.kr, mychun@boao.re.kr)}

\SIMBADobj{NGC 2506}
\SIMBADobj{Cl* NGC 2506 KC V1}
\SIMBADobj{Cl* NGC 2506 KC V2}
\SIMBADobj{Cl* NGC 2506 KC V3}
\WEBDAobj{NGC 2506}
\IBVStyp{ DSCT }
\IBVSkey{delta Scuti star, photometry}
\IBVSabs{We present observational results of three new delta Scuti type}
\IBVSabs{variable stars in a field of the old open cluster NGC 2506.}

\begintext

During our observational survey to search for low-amplitude pulsating
stars (e.g., $\delta$ Scuti stars and $\gamma$ Dor stars) in open
clusters, we discovered three new $\delta$ Scuti type variable stars in
a field of the old open cluster NGC 2506.  Kaluzny \& Shara (1988) also
monitored stars in a wider area of the cluster than our observation
but found no variable stars, probably due to their limited data sets
(only nine exposures for three nights).

Time-series $V$ CCD photometry was performed for ten nights from
December 11th, 1996 to March 5th, 1997.  The observations were done
with a SITe 1024 CCD camera attached to the 1.8-m telescope at
Bohyunsan Optical Astronomy Observatory (BOAO). Field of view of a CCD
image is $5\farcm8\times5\farcm8$ at $f/8$ Cassegrain focus of the
telescope.  Typical photometric seeing was less than 2\farcs0.  We also
carried out UBVI photometry to study physical properties of variable
stars in the cluster (Kim {\it et al.}\ 2000 for detailed results).

Data analyses such as CCD image process, PSF (Point Spread Function)
photometry and the ensemble normalization, have been carried out by the
methods described by Kim {\it et al.}\ (1999).  We examined light
variations of 590 stars in the observed field using 304 CCD frames and
found three new pulsating stars. Finding chart of these new pulsating
stars is shown in Figure~1.

Power spectra and light curves are displayed in Figures~2 and~3,
respectively.  Each pulsating star has a dominant frequency.  Three
pulsating stars are located within the $\delta$~Scuti instability strip
in the color-magnitude diagram (Figure~4).  It should be noted that V1,
a probable cluster member deduced from the membership probability
(Mermilliod 1992), is located at a bluer and brighter region than the
main-sequence turn-off point of the cluster\,;\, i.e., it might be
identified as a pulsating blue straggler star.

Observational parameters of the three new $\delta$ Scuti stars are
summarized in Table~1. Two $B$ and $V$ deep-exposured data were used to estimate the color index. Photometric errors are less than 0\fmm01.

\begin{center}
{\normalsize Table 1: Observational parameters of the new $\delta$ Scuti type variable stars}
\vskip2mm
\begin{tabular}{@{}cccccccccc@{}}
\hline
\noalign{\vskip1pt}
& ID$^1$ & RA${_{2000}}^1$ & DEC${_{2000}}^1$ & $\overline{V^2}$ & $\overline{B-V^2}$ & Period & Epoch$^3$ & $\Delta V$ & ${P_\mu}^1$ \\
\hline
V1 & 5462 & 7\hr59\mm58\fsec1 & $-$10\deg45\arcm55\arcs & 13\fmm69 & 0\fmm21 & 0\fday0678 & 431.242 & $\sim 0\fmm03$ & 0.71 \\ 
V2 & 5467 & 7\hr59\mm53\fsec6 & $-$10\deg45\arcm49\arcs & 14\fmm50 & 0\fmm29 & 0\fday0921 & 430.209 & $\sim 0\fmm17$ & 0.91 \\ 
V3 & 5589 & 7\hr59\mm57\fsec8 & $-$10\deg47\arcm21\arcs & 14\fmm75 & 0\fmm35 & 0\fday0815 & 513.082 & $\sim 0\fmm11$ & 0.00 \\ 
\hline
\end{tabular}

\normalsize
\begin{tabular}{l}
$^1$ Identification, coordinates and membership probability (Mermilliod 1992) \\
$^2$ Photometric data from Kim {\it et al.}\ (2000) \\
$^3$ Epoch at maximum brightness (H.J.D.${} - 2450000.0$) \\
\end{tabular}
\end{center}

\IBVSetable{4964-t1.tex}
\IBVSecomment{Elecronic version of the table with WEBDA links}

\IBVSfig{8cm}{4964-f1.ps}{Observed CCD field
($5\farcm8\times5\farcm8$) of the open cluster NGC 2506. Three
variable stars discovered in this study are represented by open
circles. North is up and east is to the left}

\IBVSfig{12cm}{4964-f2.ps}{Power spectra of the three variable stars.
The spectral window is shown in the top panel. The dominant frequency
is 14.742 c/d for V1, 10.854 c/d for V2 and 12.264 c/d for V3}

\IBVSfig{9.0cm}{4964-f3.ps}{Light curves of the three pulsating stars. Data
points are differently marked for each observing night. Typical
observation errors are represented by error bars}

\IBVSfig{7.0cm}{4964-f4.ps}{Positions of pulsating stars in the
color--magnitude diagram of NGC 2506, using photometric data by Kim {\it
et al.}\ (2000). Thick and thin lines denote an empirical ZAMS (Sung \&
Bessell 1999) and a theoretical isochrone (Girardi {\it et al.}\ 2000),
respectively.  Thick solid bars, nearly perpendicular to the ZAMS,
represent $\delta$ Scuti instability strip (Breger 1979).  The
pulsating stars are marked by star symbols. We adopted the reddening of
$E(B-V) = 0.04$, distance modulus of $(V-M_V)_\circ = 12.5$ and age
of $\log t = 9.25$ (Kim {\it et al.}\ 2000)}

\eject 

\references

Breger, M., 1979, {\it PASP}, {\bf 91}, 5

Girardi, L., Bressan, A., Bertelli, G., Chiosi, C., 2000, {\it A\&Ap Suppl.}, {\bf 141}, 371

Kaluzny, J., Shara, M.M., 1988, {\it AJ}, {\bf 95}, 785

Kim, S.-L., Park, B.-G., Chun, M.-Y., 1999, {\it A\&Ap}, {\bf 348}, 795

Kim, S.-L., Chun, M.-Y., Kim, S.-C., {\it et al.}, 2000, {\it JKAS}, to be submitted

Mermilliod, J.C., 1992, in {\it Open cluster data base, BDA} (http://obswww.unige.ch/webda)

Sung, H., Bessell, M.S., 1999, {\it MNRAS}, {\bf 306}, 361

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