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\IBVSheadDOI{62}{6194}{10 January 2017}

\IBVStitletl{Search for variability of five central stars}{ of planetary nebulae}

\begin{center}
\IBVSauth{Paunzen, E.$^{1}$; Netopil, M.$^1$; Rode-Paunzen, M.$^2$}
\end{center}

\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}

\SIMBADobj{Kronberger PN J1944.9+2245}
\SIMBADobj{NGC 6853}
\SIMBADobj{NGC 7008}
\SIMBADobj{NGC 7076}
\SIMBADobj{NGC 7354}

\IBVSkey{binary, CCD photometry, Planetary Nebula}

\IBVStyp{ PN }

\IBVSabs{The central stars of planetary nebulae are interesting objects}
\IBVSabs{which are important to understand the very late stages of stellar}
\IBVSabs{evolution. They exhibit variability due to binarity, stellar winds,}
\IBVSabs{and pulsation. We searched for variability in five of}
\IBVSabs{such objects and found only upper limits. However, also this null}
\IBVSabs{result is important for the understanding of these stars.}


\begintext

\section*{Introduction}

Planetary nebulae (PN) are the next-to-last stages of evolution of stars with main sequence masses between 0.8 and
8\,$M_{\odot}$. As the core slowly collapses, the outer layers are ejected forming the typical
nebula-like structures. 
Nowadays it is beyond doubt that there is a connection between binary central stars and PN shaping 
(Hillwig et al. 2016). However, due to projection effects, even a spherical symmetric shape does
not imply an apparent single star nature. It is therefore important to search for signs of binarity
using photometric time series. One would either expect classical eclipses 
or reflection effects due to the orbital motion of the components. 

Another type of variability is attributed to variations in the stellar wind, but also
pulsation of some objects has been suggested as a possible explanation (Handler et al. 1997).
These variations have been found in all type of PN central stars. For example,
HD 35914, the variable central star of the PN IC 418 shows two distinct kinds of variability: 
irregular light modulation with a time scale of days (amplitude of about 0.3\,mag), as well as cyclic variations
with a time scale of hours (0.02\,mag). 

De Marco et al. (2008) summarized the back then known variability properties of PNe (Figure 4 therein) including
all above described mechanisms. The observed periods range up to three days with amplitudes up to two magnitudes.

Here, we present photometric observations of five central stars of PNe, Kronberger PN J1944.9+2245, NGC 6853, 
NGC 7008, NGC 7076, and NGC 7354 in order to
search for variability on time scale from hours to days. We were not able to detect any signs of variability
with upper limits between 0.01 and 0.12\,mag, respectively.

\section*{Observations and data reduction}

The observations were performed in August and September 2010 at the Hvar Observatory, University of
Zagreb (Croatia), using the 1\,m Austrian-Croatian Telescope (ACT). The telescope was equipped with an Apogee 
Alta U47 CCD camera of 1024$\times$1024 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 between 45 and 120 s, 
depending on the weather conditions and the brightness of the target. After the basic CCD reductions 
(bias-subtraction and flat fielding), we applied aperture photometry within IRAF because all
targets are in non-crowded fields. The sizes of the applied apertures were inspected manually for all 
observations to minimize the effects of the surrounding nebula. 
The further reduction steps were performed using the standard technique for time series CCD observations.

In each field, there were at least seven comparison stars of similar brightness as the target.
The differential light curves derived from at least five comparison stars within the field were
used for the time-series analysis. This should guarantee eliminating possible unknown instrumental effects.
All differential light curves were examined in more detail using the Phase-Dispersion-Minimization method (PDM)
within the software Peranso (Paunzen \& Vanmunster 2016). An analysis with a discrete Fourier algorithm
gave the same noise level over the searched frequency range as PDM. 

\begin{table}
\caption{Observation log and results. Kronberger PN J1944.9+2245 was also investigated for variations in four nights on the time scale of hours (second line).}
\begin{center}
\begin{tabular}{lccccc}\hline
PN & $V$ & HJD(start) & HJD(end) & $N$ & Upper Limit \\
   & [mag] & 2455792+   & 2455792+ &     & [mag] \\
\hline
J1944.9+2245 & 15.8 & 0.44241 & 10.37849 & 42 & 0.12 \\
J1944.9+2245 & & 44.25059 & 49.27524 & 131 & 0.10 \\
NGC 6853 & 14.1 & 0.45868 & 45.31385 & 52 & 0.08 \\
NGC 7008 & 12.8 & 0.47488 & 49.40449 & 64 & 0.01 \\
NGC 7076 & 13.4 & 0.48364 & 44.41168 & 38 & 0.06 \\
NGC 7354 & 16.1 & 0.49543 & 49.42055 & 44 & 0.07 \\
\hline
\end{tabular}
\end{center}
\end{table}

\section*{Analysis and conclusions}

For all five targets, we searched for long-term variations on the
time scales of days. For this, three to five consecutive observations
in one night were secured. The time-series analysis was performed using
all individual measurements, but also for the mean of all individual
nights. In Table 1, the results are summarized. We were not able
to detect any statistically significant (4\,$\sigma$ above the noise level)
signal in the light curves. The upper limits range from 0.01 to 0.12\,mag.
Hillwig et al. (2015) presented variability caused by binarity for the central
star of Abell 65. The amplitude in the $I$ filter for this object is about
one magnitude with a period very close to one day. In addition, the characteristics
for other known binaries among PN central stars are very similar (De Marco et al. 2008).
Therefore we conclude that none of the investigated objects shows eclipses
and irradiation effects on the investigated time scale of up to 50 days.

The object Kronberger PN J1944.9+2245 was also investigated during four nights
searching for variation on a time scale up to a few hours. The noise level is rather
high (0.1\,mag) compared to the cyclic variations (0.02\,mag) found by
Handler et al. (1997) for the central star of IC 418. We can therefore not exclude 
such variations for this object. 

We encourage further photometric observations of PN central stars to detect to-date
unknown binary systems which is a perfect suited project for
smaller telescopes. The expected variations can reach even up to a few magnitudes which
is observable in non-perfect conditions and sites. With the prospects of precise
parallaxes and thus distances from the Gaia, such observations will significantly 
contribute to our knowledge of PNe.

\bigskip

{\bf Acknowledgements:} The work was supported by the Czech Grant Agency under the project 14-26115P.

\references

De Marco, O., Hillwig, T. C., Smith, A. J.,  2008, {\it AJ}, {\bf 136}, 323 \DOI{10.1088/0004-6256/136/1/323}

Handler, G., et al., 1997, {\it A\&A}, {\bf 320}, 125 

Hillwig, T.~C., et al., 2015, {\it AJ}, {\bf 150}, 30 \DOI{10.1088/0004-6256/150/1/30}

Hillwig, T.~C., et al., 2016, {\it ApJ}, {\bf 832}, 125 \DOI{10.3847/0004-637X/832/2/125}

Paunzen, E., Vanmunster, T., 2016, {\it AN}, {\bf 337}, 239 \DOI{10.1002/asna.201512254}

\endreferences

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