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\IBVShead{6113}{10 August 2014}

\IBVStitletl{ASAS 000709+2621.5 is an overcontact eclipsing binary,}{ not a \boldmath{$\delta$} S\lowercase{ct} variable}


\IBVSauth{KJURKCHIEVA, D. P.$^1$; DIMITROV, D. P.$^2$; IBRYAMOV, S. I.$^2$}

\IBVSinst{Department of Physics, Faculty of Natural Sciences, Shumen University, 9700 Shumen,
Bulgaria, e-mail: d.kyurkchieva@shu-bg.net}
\IBVSinst{Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 72 Tsarigradsko Shose Blvd.,
1784 Sofia, Bulgaria, e-mail: dinko@astro.bas.bg; sibryamov@astro.bas.bg}


\SIMBADobj{ASAS J000709+2621.5}

\IBVStyp{ SB1 }

\IBVSkey{photometry}

\IBVSabs{We present \emph{VI} photometric observations of the star}
\IBVSabs{ASAS 000709+2621.5. They did not confirm its previous}
\IBVSabs{classification as a delta Sct star but exhibited its }
\IBVSabs{binary nature. Our conclusion was based on several criteria: }
\IBVSabs{the light minima are sharper than maxima; different depths }
\IBVSabs{of the light minima; low temperature; successful reproducing }
\IBVSabs{of the Rozhen data by partial eclipses. Our light curve }
\IBVSabs{solution revealed that ASAS 000709+2621.5 is overcontact }
\IBVSabs{system with parameters: orbital inclination i=61.3^0; }
\IBVSabs{mass ratio q=0.953; temperatures of the components }
\IBVSabs{T_1=5940 K and T_2=5673 K; relative radii r_1=0.395 }
\IBVSabs{and r_2=0.386; relative luminosities l_1=0.55 and }
\IBVSabs{l_2=0.45.}

\begintext

Recently the discovery of new variable stars has been accelerating
due to large observing programs such as ROTSE, MACHO, ASAS, OGLE,
etc. However the low precision of these data as well as their
automatic reduction and fitting led to the wrong classification of many
targets. This refers especially to the cases of almost sinusoidal
light curves which are appropriate both for $\delta$ Sct variables
and for W UMa systems. Their right classification often requires
follow-up observations and analysis.

The star ASAS 000709+2621.5 (further shortly ASAS 0007) is such a
case. It was classified as a $\delta$ Sct variable with a
pulsation period of $P$=0.2017 d and amplitude of 0.49 mag on
the base of the ASAS data (Pojmanski 2000). We built folded curves
of these data with periods 1$P$ and 2$P$ (Fig. 1) but
they did not allow us to conclude if the target is a $\delta$ Sct
variable or a W UMa binary.

To answer this question we carried out follow-up photometric
observations of ASAS 0007 with the 60-cm Cassegrain telescope
using the FLI PL09000 CCD camera ($3056 \times 3056$ pixels, 12
$\mu$m/pixel, field of view $27.0 \times 27.0$ arcmin with focal reducer)
of the Rozhen National Astronomical Observatory (Table 1). The
average photometric precision per data point was below 0.01 mag.

\vskip 0.5cm
\centerline{Table 1. Journal of the Rozhen observations} \vskip
3mm
\begin{center}
\begin{tabular}{cccc}
\hline
Date & exposures (s)  & filters & number \\
\hline
 Aug 20 2010  & 60, 60 & $V$, $I$ & 120, 120\\
 Aug 22 2010  & 60, 60 & $V$, $I$ & 176, 177\\
 Aug 23 2010  & 60, 60 & $V$, $I$ & 16, 16\\
 Sept 17 2010 & 70     & $I$    & 420\\
\hline
\end{tabular}
\end{center}
\vskip 0.5cm \centerline{Table 2. Coordinates, magnitudes and
colors of the target and standard stars} \vskip 3mm
\begin{center}
\begin{tabular}{ccccrrrc}
\hline
Star & GSC1ID & RA (2000) & DEC (2000)  & $V$ & $V-I$ \\
\hline
ASAS 0007& 0173200262 & 00 07 09.00 & +26 21 30.0 & 11.55 & 0.86\\
C1 & 0173200564 & 00 07 24.71 & +26 16 38.1 & 12.87 & 1.20 \\
C2 & 0173201665 & 00 07 15.61 & +26 23 33.1 & 11.10 & 1.25 \\
C3 & 0173200140 & 00 07 10.20 & +26 15 49.9 & 14.14 & 1.25 \\
\hline
\end{tabular}
\end{center}

\IBVSfig{11cm}{6113-f1.eps}{Folded light curves of the ASAS data with
periods 1$P$ (top) and 2$P$ (down)}
\IBVSfigKey{6113-f1.eps}{ASAS 000709+2621.5}{light curve}

\IBVSfig{8cm}{6113-f2.eps}{The field $10 \times 10$ arcmin around ASAS
0007}
\IBVSfigKey{6113-f2.eps}{ASAS 000709+2621.5}{finding chart}

The standard procedures were used for reduction of the photometric
data. The standard stars in the observed field (Fig. 2) were
chosen by the criterion to be constant within 0.01 mag during the
observations in all filters and all nights. Table 2 presents their
\emph{V} magnitudes and $V-I$ colors from USNO-B1 catalog.

The periodogram analysis of our $I$ data (which are considerably
more numerous than the \emph{V} data, see Table 1) with a software
\textsc{Persea} led to the ephemeris
\begin{equation}
{\rm HJD (MinI)}=2455429.744 + 0.4034 \times E
\end{equation}
and Figure 3 presents the corresponding folded light curves of ASAS 0007.

The amplitudes of the light variability in \emph{V} and \emph{I}
filters are correspondingly 0.32 mag and 0.30 mag. The previous
bigger \emph{V} amplitude of 0.49 mag (Pojmanski 2000) probably due to the rough
automate reduction of the ASAS data.

The qualitative analysis of the Rozhen data revealed binary
nature of ASAS 0007 rather than $\delta$ Sct variability due to the
following reasons:

(a) There are two light minima with different depths (visible in \emph{I} filter);

(b) The brightness minima seem slightly sharper than the maxima;

(c) We calculated the de-reddened color index
$(V-I)_{0}$=0.86 of the target by the Galactic Reddening
and Extinction Calculator (Schlafly \& Finkbeiner 2011). It
corresponds to temperature $T_{m}$=5870 K according to the tables
of VandenBerg \& Clem (2003). This value is quite low for a
$\delta$ Sct star (see Table 2 and Figure 2 of McNamara 2000) but
appropriate for a W UMa star.

Another important criterion for the binary nature of ASAS 0007
could be the successful reproducing of its light curve by
eclipses. That is why we tried to model the Rozhen data of ASAS
0007 using the code \textsc{phoebe} (Pr\'sa $\&$ Zwitter 2005) by
the following procedure.

Initially the primary temperature was fixed as $T_{1}$=$T_{m}$. We
adopted coefficients of gravity brightening 0.32 and reflection
0.5 appropriate for late stars while the limb-darkening
coefficients for each component were taken from the tables of
VanHamme (1993) according to its temperature. The fitted
parameters were: secondary temperature $T_{2}$, mass ratio $q$,
orbital inclination $i$ and potentials $\Omega_{1,2}$ (and
correspondingly relative radii $r_{1,2}$ and fillout factors
$f_{1,2}$). Finally we varied the
temperatures of the two components around $T_{m}$ to reach a best
fit of the observations.

The parameter values corresponding to our light curve solution
are: orbital inclination $i=61\fdeg30\pm0\fdeg06$; mass
ratio $q=0.953\pm0.003$; temperatures $T_{1}=5940\pm113$ K and
$T_{2}=5673\pm105$ K; relative radii
$r_{1}^{\rm mean}=0.395\pm0.005$ and $r_{2}^{\rm mean}=0.386\pm0.005$
(potentials $\Omega_{1}=\Omega_{2}=3.595\pm0.007$); relative
luminosities $l_{1}=0.55$ and $l_{2}=0.45$. The cited values of
the parameter errors are the formal ones obtained by
\textsc{phoebe}.

The synthetic curves corresponding to these parameters (Fig. 3)
revealed that the Rozhen data can be well-reproduced by partial
eclipses of an overcontact binary (with fillout factor
$f_{1,2}$=0.147) whose components are not in thermal contact.
Hence, the analysis of our observations of ASAS 0007 allowed us
confidently to conclude that it is an eclipsing star but not a
$\delta$ Sct variable, as it was previously classified.

This investigation is a new emphatic illustration of the proposition that the morphological 
classifications given in large data sets, especially for variable
stars with almost sinusoidal light variability, might be wrong.
Hence, the results of the statistical investigations on the base
of such classifications should be assumed rather as
approximate estimations.

\IBVSfig{6cm}{6113-f3.eps}{Rozhen $VI$ folded light curves of
ASAS 0007}
\IBVSfigKey{6113-f3.eps}{ASAS 000709+2621.5}{light curve}

{\bf Acknowledgement:} This research was supported partly by
funds of the project RD-08-244 of Shumen University. It used the
SIMBAD database, operated at CDS, Strasbourg, France, USNO-B1.0
catalogue ({\tt http://www.nofs.navy.mil/data/fchpix/}), and NASA's
Astrophysics Data System Abstract Service.

\references

McNamara D., 2000, {\it ASPC}, {\bf 210}, 373

Pojmanski G., 2000, {\it AcA}, {\bf 50}, 177

Pr\'sa A., Zwitter T., 2005, {\it ApJ}, {\bf 628}, 426

Schlafly E., Finkbeiner D., 2011, {\it ApJ}, {\bf 737}, 103

VandenBerg D., Clem J., 2003, {\it AJ}, {\bf 126}, 778

Van Hamme W., 1993, {\it AJ}, {\bf 106}, 2096

\endreferences

\end{document}
