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\IBVShead{5037}{7 March 2001}

\IBVStitle{Variable BSS Candidates in M\,3 Proved to be Quasars}


\IBVSauth{Meusinger, H.$^1$; Scholz, R.-D.$^2$; Irwin, M.$^3$}

\IBVSinst{
Th\"uringer Landessternwarte Tautenburg, 07778 Tautenburg, 
Germany, e-mail: meus@tls-tautenburg.de}
\IBVSinst{
Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, 
Germany,\\ \indent e-mail: rdscholz@aip.de}
\IBVSinst{
Insitute of Astronomy, Madingley Road, Cambridge CB3\,1HA, UK,
e-mail: mike@ast.cam.ac.uk}

\SIMBADobj{NSV6388} 
\SIMBADobj{RX J1342.5+2837} 
\SIMBADobj{[HB93] 1340+287}

\NEDobj{NSV6388}
\NEDobj{RX J1342.5+2837}
\NEDobj{FBQS J1342+2828}

\begintext

The discovery of eclipsing binaries among blue straggler stars 
(BSS) is important to check ideas for the origin of the 
BSS phenomenon (binary merging versus collisional merging, 
e.g.\ Bailyn 1995) and, moreover, for the understanding of the 
dynamical evolution of dense stellar systems (Hut et al.\ 1992). 
In M\,3, the radial BSS distribution seems to be consistent with a
collisional origin in the centre and binary
origin in the outskirts (Bailyn \& Pinsonneault 1995).

\IBVSfig{10cm}{5037-f1.ps}{
Colour--magnitude diagram of M\,3. Strongly variable BSS candidates are
marked by octagons, other strongly variable stars detected by the VPMS 
are indicated by open squares}

In the framework of a variability--proper motion survey (VPMS) for
quasars in the field centred on M\,3 (Scholz et al.\ 1997), we detected
three significantly variable star-like objects in the cluster halo
region having typical BBS colours and magnitudes. Figure~1 shows the
colour--magnitude diagram (CMD) for all star-like objects with a high
cluster membership probability which have: (1)~cluster-centric
distances $5\arcm \le r \le 24\arcm$ (the innermost region is excluded
to minimize effects of image crowding), (2)~no nearby neighbour within
a $7\arcs$ distance, (3)~proper motion vectors close to the centroid of
M\,3 (defined by stars selected by the first two constraints and having
colour indices compatible with the cluster CMD), i.e.\
$|\vec{\mu}-\vec{\mu}_{\rm M3} | \le 6$~mas\,yr$^{-1}$. Photometric
and astrometric data were derived from Tautenburg Schmidt plates
digitized by means of the APM facility, Cambridge. Johnson $B$
magnitudes were measured on 57 plates, taken in the years 1964 to
1994. A further 6 $V$ plates were used to obtain $B-V$ colour
indices. Proper motions were derived from 81 plates altogether with a
baseline of three decades. Variability was quantified by means of a
simple yet powerful variability index $I_{\rm var}$ (for details, see
Scholz et al.\ 1997). The three BSS candidates marked in Fig.~1 and
listed in Table~1 proved to be variable on a significance level larger
than 99.99\%.

\begin{table}[h]
\caption{Basic data for three variable BSS candidates in M\,3}
\begin{center}
\begin{tabular}{lrrr}
\hline
VPMS \# & 12\,006 & 12\,465 & 15\,224 \\
R.A. (J2000.0) & 13\hr41\mm07\fsec5 & 13\hr42\mm30\fsec9 & 13\hr42\mm54\fsec5 \\
DEC (J2000.0) & +28\deg39\arcm36\arcs & +28\deg37\arcm25\arcs & +28\deg28\arcm06\arcs \\
$B$ & 18.9 & 18.1 & 18.4 \\
$B-V$ & 0.40 & 0.28 & 0.35 \\
$I_{\rm var}$ & 2.8 & 3.4 & 2.3 \\
SIMBAD identifiers & NSV\,6388 & RX\,J1342.5+2837 & [HB93]\,1340+287 \\
spectroscopic identification & QSO ($z=1.580$) & QSO ($z=1.272$) & QSO ($z=1.037$) \\
\hline
\end{tabular}
\end{center}
\end{table}

Follow-up low-resolution spectroscopy with CAFOS at the 2.2-m telescope
of the German--Spanish Astronomical Centre on Calar Alto surprisingly
revealed that all three variable BSS candidates are quasars with
redshifts $z\approx 1$--1.5 (Table~1). Their absolute proper
motions are consistent with both the cluster proper motion and the
zero-proper motion constraint for quasars due to the small absolute
proper motion of M\,3. Of course, [HB93] 1340+287 was catalogued as a
quasar long before (Hewitt \& Burbidge 1989); our observations have
just confirmed the earlier redshift measurement.

We conclude that samples of {\it variable\/} BSS candidates in the
outer regions of high-latitude globular clusters can be substantially
contaminated by low-redshift quasars.

\references

Bailyn, C.D., 1995, {\it ARA\&A}, {\bf 33}, 133

Bailyn, C.D., Pinsonneault, M.C., 1995, {\it ApJ}, {\bf 439}, 705

Hewitt, A., Burbidge, G., 1989, {\it ApJS}, {\bf 69}, 1

Hut, P., McMillan, S., Goodman, J., Mateo, M., Pryor, C., {\it et al.},
1992, {\it PASP}, {\bf 104}, 981

Scholz, R.-D., Meusinger, H., Irwin, M., 1997, {\it A\&A}, {\bf 325}, 457

\endreferences

\end{document}

