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\IBVShead{6144}{10 June 2015}

\IBVStitle{Search for variables in the open cluster King 12}

\IBVSauth{Paunzen, E.$^{1}$; Netopil, M.$^1$; Rode-Paunzen, M.$^2$; Bozic, H.$^3$}

\IBVSinst{Department of Theoretical Physics and Astrophysics, Masaryk University,
Kotl\'a\v{r}sk{\'a} 2, 611~37 Brno, Czech Republic}
\IBVSinst{Institut f{\"u}r Astrophysik der Universit{\"a}t Wien, 
T{\"u}rkenschanzstr. 17, A-1180 Wien, Austria}
\IBVSinst{Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kaciceva 26, HR-10000 Zagreb, Croatia}

\SIMBADobj{King 12}
\WEBDAobj{King 12}

\IBVSkey{variables, CCD photometry}

\IBVSabs{We present the time series analysis of CCD }
\IBVSabs{photometry from the 1 m telescope at the Hvar }
\IBVSabs{Observatory (Croatia) for 54 stars in the area }
\IBVSabs{of the young open clusters King 12. We found no}
\IBVSabs{new variable but list upper detection limits.}

\begintext

Open clusters are physically related groups of stars held together by mutual 
gravitational attraction. Therefore, these populate a limited region of space, 
typically much smaller than their distance from the Sun, so that the members are all 
approximately at the same distance. They are believed to originate from large cosmic gas and 
dust clouds, and to continue to orbit the Milky Way through the disk. 
Also, as all the stars in a cluster are formed from the same diffuse nebula, 
they are all of similar initial chemical composition.
In many clouds visible as bright diffuse nebulae, star formation 
still takes place, so that we can observe the birth of new young 
star clusters (Massi et al. 2015).

Variable stars play an important role in stellar astrophysics. They offer, in general,
the only possibility to look inside stars (asteroseismology), and they represent an 
important way to measure distances (Zejda et al. 2012). 

We searched for variables in the central region of the young open cluster
King 12 (C 2350+616) which, to our knowledge, was never done, up to now. This cluster
is located in the Galactic disk ($l=116\fdeg121 $ and $b=-0\fdeg151$) at a distance of about 2.9\,kpc from the Sun 
(Lata et al. 2014). The most interesting fact is that it still contains pre-main sequence stars at an age of  
approximately 12\,Myr (Davidge 2012). 

The observations were performed in August and September 2010 at the Hvar Observatory, University of
Zagreb, using the 1 m Austrian-Croatian Telescope (ACT). The telescope was equipped with a Apogee 
Alta U47 CCD camera of $1024\times1024$ pixels, resulting in a field-of-view of about 3\arcm~square.
The integration times for the observations in the Bessell $I$ filter system were set either to 20 or 45 s, 
depending on the weather conditions. In total, 674 frames of about 500 minutes of photometry in six nights
were secured. After the basic CCD reductions (bias-subtraction and flat fielding),
we applied point spread function (PSF) photometry within IRAF. The further reduction steps were performed 
using the standard technique for time series CCD observations. The complete observation log is listed in Table 1.

All differential light curves were examined in more detail using the Phase Dispersion Minimization method (PDM, Stellingwerf 1978)
within the software Peranso\footnote{\tt http://www.peranso.com/}. An analysis with a discrete Fourier algorithm
gave the same noise level over the searched frequency range as PDM. 

Primarily, we were searching for variations on times scales of a few hours (typical $\delta$~Scuti type pulsation) but
also for very high amplitude variations on the time scales of weeks and months. For the latter, we would not
be able to retrieve the exact periods put to sort out good candidates. In total, we investigated 54 stars in the 
central part of King~12.   

Figure 1 shows the upper limits of non-variability for the investigated stars. The
dependency of them on the apparent magnitude is caused by the photon noise. The detection limits is about 8\,mmag
for the brightest stars and decreases to about 0.04\,mag for the faintest ones. We found no star
which shows a statistically significant peak in its frequency spectrum in the range from 12 h down to
20 min. Of course, we are not able to exclude variability on other time scales and/or with lower amplitudes
than reported here.

\begin{table}[!t]
\centerline{\bf Table 1: Observation log.}
\begin{center}
\small
\begin{tabular}{ccc|ccc}\hline
JD(start) & JD(end)   & $N$ & JD(start) & JD(end)   & $N$ \\
2455400+  & 2455400+  &     & 2455400+  & 2455400+  &     \\
\hline
20.464288 & 20.620203 & 266 & 60.381684 & 60.405388 & 33 \\
21.554554 & 21.620735 & 115 & 63.424560 & 63.447975 & 82 \\
26.545666 & 26.611418 & 118 & 67.348276 & 67.363623 & 60 \\
\hline
\end{tabular}
\normalsize
\end{center}
\end{table}

\IBVSfig{10cm}{6144-f1.eps}{Upper limits of variability of stars in the field of King 12.}
\IBVSfigKey{6144-f1.eps}{King 12}{other}

{\bf Acknowledgements:} This project is financed by the SoMoPro II programme \linebreak (3SGA5916). The research leading
to these results has acquired a financial grant from the People Programme
(Marie Curie action) of the Seventh Framework Programme of EU according to the REA Grant
Agreement No. 291782. The research is further co-financed by the South-Moravian Region. 
It was also supported by the grants 7AMB14AT015, 14-26115P, and 
the financial contributions of the Austrian Agency for International 
Cooperation in Education and Research (BG-03/2013 and CZ-09/2014). 
HB acknowledges financial support by Croatian Science Foundation under
the project 6212 ``Solar and Stellar Variability''.
This research has made use of the WEBDA database, operated at the Department of 
Theoretical Physics and Astrophysics of the Masaryk University.

\references

Davidge, T. J. 2012, {\it ApJ}, {\bf 761}, 155   

Lata, S., Pandey, A. K., Sharma, S., Bonatto, C., Yadav, R. K. 2014, {\it New Astronomy}, {\bf 26}, 77

Massi, F., Giannetti, A., Di Carlo, E., Brand, J., Beltr{\'a}n, M. T., Marconi, G. 2015, {\it A\&A}, {\bf 573}, A95

Stellingwerf, R. F. 1978, {\it ApJ}, {\bf 224}, 953

Zejda, M., Paunzen, E., Baumann, B., Mikul{\'a}{\v s}ek, Z., \& Li{\v s}ka, J. 2012, {\it A\&A}, {\bf 548}, A97

\endreferences

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