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\begin{document}

\IBVShead{6061}{3 July 2013} 

\IBVStitletl{Discovery of irregular variability of five stars in}{ the vicinity of the young stellar object V645~Cygni}

\IBVSauth{Sobolev, A. M.; Gorda, S. Yu.; Davydova, O. A.}

\IBVSinsto{Astronomical Observatory, Ural Federal University,
Russia} \vskip -1mm \IBVSinsto{E-mail: {\tt Andrej.Sobolev@usu.ru, Stanislav.Gorda@usu.ru}}

\SIMBADobj{V645 Cyg}
\IBVStyp{ eclipsing binary } 
\IBVSkey{IR-objects, young star}

\begintext

We carried out a search for irregular variable stars in the vicinity
of the young massive star V645~Cyg. The search was performed on
the basis of a set of CCD frames obtained at the Astronomical
Observatory of the Ural Federal University. The data was collected
in the course of the 1.5-year photometric monitoring of the young
variable star V645~Cyg in filters $V$ and $R_c$. All frames were
obtained with the AZT-3 reflector ($D = 0.45$ m, $F_{\rm Newton} =
2.0$ m) equipped with Alta-U6 CCD camera (Kodak KAF-1001E,
$1048 \times 1048$, 24-micron chip). The frames contain information on
sources within about $40' \times 40'$ fields centered approximately at
V645~Cyg. These sources include several suspected young stellar
objects (YSOs), e.g. IRAS~21377+4955 and IRAS~21389+5003, which
are poorly studied at optical wavelengths.

Search for the YSOs in the V645~Cyg vicinity is necessary to find
other young stars which could be born together with this massive
star which recently emerged from its cocoon. At present there are
no firmly established YSOs in the vicinity of V645~Cyg. This is
enigmatic because the current paradigma implies that ``massive stars are seldomly (if at all) formed in isolation" (Zinnecker and Yorke 2007).

In this study we considered objects in $15'$ vicinity of V645~Cyg.
YSOs are characterized by being brighter in infrared than expected for the main sequence stars.
In order to identify YSO candidates in the obtained frames we
carried out photometry of a number of selected stars which show
infrared colour indices $H - K_s > 0\fmm3$ in 2MASS catalog.
The area is rich in the sources
that are surely more distant than V645~Cyg. In order to avoid
them we introduced the following criterion: $K_s < 11\mm$.
One object (2MASS~21385638+5012061) violates this criterion and was
included in the list because of its high red colour index
$V - R = 1\fmm98$ in the NOMAD1 catalogue.
We did not introduce any stringent criterion on the infrared colour indices
in order to avoid possible elimination of Orion variables of IN type, etc.
Some young variable stars of this type have 2MASS colours close to
the main sequence stars and have good chances to be associated with
the massive young variable V645~Cyg. 
Another selection
criterion was the optical brightness of objects $R < 14\mm$. The
latter criterion is necessary to achieve the photometric accuracy of the order of $0\fmm01$ for the frames obtained with
exposure times adjusted for the V645~Cyg photometry. The list
includes three fainter objects which allowed sufficiently high
photometric accuracy.

In total, 14 stars were selected for analysis. We included
V645~Cyg in the analysis because it satisfies our selection
criteria. Except for
V645~Cyg the list includes 2 other members of the IRAS point
source catalogue, the rest of the target stars have considerable
red colour indices.

\small
\begin{table}
\centerline{{\bf Table 1.} List of target stars and comparison stars. Stellar magnitudes are taken from the NOMAD1 catalogue}
\begin{center}
\begin{tabular}{c@{\quad}c@{\quad}c@{\quad}
c@{\quad}|@{\quad}c@{\quad}c@{\quad}c@{\quad}}
 \hline
\multicolumn{4}{c|@{\quad}}{Objects} & \multicolumn{3}{c}{Comparison stars (2MASS)}\\
\hline
$\#$ &   2MASS  &  $m_V$ & $m_R$ &  $C_1$  &  $C_2$  & $C_3$ \\
\hline
1 & 21395825+5014209 & $\sim$13 & $\sim$12 & 21401743+5013545 & 21395248+5015526 & 21393905+5012016 \\
& (V645 Cyg) &&&& \\
2 & 21392977+5009286 & 14.31 & 12.36 & 21393905+5012016 & 21394305+5010491 & 21393643+5008273 \\
%  & &  NSV 25721 & & & & \\
& (IRAS~21377+4955) &&&& \\
3 & 21404646+5016489 & 12.61 & 10.72 & 21403765+5016258 & 21405102+5017094 & 21394758+5017008 \\

& (IRAS~21389+5003) &&&& \\
4 & 21385638+5012061 &  15.53 & 13.55  & 21391128+5011193 & 21385214+5011225 & 21384244+5012131 \\
5 & 21390639+5004544 &  13.01 & 11.15 & 21384996+5007368 & 21391834+5003325 & 21390691+5003206 \\
6 & 21393188+5006596 &  16.00 & 14.75 & 21394160+5009340 & 21393870+5006470 & 21391935+5008547 \\
7 & 21402043+5020007 &  15.04 & 13.84 & 21403180+5019153 & 21403029+5017160 & 21401798+5022063 \\
8 & 21403143+5021300 &  15.73 & 14.51 & 21401858+5022036 & 21403180+5019153 & 21405252+5022075 \\
9 & 21403966+5010088 &  16.04 & 14.29 & 21404627+5010344 & 21404549+5007301 & 21403186+5009173 \\
10 & 21404091+5013584 &  13.24 & 11.33 & 21410979+5012361 & 21403765+5016258 & 21404271+5010334 \\
11 & 21410408+5010083 &  12.90 & 11.60 & 21410979+5012361 & 21410818+5008461 & 21404271+5010334 \\
12 & 21410428+5014152 &  14.41 & 12.88 & 21411409+5016534 & 21405102+5017094 & 21410979+5012361 \\
13 & 21410943+5018008 &  11.38 & 10.51 & 21411409+5016534 & 21410645+5017483 & 21412779+5018164 \\
14 & 21411121+5010380 &  13.91 & 12.83 & 21410818+5008461 & 21410979+5012361 & 21412626+5009326 \\
\hline
\end{tabular}
\end{center}
\end{table}

\large

One of the IRAS sources (IRAS~21377+4955, 2MASS~21392977+5009286)
was previously identified as a carbon star (Alksnis and
Alksne, 1988) and suspected to be variable
(NSV~25721). The list of IRAS sources and other target stars  is
given in Table 1 in order of increasing right ascension.
Designations of the stars are given according to the 2MASS
catalog. For each target star we selected three stars of
comparison in its near vicinity ($r < 5'$), see Table 1.

The reduction of the CCD frames was carried out using $Muniwin$
software \hfill\break ({\tt http://c-munipack.sourceforge.net)}. After that we calculated values of the night-average
differences of the brightness for each star with respect to the
three comparison stars. Before averaging all the values were
reduced to a particular comparison star (C1 stars in Table 1).

In order to determine the degree of brightness variability for the
investigated stars we used a simple criterion similar to the Stetson
index (Stetson, 1996). Namely, we considered ratio of the root
mean square (rms) brightness differences for the target stars to
rms brightness differences for the comparison stars calculated
for the whole period of observations. In this case the 
dispersion of the brightness differences for the stars of
comparison characterizes accuracy of the brightness measurements
for the target stars.

\IBVSfig{10cm}{6061-f1.eps}{The light curves of the IRAS point sources. The bars correspond to the rms scatter of brightness during one night. } 
\IBVSfigKey{6061-f1.eps}{IRAS 21389+5003; IRAS 21377+4955}{lightcurve}

\IBVSfig{10cm}{6061-f2.eps}{The light curves of 2MASS 21410943+5018008 ($\#13$, left) and 2MASS 21403966+5010088 ($\#9$, right). The bars correspond to the rms scatter of brightness during one night. } 
\IBVSfigKey{6061-f2.eps}{2MASS J21410943+5018008; 2MASS J21403966+5010088}{light curve}

As a criterion for the existence of variability we adopted
3$\sigma$ criterion. This means that the star was considered to be
variable if the rms difference of the brightness of a target star in
both filters exceeds the corresponding rms difference for the comparison stars
 more than three times. Tables 2 and 3 contain values of the rms
brightness difference for the target stars ($\sigma_{\rm var}$) and
the comparison stars ($\sigma_{\rm comp}$), correspondingly. It is
seen that $\sigma_{\rm comp}$ in filter $R_c$ is of the order of
$0\fmm01$ and less. The accuracy of measurements in filter $V$ in a few
cases is about twice worse because the target stars are fainter in this passband.

Analysis of the data from Tables 2 and 3 shows that the 3$\sigma$
criterion of variability is satisfied for 6 target stars including
V645~Cyg. The other target stars with one exception did not show
significant variability of brightness. This exception is
2MASS~21402043+5020007 ($\# 7$) for which the 3$\sigma$ criterion
is satisfied only in filter $R_c$. We consider the detection of
variability of this star uncertain.

\small
\begin{table}
\centerline{{\bf Table 2.} Target stars with detected variability}
\begin{center}
\begin{tabular}{c@{\quad}c@{\quad}c@{\quad}
c@{\quad}|@{\quad}c@{\quad}c@{\quad}|@{\quad}c@{\quad}c@{\quad}|c|c}
 \hline
 & & & &\multicolumn{2}{c|@{\quad}}{V} & \multicolumn{2}{c|}{R}& V & R \\
 \hline
$\#$& 2MASS & IRAS& NSV & $\sigma_{\rm var}$ & $\sigma_{\rm comp}$ & $\sigma_{\rm var}$ & $\sigma_{\rm comp}$ & $\sigma_{\rm var}/\sigma_{\rm comp}$& $\sigma_{\rm var}/\sigma_{\rm comp}$\\
\hline
1 & 21395825+5014209 &(V645 Cyg) & & 0.097 & 0.011 & 0.077 & 0.009 & 8.8 & 8.6 \\
2 & 21392977+5009286 &  21377+4955 &  25721 & 0.104 & 0.009 & 0.093 & 0.006 & 11.2 & 16.0 \\
3 & 21404646+5016489 &  21389+5003 & & 0.265 & 0.015 & 0.190 & 0.008 & 17.5 & 23.8 \\
5 & 21390639+5004544 & & & 0.217 & 0.013 & 0.237 & 0.010 & 16.5 & 23.7 \\
9 & 21403966+5010088 & & & 0.187 & 0.007 & 0.090 & 0.006 & 25.9 & 15.6 \\
13 & 21410943+5018008 & & & 0.047 & 0.008 & 0.063 & 0.004 & 5.8 & 15.2 \\
\hline
\end{tabular}
\end{center}
\end{table}


\begin{table}
\centerline{{\bf Table 3.} Target stars without detected variability}
\begin{center}
\begin{tabular}{c@{\quad}
c@{\quad}|@{\quad}c@{\quad}c@{\quad}|@{\quad}c@{\quad}c@{\quad}|c|c}
 \hline
 &  &\multicolumn{2}{c|@{\quad}}{V} & \multicolumn{2}{c|}{R}& V & R \\
 \hline
$\#$& 2MASS & $\sigma_{\rm var}$ & $\sigma_{\rm comp}$ & $\sigma_{\rm var}$ & $\sigma_{\rm comp}$ & $\sigma_{\rm var}/\sigma_{\rm comp}$& $\sigma_{\rm var}/\sigma_{\rm comp}$\\
\hline
4 & 21385638+5012061 & 0.042 & 0.022 & 0.013 & 0.010 & 1.9 & 1.3 \\
6 & 21393188+5006596 & 0.028 & 0.017 & 0.023 & 0.011 & 1.7 & 2.1 \\
7 & 21402043+5020007 & 0.039 & 0.013 & 0.056 & 0.011 & 3.0 & 5.1 \\
8 & 21403143+5021300 & 0.028 & 0.010 & 0.014 & 0.008 & 2.7 & 1.8 \\
10 & 21404091+5013584 & 0.008 & 0.009 & 0.015 & 0.006 & 0.9 & 2.4 \\
11 & 21410408+5010083 & 0.016 & 0.010 & 0.013 & 0.007 & 1.5 & 1.9 \\
12 & 21410428+5014152 & 0.018 & 0.011 & 0.016 & 0.008 & 1.6 & 2.0 \\
14 & 21411121+5010380 & 0.016 & 0.012 & 0.014 & 0.009 & 1.3 & 1.6 \\
\hline
\end{tabular}
\end{center}
\end{table}

\large

\IBVSfig{10cm}{6061-f3.eps}{The light curves of 2MASS 21390639+5004544 ($\#5$, left) and V645 Cyg (right). The bars correspond to the rms scatter of brightness during one night. }
\IBVSfigKey{6061-f3.eps}{2MASS J21390639+5004544; V645 Cyg}{light curve}

We analysed the changes of brightness of newly found variable stars
and V645~Cyg during particular nights. In several cases
observations during one night lasted for up to 6 hours. In most
cases variability had a character of monotonic brightness
changes. In a few cases one brightness extremum was registered during a single night.

We examined the brightness data of the six
variable stars for possible periodicity. For this purpose we
implemented the Lafler-Kinman method realized in the package
winefk3 written by Goranskij V.P. (the program with comments in English can be downloaded from http://variablestars.ru/FILES/winefk.rar). Brightness changes for all newly
found variable stars and V645~Cyg were found to be non-periodic.

We found that the bright infrared point sources IRAS~21377+4955
and \linebreak IRAS~21389+5003 show brightness variability of irregular type
with amplitudes up to $0\fmm8$. For IRAS~21389+5003 the variability
of brightness is registered for the first time. For another bright
infrared source IRAS~21377+4955 only photographic
estimates of the optical brightness were reported (Alksnis and Alksne,
1988). In this study we present results of much more accurate CCD
measurements of the brightness in filters $V$ and $R_c$ for a period of 1.5 years.



Far infrared colour indices of IRAS~21377+4955 and IRAS~21389+5003
are characteristic of the YSOs. Brightness of these stars as point sources of the WISE (Wide-Field Infrared Survey Explorer) survey increases with wavelength (the longest wavelength of the survey is 22 microns).
However, combination of the colour indices and irregular type of variability cannot be used as a strict proof of the YSO nature of the sources.
For example, IRAS 21377+4955 was identified as a carbon star on the basis of its photographic spectrum in the red part of the optical range (Alksnis and Alksne, 1988). However, no other carbon stars were reported in our field though it was inspected by Alksnis and Alksne (1988). Additional spectroscopy is necessary to establish the nature of the sources.
In this study we have registered the irregular type of optical variability of the mentioned IRAS sources by means of CCD photometery.

Brightness changes of similar character were detected for 3
other target stars and V645~Cyg. Variability of brightness had
considerable amplitudes $\Delta m=|m_{max}-m_{min}|$:
\begin{itemize}
\item $\Delta m\sim 1\fmm0$ -- 2MASS~21390639+5004544 ($\#$5),
\item $\Delta m\sim 0\fmm9$ -- 2MASS~21403966+5010088 ($\#$9),
\item $\Delta m\sim 0\fmm3$ -- 2MASS~21410943+5018008 ($\#$13),
\item $\Delta m\sim 0\fmm4$ -- 2MASS~21395825+5014209 (V645~Cyg).
\end{itemize}

The light curves of the 5 target stars and V645~Cyg with variability
detected in filters $V$ and $R_c$ are shown in Figures 1-3.
Measured values of the brightness changes for the 5 newly found
variable stars and V645~Cyg are presented in Table 4 (online only).

Thus, in the course of the current study we have found 4 new
variable stars in $15'$ vicinity of the young massive
variable star V645~Cyg and confirmed variability of one suspected
variable. All of these stars show variability of irregular type
and have considerable infrared colour indices characteristic of the
young stellar objects. Amplitudes of variability of the newly
detected variables are similar to those of some Orion variables of
IN type. According to the General Catalogue of Variable Stars (GCVS) 
the amplitudes of the IN type variables have values in the range from 0.5 to 1-2
magnitudes on the time scale of several tens to hundreds of days.
The nature of these sources and their
association with V645~Cyg will be discussed in a forthcoming
paper.

The study was partly supported by the Russian federal task program
``Research and operations on priority directions of development of
the science and technology complex of Russia for 2007-2013" (state
contract No.~14.518.11.7064). The authors are grateful to the anonymous referee who helped to improve the quality of our paper. 


\references

Alksnis, A. and Alksne, Z., 1988, Carbon stars in a field in Cygnus, Riga: Zinatne, 12. \BIBCODE{1988csfc.book.....A}

Stetson, P. B., 1996, {\it PASP}, {\bf 108}, 851. 

Zinnecker, H. and Yorke, H.W., 2007, {\it Ann. Rev. Astron. Astrophys}., {\bf 45}, 481.

\endreferences

\end{document}
