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\begin{document} 

\IBVShead{5879}{4 March 2009} 

\IBVStitletl{New Photometry of Blue Stragglers}{in Four Galactic Open Clusters}

\IBVSauth{Ta\c{s}, G\"{u}nay; Evren, Serdar; \"{O}zdarcan, Orkun; Dal, Hasan A.; S\.{i}pah\.{i}, Es\.{i}n}

\IBVSinsto{Ege University Observatory, 35100 Bornova - \.{I}zmir - TURKEY} 

\SIMBADobjAlias{IC 4725}{M25}
\SIMBADobj{Melotte 111}
\SIMBADobj{NGC 1342}
\SIMBADobj{NGC 6871}
\WEBDAobj{IC 4725}
\WEBDAobj{Melotte 111}
\WEBDAobj{NGC 1342}
\WEBDAobj{NGC 6871}
\IBVStyp{ galactic open cluster, blue straggler }
\IBVSkey{photometry} 

\begintext 
Blue straggler stars are very interesting subjects according to the stellar evolution theory. The appearance of the stars is not expected
on the main-sequence and on the bluer side of the cluster turn-off point according to the standard evolution theory. Although there are some scenarios %%@
that have tried to clarify their locations on the H-R diagram, their origins are still considered as uncertain. In this context, the variation %%@
mechanisms of blue stragglers can provide a clue to explain their existence and to derive their physical parameters. Hrivnak (1977), Lapasset and Ahumada %%@
(1996), Mateo (1993), Maitzen et al. (1981) can be given as example studies to such attempts. The locations of stars in color-magnitude and %%@
color-color diagrams are significant to determine the photometrical cluster membership probability of the stars but a more accurate photometry is %%@
needed. Hence, we initiated an observation program for blue straggler stars of four galactic open clusters. The observations were carried out with a %%@
48-cm Cassegrain telescope equipped with a three-channel photometer, using broad-band Johnson $UBV$ filters, at Ege University Observatory during the %%@
2006-2007 observing season. The blue straggler stars, which catalogued in WEBDA database for four open clusters, namely IC 4725, Melotte 111, NGC %%@
1342 and NGC 6871, were observed with the comparison stars chosen from same clusters. The observations were reduced from the effects of %%@
atmospheric extinction; and then heliocentric corrections to the observing times were applied. The standard stars with appropriate color and %%@
brightness, taken from the lists of Landolt (1992) and Harmanec et al. (1994), were used for transformation to the standard system. The galactic and %%@
equatorial coordinates (from SIMBAD) and $E(B-V)$ color excesses (from WEBDA) of the four open clusters can be seen in Table 1.

\vskip 0.5cm

\centerline{Table 1. Coordinates and $B-V$ color excesses of the four open clusters.}
\begin{center}
\begin{minipage}{13.5cm}
%\begin{table}
\begin{tabular}{cccccc}
\hline
\multicolumn{1}{c}{ } & \multicolumn{2}{c}{Galactic coordinates} & \multicolumn{2}{c}{Equatorial coordinates} & \multicolumn{1}{c}{} \\
%\cline{1-6}
\hline
Cluster & \em l    & \em b  & RA (2000) & DEC (2000)& {$E(B-V)$} \\
        &    (\deg)            &     (\deg)         & ~(h~~m)          &~~~(\deg~~~\arcm)& (mag) \\
\hline
IC 4725     & 13.69  & $-$04.42 & 18 32 & $-$19 07 & 0.476\\
Melotte 111 & 222.51 & $+$83.40 & 12 22 & $+$25 50 & 0.013\\
NGC 1342    & 155.19 & $-$15.12 & 03 33 & $+$37 25 & 0.319\\
NGC 6871    & 72.65  & $+$02.05 & 20 06 & $+$35 47 & 0.443\\
\hline
\end{tabular}
%\end{table}
\end{minipage}
\end{center}
\vskip0.35cm

We determined the interstellar reddening in the line of sight by the color-color diagram. The estimations of color excesses in $U-B$ and $B-V$ were made %%@
by using the sliding fit method and adopting the slope of the reddening line as 0.72. The results for both blue stragglers and the comparison stars %%@
are listed in Table 2. 'BS' in the fourth column (Star type) denotes blue straggler candidates listed in the WEBDA database.

\vskip 0.5cm

\centerline{Table 2. New photoelectric photometry of blue stragglers and comparison stars} \centerline{in four galactic open clusters.} 
\small
\begin{center}
\begin{tabular}{lcrcccccc}
\hline
Cluster	&	WEBDA	&	HD/BD	&	Star	&	$V$	&	$B-V$	&	$U-B$	&	$E(B-V)$	&	$E(U-B)$	\\
	&	 No.	&	 No.	&	 type	&	(mag)	&	(mag)	&	(mag)	&	(mag)	&	(mag)	\\
\hline	&		&			&		&		&		&		&		&		\\
NGC1342	&	1	&	21773	&		&	8.490 (6)	&	0.416 (10)	&	 -0.133 (13)	&	0.59	&	0.43	\\
NGC1342	&	2	&	21728	&	BS	&	8.815 (5)	&	0.235 (10)	&	 -0.142 (13)	&	0.35	&	0.26	\\
NGC1342	&	3	&	21785	&	BS	&	9.273 (5)	&	0.247 ~(8)	&	0.141 (10)		&	0.27	&	0.20	\\
NGC1342	&	4	&	275501	&	 	&	9.362 (6)	&	1.214 (11)	&	1.108 (16)		&	0.34	&	0.25	\\
NGC1342	&	6	&	275509	&		&	9.652 (8)	&	1.109 (12)	&	0.895 (20)		&	0.28	&	0.20	\\
NGC1342	&	7	&	275507	&		&	10.160 (6)	&	1.123 (11)	&	0.824 (19)		&	0.34	&	0.24	\\
NGC1342	&	8	&	275510	&		&	10.356 (5)	&	0.324 (10)	&	0.203 (17)		&	0.35	&	0.26	\\
NGC1342	&	10	&	275502	&		&	10.719 (7)	&	0.430 (11)	&	0.331 (17)		&	0.29	&	0.21	\\
NGC1342	&	11	&	275508	&		&	10.669 (7)	&	0.390 ~(9)	&	0.224 (11)		&	0.20	&	0.14	\\
NGC1342	&		&	275495	&		&	9.364 (5)	&	0.446 ~(9)	&	 -0.070 (10)	&	0.60	&	0.44	\\
\hline	&		&			&		&		&		&		&		&		\\
NGC 6871&	4	&	190864	&	BS	&	7.764 (4)	&	0.155 ~(9)	&	 -0.809 (10)	&	0.48	&	0.34	\\
NGC 6871&	5	&	227634	&	BS	&	7.923 (9)	&	0.203 (14)	&	 -0.676 (11)	&	0.49 	&	0.35	\\
NGC 6871&	7	&	227586	&	BS	&	8.803 (6)	&	0.158 (11)	&	 -0.694 (14)	&	0.45	&	0.32	\\
NGC 6871&	8	& +35$\deg$ 3956	&	BS	&	8.833 (6)	&	0.165 (15)	&	 -0.627  (18)	&	0.43	&	0.31	\\
NGC 6871&	1163	&	227767	&		&	8.871 (3)	&	0.016 (8)	&	 -0.816 (11)	&	0.31	&	0.22	\\
NGC 6871&	1866	&	191201	&		&	7.257 (4)	&	0.110 (9)	&	 -0.808 (10)	&	0.43	&	0.31	\\
\hline	&		&		&		&		&		&		&		&		\\
Melotte 111	&	89	&	107655	&		&	6.243 (6)	&	0.009 (10)	&	 -0.010 (11)		&	0.04	&	0.03	\\
Melotte 111	&	136	&	108449	&		&	8.271 (7)	&	0.263 (11)	&	0.043 (16)			&	0.34	&	0.24	\\
Melotte 111	&	139	&	108486	&		&	6.684 (3)	&	0.170 ~(9)	&	0.101 (12)			&	0.20	&	0.14	\\
Melotte 111	&	146	&	108662	&	BS	&	5.311 (3)	&	 -0.020 ~(8)	&	 -0.150 (10)	&	0.05	&	0.03	\\
\hline	&		&		&		&		&		&		&		&		\\
IC 4725	&	6	&	-19$\deg$ 5032	&		&	9.553~~(7)	&	0.192 (11)	&	 -0.140 (12)	&	0.31	&	0.22	\\
IC 4725	&	50	&	170682	&	BS	&	7.907~~(8)	&	0.290 (19)	&	 -0.071 (23)	&	0.41	&	0.29	\\
IC 4725	&	91	&	170719	&	BS	&	8.060 (12)	&	0.254 (20)	&	 -0.056 (22)	&	0.36	&	0.26	\\
IC 4725	&	97	&	 -19$\deg$ 5044	&	BS	&	8.724 (15)	&	0.320 (22)	&	 -0.056 (21)	&	0.44	&	0.32	\\
IC 4725	&	163	&	170835	&	BS	&	8.765 (11)	&	0.200 (16)	&	 -0.480 (18)	&	0.43	&	0.31	\\
IC 4725	&	167	&	170836	&	BS	&	8.952 (12)	&	0.267 (19)	&	 -0.120 (24)	&	0.40	&	0.29	\\
IC 4725	&	233	&	170763	&		&	8.948~~(5)	&	0.261~~(8)	&	 -0.028 (10)	&	0.36	&	0.26	\\
\hline																	
\end{tabular}
\end{center}

\IBVSetable{5879-t2.tex}
\IBVSecomment{Table 2. with WEBDA links}

\bigskip
\large

Ahumada and Lapasset (2007) suggested new criteria to select the blue straggler stars in published data in WEBDA database; and accordingly, they %%@
excluded the stars NGC 1342 1 and 10. Both stars are also listed in Table 2 to present their new photometry. On the other hand, NGC 1342 4 (HD 275501) %%@
is tabulated by its photometric values, using Ahumada and Lapasset (2007)'s study, as $V = 9\fmm21$, $B-V = 0\fmm24$, $U-B = 0\fmm17$. The %%@
literature related to this star (Hoag et al. 1961, Jennens and Helfer 1975, Francic 1989, Svolopoulos 1961, Bersier 1996, Pena et al. 1994) and our %%@
observations showed that HD 275501 is a late type star. It is thought that there is a misprint; and the values mentioned above for this star were %%@
similar to those of NGC 1342 3. NGC 1342 3 is believed to be wrongly labeled as NGC 1342 4 in the catalogue of Ahumada and Lapasset (2007) since its %%@
location on the H-R diagram was more appropriate to be a blue straggler candidate. Thus, NGC 1342 3 was labeled as blue straggler star in Table 2. The %%@
means of the color excesses listed in Table 2 were calculated for each cluster and then adopted as cluster's color excess. NGC 1342 1, on the other %%@
hand, was excluded since its color excesses have a very different value than other blue stragglers in the same cluster. This can be a photometric evidence %%@
for NGC 1342 1 that it may not be a member of the cluster. Consequently, we determined $E(B-V)$ reddenings of 0.40, 0.33, 0.46, and 0.05 mag for the %%@
clusters IC 4725, NGC 1342, NGC 6871, and Melotte 111, respectively. 

The photometry of stars listed in Table 2 were checked for variability. As for the long-term observations of the blue %%@
stragglers included in this study; we detected that one of them, namely HD 21728, showed light variations.

Although HD 21728 has been classified as a chemically peculiar star (e.g. Svolopoulos 1961, Young and Martin 1973, Abt 1985, Renson 1992), its %%@
photometric variation was not identified up to now. We found a light variation with a period of 5.340 days, using the Phase Dispersion Minimization %%@
Technique of PERANSO period analysis program, for the star. The amplitude of the variation was 0.04 and 0.05 mag in $B$ and $V$ filters, %%@
respectively. We calculated the phases of the light and color curves presented in Figure 1 using the following light elements:

%\begin{center}
$$\mathrm{HJD} = 2\,454\,048.2562 + 5\fday340 (28) \times~E,$$
%\end{center}

\noindent where T$_{0}$ is the time of first observation point and P is the period determined from period analysis of $V$ data. The star's %%@
light curve has asymmetric shape, while its color curve does not establish a clear variation as can be seen from Figure 1. It is known that the light %%@
curve shapes of the chemically peculiar stars can change in different filters (e.g. Maitzen 1989, Adelman 1997). The determination of physical %%@
parameters of blue stragglers is significant to test their formation scenarios and to comprehend their origin. Consequently, their photometric or %%@
spectral variability is a fairly useful tool. The magnetic peculiar stars, which are member of cluster, are also very valuable objects to study their %%@
evolution (North et al. 2008). In this context, additional multicolor broad and intermediate band photometry of HD 21728 is considered as necessary %%@
for future studies. 

\IBVSfig{5.7cm}{5879-f1.eps}{The first light and color curves of HD 21728.}
\IBVSfigKey{5879-f1.eps}{HD 21728}{light curve}
\vskip -0.65 cm

The authors would like to thank to Ege \"{U}niversitesi, Ara\c{s}t{\i}rma Fon Saymanl{\i}\u{g}{\i} for supporting this study with the Project No. 2005 %%@
/ FEN / 050.

\IBVSedata{5879-t3.txt}
\IBVSdataKey{5879-t3.txt}{HD21728}{photometry}
\IBVSecomment{time-series photometry}

\IBVSedata{5879-t4.txt}
\IBVSdataKey{5879-t4.txt}{HD21728}{photometry}
\IBVSecomment{time-series photometry}


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\end{document}
