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\IBVShead{4970}{26 October 2000}

\IBVStitle{An SX P\lowercase{he} Star in the Globular Cluster M15}

\IBVSauth{Jeon, Young-Beom$^{1,3}$; Kim, Seung-Lee$^1$; Lee, Ho$^2$; Lee, Myung Gyoon$^3$}

\IBVSinst{Korea Astronomy Observatory, Taejon, 305-348, Korea}
\IBVSinst{Dept.\ of Earth Science Education, Korea National University of Education, Choongbuk, 363-791, Korea}
\IBVSinst{Astronomy Program, SEES, Seoul National University, Seoul, 151-742, Korea}

\SIMBADobj{M15}
\SIMBADobj{Cl* NGC 7078 SXP 1}
\IBVStyp{ SXPHE }
\IBVSkey{SX Phe star, photometry}
\IBVSabs{We present observational results of a newly discovered SX Phe star}
\IBVSabs{in the globular cluster M15.}

\begintext

We present observational results of a newly discovered SX Phe star, SX1
(RA$_{2000} = 21\hr29\mm39\fsec4$, DEC$_{2000} =
12\deg11\arcm43\farcs4$, $V = 18\fmm450$, $B-V = 0\fmm225$, from our
observation), in a field located 102\farcs4 north and  283\farcs5 west
from  the center of  the globular cluster  M15 (21\hr29\mm58\fsec3,
+12\deg10\arcm01\arcs, Djorgovski \& Meylan 1993). We could  not find
this star at  the recent catalogue  of variable stars in globular
clusters (Clement 2000).

Time-series $BV$ CCD photometry was performed over four nights from
August 12th to 16th, 1999.  The observations were done with an SITe
$2048 \times 2048$ CCD camera attached to the 1.8-m telescope at the
Bohyunsan Optical Astronomy Observatory (BOAO). The size of the field
of view of a CCD image is $11\farcm6\times11\farcm6$ at the $f/8$ Cassegrain
focus of the telescope.

Using IRAF/CCDRED package, we processed images to correct overscan
regions, trim sections, subtract bias frames and correct
flat field images.  Instrumental magnitudes were obtained using the
Point Spread Function fitting photometry routine in IRAF/DAOPHOT
package (Stetson 1987, Massey \& Davis 1992).  We applied the ensemble
normalization technique (Gilliland \& Brown 1988, Kim {\it et al.}\
1999) to standardize the instrumental magnitudes of all stars in the
time-series CCD frames.

\begin{table}[b]
\caption{Results of the multiple frequency analysis}
\begin{center}
\begin{tabular}{cccrrcccrr}
\hline
& \multicolumn{4}{c}{$B$-band} & & \multicolumn{4}{c}{$V$-band}  \\
\cline{2-5}\cline{7-10}
\noalign{\vskip2pt}
\strut
& Freq.\ (c/d) & Amp.$^1$ & Phase$^1$ & $S/N$$^2$ & & Freq.\ (c/d) & Amp.$^1$  & Phase$^1$ & $S/N$$^2$ \\
\hline
$f_1$ & 24.626  & 0\fmm054  & 3.67 & 13.7 & & 24.626 & 0\fmm049 & 3.62 & 14.2\\
$f_2$ & 24.350  & 0\fmm031  & $-$1.21 &  6.7 & & 24.353 & 0\fmm025 & 3.63 & 5.9 \\
\cline{2-5}\cline{7-10}
\noalign{\vskip2pt}
s.d.$^3$ & & 0\fmm023 & & & & & 0\fmm020  \\
\hline
\multicolumn{10}l{\normalsize $^1$ $B$ or $V = {\rm constant} + \Sigma_j A_j \cos {2 \pi f_j (t - t_0) + \phi_j},\quad t_0 = {\rm HJD}~2451400$} \\
\multicolumn{10}l{\normalsize $^2$ Amplitude signal to noise ratio introduced by Breger {\it et al.}\ (1993)} \\
\multicolumn{10}l{\normalsize $^3$ Standard deviation after fitting synthetic curves to the data} \\
\end{tabular}
\end{center}
\end{table}

\IBVSfig{10.0cm}{4970-f1.ps}{A greyscale map of a $V$-band CCD image
of the globular cluster M15.  The new SX Phe star SX1 is denoted by `V' in
the center of the small circle.}

\IBVSfig{8.0cm}{4970-f2.ps}{Light curves of SX1, $B$-band (left) and
$V$-band (right). Synthetic curves obtained from the multiple frequency
analysis (see Table~1) are superimposed to the data.}

\IBVSfig{10.0cm}{4970-f3.ps}{Power spectra of SX1 for the $B$-band
(left) and $V$-band (right). Window spectra are in the top panel.  Two
closely-separated frequencies, $f_1$ and $f_2$, are obviously found.}

\IBVSfig{8.0cm}{4970-f4.ps}{$V$ amplitude versus period diagram:
star symbol for the new SX Phe star SX1, small dots for $\delta$ Sct
stars (Rodr\'{\i}guez {\it et al.}\ 2000), open triangles for field SX
Phe stars (Rodr\'{\i}guez {\it et al.}\ 2000) and open circles for SX
Phe stars in other globular clusters (Rodr\'{\i}guez \& Lopez-Gonzalez
2000).}

\IBVSfig{10.0cm}{4970-f5.ps}{ Position of SX1 in the color-magnitude
diagram of the globular cluster M15.  Note that it is located at the
blue straggler region.}

\vskip-55pt
A finding chart of SX1 is shown in Figure 1.  Light curves of SX1 are
displayed in Figure~2, which shows amplitude modulating features
implying the excitation of closely-separated pulsating frequencies.  We
have obtained power spectra for SX1 from the multiple frequency
analysis (Kim \& Lee 1996), as shown in Figure 3. Table 1 lists the
results of the analysis.  Considering the position in the
period--amplitude diagram (Figure~4) and the color--magnitude diagram
(Figure~5), SX1 might be identified as an SX Phe star and/or a
pulsating blue straggler star in the cluster.

Detailed analysis of color variations, pulsation modes and the
period--luminosity relation will be given elsewhere.



\references

Breger, M., Stich, J., Garrido, R., et al., 1993, {\it A\&A}, {\bf 271}, 482
 
Clement, C.M., 2000, {\it Catalogue of variable stars in globular clusters} \\
(http://www.astro.utoronto.ca/\~{}cclement/papers.html)

Gilliland, R.L., Brown, T.M., 1988, {\it PASP}, {\bf 100}, 754

Kim, S.-L., Lee, S.-W., 1996, {\it A\&A}, {\bf 310}, 831

Djorgovski, S., \& Meylan, G. 1993, {\it ASP Conference Series}, {\bf 50}, 325, in Structure and Dynamics of Globular Clusters, eds. S. Djorgovski \& G. Meylan
%Changed: Fri Jun  3 09:56:08 MEST 2005

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

Massey, P., Davis, L.E., 1992, {\it A User's Guide to Stellar CCD photometry with IRAF}

Rodr\'{\i}guez, E., Lopez-Gonzalez, M.J., 2000, {\it A\&A}, {\bf 359}, 597

Rodr\'{\i}guez, E., Lopez-Gonzalez, M.J., L\'opez de Coca, P., 2000, {\it A\&AS}, {\bf 144}, 469

Stetson, P., 1987, {\it PASP}, {\bf 99}, 191 


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