\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}

\IBVShead{5574}{12 November 2004}

\IBVStitle{Differential Photometry of AW V\lowercase{ir} in April 2004}


\IBVSauth{Zboril, M.$^{1}$; Djura\v{s}evi\'{c}, G.$^{2}$}

\IBVSinst{Astronomical Institute, Tatransk\'{a} Lomnica, 059 60, Slovakia,
e-mail: zboril@astro.sk}
\IBVSinst{Astronomical Observatory, Volgina 7, 11160, Belgrade, Yugoslavia and\\ 
          Isaac Newton Institute of Chile, Yugoslav branch}

\SIMBADobj{AW Vir}
\IBVStyp{  RS, BY  }
\IBVSkey{photometry}
\IBVSabs{The BV photometry of active over-contact binary AW Vir}
\IBVSabs{is presented and analysed.}                           

\begintext

The solar analogy of activity has been widely studied on a variety of active late-type
stars. Here we present the study of activity centers (spots) on W-type overcontact binary
AW Vir.\\
AW Vir (GSC\,00303-00887,F8, m$_{V}$=11.0 ) is an eclipsing binary on which 
the photographic variability was reported in 1935 (Hoffmeister 1935). 
Since then, few both photo-electric and CCD observations have been made to obtain
the minima times (2002 data, e.g. Agerer \& Hubscher 2003). Detailed light curve
analysis was made by Niarchos et al. (1997), who derived the mass ratio $q$,
the masses and radii of components and found one cool spot on the primary 
(more massive and
cooler) component (February 1982 dataset). Qian (2003) paid attention to the period changes 
of the system ($\sim$ 10$^{-8}$ d.y$^{-1}$) and introduced a quadratic term in the minima ephemeris.
He still found the $O-C$ diagram considerably scattered.
We obtained new observations in April 2004 to fill in the gap in the observations and
to continue tracing the activity
signatures (spots) on this system. Such studies are important as the tidal
effects on spots should be present on systems similar to AW Vir and, in addition, 
magnetic-induced activity cycles (magnetic flux tubes evolution) are of interest as well.
The star was monitored during the spring season 2004 using the
0.6m telescope at Star\'{a} 
Lesn\'{a} Observatory equipped with a single channel photometer.

\begin{table}[th]
\begin{center}
\caption{The log of observations.}
\begin{tabular}{cccc}\hline
Date & Filter & Obs. & S$-$CH \\ \hline
14.4.2004 & BV & SL & 0.02\\
15.4.2004& BV & SL & 0.03\\
21.4.2004& BV & SL & 0.03\\
22.4.2004& BV & SL & 0.05\\
\hline
\end{tabular}
\end{center}
\end{table}

The standard differential BV photometry with the sequence 
\dots S -- V -- CH \dots was made as well as standard reduction process 
including the corrections for differential extinction. 
The check star (CH) was SAO\,119 944 and the standard star (S) was SAO\,119 935.
The following linear ephemeris has been used

\begin{equation}
{\rm MinI} = {\rm HJD}~ 2~445~022.6528 + 0\fday35399712~\times E
\end{equation}
being very close to previous determinations.

The complete dataset is presented in Table 1 (raw data are available only electronically via IBVS web-page as files 5574-t4 -- t10.txt).
The final analysis was made using the code of Djura\v{s}evi\v{c}, see Djura\v{s}evi\v{c} et al. (2004) for the details.

\IBVSfig{8.0cm}{5574-f1.eps}{Observed (LCO) and final synthetic (LCC) light curves of AW Vir with final $O-C$ residuals obtained by analysing B and V observations {\bf (left - two spots hypothesis ; middle - one spot hypothesis; right - without spots)} and the corresponding view of the system obtained with the resulting parameters.}
\IBVSfigKey{5574-f1.eps}{AW Vir}{light curve}


\IBVSedata{5574-t4.txt}
\IBVSdataKey{5574-t4.txt}{AW Vir}{photometry}

\IBVSedata{5574-t5.txt}
\IBVSdataKey{5574-t5.txt}{AW Vir}{photometry}

\IBVSedata{5574-t6.txt}
\IBVSdataKey{5574-t6.txt}{AW Vir}{photometry}

\IBVSedata{5574-t7.txt}
\IBVSdataKey{5574-t7.txt}{AW Vir}{photometry}

\IBVSedata{5574-t8.txt}
\IBVSdataKey{5574-t8.txt}{AW Vir}{photometry}

\IBVSedata{5574-t9.txt}
\IBVSdataKey{5574-t9.txt}{AW Vir}{photometry}

\IBVSedata{5574-t10.txt}
\IBVSdataKey{5574-t10.txt}{AW Vir}{photometry}

\IBVSedata{5574-t11.txt}
\IBVSdataKey{5574-t11.txt}{AW Vir}{photometry}


\begin{table}[th]
\small
\vspace*{-1.5cm}
\begin{center}
\caption{Times of minima.}
\begin{tabular}{cccccc}\hline
Date & Filter & Min. & Min. type & $O-C$ & $(O-C)$ \\ \hline
14.4.2004 & B & 53110.4280 &I & 0.26 & 0.25 \\
14.4.2004 & V & 53110.4278 &I & 0.25 & 0.25\\
15.4.2004 & B & 53111.4894 &I & 0.26 & 0.25\\
15.4.2004 & V & 53111.4897 &I & 0.26 & 0.25\\
21.4.2004 & B & 53117.3291 &II& 0.08 & 0.07 \\     
21.4.2004 & V & 53117.3308 &II & 0.08 & 0.07\\     
22.4.2004 & B & 53118.3935 &II & 0.08 & 0.07\\
22.4.2004 & V & 53118.3961 &II & 0.09 & 0.08\\
\hline
Note: $(O-C)$ by Qian (2003).\\
\end{tabular}
\end{center}
\end{table}

\vspace*{-0.5cm}

The results are presented in Table 3. The shape of the light curve changed 
since the
previous study (Niarchos et al. 1997) and we found the activity centers on the less-massive and
hotter component. Though we could not decide between single and/or two-spot models, the spots
cooler than the surrounding photosphere
at intermediate latitudes confirm the tidal effect just with their locations. The system
without spots gives the worst fit to the light curves. In addition, the mass
ratio of the components had to be changed. The spots on the more-massive components are
unrealistic as well, what is demonstrated with the convergence properties of the modelling. The times
of minima do not fit well the quadratic ephemeris of Qian (2003) and at least suggest the importance of spots in the system. The overcontact
factor for the binary is up to 10\%. The times of minima derived by us are presented in Table 2.


\begin{table}[!ht]
\small
      \caption{Results of the simultaneous analysis of the AW Vir B,V light 
curves obtained by solving the inverse problem for the Roche model with  
and without spots on the less-massive (hotter) component.}       
        \label{TabAWVir}
      \[
        \begin{array}{llll}
            \hline
            \noalign{\smallskip}
Quantity & {\rm Hip. \ I. - two  \ spots} & {\rm Hip. \ II. - one \ spot} & {\rm Hip. \ III. - without \ spots}  \\
            \noalign{\smallskip}
            \hline
            \noalign{\smallskip}      
   n                    & 202              & 202             & 202             \\                    
{\rm \Sigma(O-C)^2}     & 0.1924           & 0.1954          & 0.2453          \\                    
{\rm \sigma}            & 0.0309           & 0.0312          & 0.0349          \\                    
f{\rm _{h,c}}           & 1.0              & 1.0             & 1.0             \\  
{\rm A_{h,c}}           & 0.5              & 0.5             & 0.5             \\ 
{\rm \beta_{h,c}}       & 0.08             & 0.08            & 0.08            \\ 
{\rm T_h}               & 6200             & 6200            & 6200            \\ 
{\rm A_{s1}=T_{s1}/T_h} &  0.65  \pm 0.08  &  0.66  \pm 0.12 &                 \\ 
{\rm \theta_{s1}}       &  33.8  \pm 1.8   &  25.2  \pm 1.8  &                 \\ 
{\rm \lambda_{s1}}      & 351.2  \pm 2.2   & 344.1  \pm 4.1  &                 \\ 
{\rm \varphi_{s1}}      &  45.2  \pm 4.7   &  39.5  \pm 5.5  &                 \\ 
{\rm A_{s2}=T_{s2}/T_h} &  0.93  \pm 0.01  &                 &                 \\ 
{\rm \theta_{s2}}       &  40.4  \pm 2.2   &                 &                 \\ 
{\rm \lambda_{s2}}      & 183.4  \pm 4.9   &                 &                 \\ 
{\rm \varphi_{s2}}      &  40.9  \pm 4.1   &                 &                 \\ 
{\rm T_c}               &  6078  \pm 23    &  6189  \pm 23   &  6100  \pm 26   \\ 
{\rm F_h}               & 1.006  \pm 0.002 & 1.020  \pm 0.002& 1.029  \pm 0.003\\ 
i \ [^\circ]            &  82.1  \pm 0.4   &  83.3  \pm 0.4  &  83.8  \pm 0.4  \\ 
q=m{\rm _c}/m{\rm _h}   &  1.32  \pm 0.04  &  1.30  \pm 0.05 &  1.67  \pm 0.06 \\ 
a^{\rm h,c}_1{\rm [B]}  &+0.3369, +0.3645  &+0.3369, +0.3395 &+0.3369, +0.3594 \\                     
a^{\rm h,c}_2{\rm [B]}  &+0.5521, +0.3976  &+0.5521, +0.5378 &+0.5521, +0.4262 \\                     
a^{\rm h,c}_3{\rm [B]}  &+0.0590, +0.2836  &+0.0590, +0.0798 &+0.0590, +0.2419 \\                     
a^{\rm h,c}_4{\rm [B]}  &-0.1039, -0.1940  &-0.1039, -0.1123 &-0.1039, -0.1773 \\                     
a^{\rm h,c}_1{\rm [V]}  &+0.4165, +0.4408  &+0.4165, +0.4187 &+0.4165, +0.4363 \\                     
a^{\rm h,c}_2{\rm [V]}  &+0.5507, +0.4218  &+0.5507, +0.5387 &+0.5507, +0.4457 \\                     
a^{\rm h,c}_3{\rm [V]}  &-0.1962, +0.0071  &-0.1962, -0.1774 &-0.1962, -0.0306 \\                     
a^{\rm h,c}_4{\rm [V]}  &-0.0039, -0.0937  &-0.0039, -0.0122 &-0.0039, -0.0771 \\                     
{\rm \Omega_{h,c}}         & 4.2273 & 4.1612 & 4.6842 \\                             
{\rm \Omega_{in}}          & 4.2471 & 4.2233 & 4.7788 \\                          
{\rm \Omega_{out}}         & 3.6798 & 3.6571 & 4.1935 \\                          
f_{\rm over}[\%]           &  3.48  & 10.98  & 16.16  \\                          
{\rm R_{h,c} [D=1]}        & 0.336, \ 0.381 & 0.341, \ 0.385 & 0.323, \ 0.408 \\
{\rm L_h/(L_h+L_c) (B;V)}  & 0.433; \ 0.432 & 0.433; \ 0.433 & 0.410; \ 0.409 \\
            \noalign{\smallskip}                          
            \hline                    
         \end{array}
      \]
\noindent {\bf Note:} $n$ - total number of the B and V observations, 
${\rm \Sigma (O-C)^2}$ - 
final sum of squares of residuals between observed (LCO) and synthetic (LCC) 
light curves, ${\rm \sigma}$ - standard deviation of the observations, 
$q=m{\rm _c}/m{\rm _h}$ - mass ratio of the components, $f{\rm _{h,c}}$, 
${\rm \beta_{h,c}}$, ${\rm A_{h,c}}$ - nonsynchronous rotation coefficients,
gravity-darkening exponents and albedo of the components,
$T_{h,c}$ - temperature of the hotter primary and cooler secondary, 
${\rm A_{S1,2}}$, ${\rm \theta_{S1,2}}$, ${\rm \lambda_{S1,2}}$ and 
${\rm \varphi_{S1,2}}$ - spots temperature coefficients, angular dimensions, 
longitudes and latitudes (in arc degrees), ${\rm F_{h,c}}$ - filling factors for 
the critical Roche lobe of the hotter (less-massive) star, $i \ [^\circ]$ 
- orbit inclination (in arc degrees), $a^{\rm h,c}_1$, $a^{\rm h,c}_2$, 
$a^{\rm h,c}_3$, $a^{\rm h,c}_4$ - nonlinear B and V limb-darkening coefficients of 
the components (Claret's formula), 
$\Omega_{h,c}$, $\Omega_{\rm in}$, $\Omega_{\rm out}$ 
- dimensionless surface potentials of the components and of the inner and 
outer contact surfaces respectively, $f{\rm _{over}}[\%]$ - degree of 
overcontact, $R_{h,c}$ - polar radii of the components in units of the 
distance between the component centre and ${\rm L_h/(L_h+L_c)}$ - (B;V) 
luminosity of the hotter star (including spots).                                   
\end{table}

\vspace*{0.5cm}

{\it Acknowledgements:} The referee is cordially thanked for valuable comments.

\references

Agerer, F. and Hubscher, J. 2003, {\it IBVS}, No.5484

Djura\v{s}evi\v{c}, G., Albayrak, B., Selam, S. O., Erkapic, S., Senavci, H. V. 2004,
{\it NewA}, {\bf 9}, 425

Hoffmeister, C. 1935, {\it Astr. Nachr.}, {\bf 255}, 401

Niarchos, P. G., Hoffmann, M., Durbeck, H. W. 1997, {\it A\&AS}, {\bf 124}, 291

Qian, S. 2003, {\it MNRAS}, {\bf 342}, 1260

\endreferences


\end{document}

