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\IBVShead{6079}{8 November 2013}

\IBVStitle{AW~UM\lowercase{a} observed with MOST satellite\IBVSfm}

\IBVSauth{Rucinski,~S.M.$^1$; Matthews, J.M.$^2$; Cameron, C.$^3$; Guenther,~D.B.$^4$; Kuschnig,~R.$^5$; Moffat,~A.F.J.$^6$; Rowe,~J.F.$^7$; Sasselov,~D.$^8$; Weiss,~W.W.$^9$}

\IBVSinst{Department of Astronomy and Astrophysics, University of Toronto, 
50 St.~George Street, Toronto, Ontario M5S~3H4, Canada.
E-mail: rucinski@astro.utoronto.ca}

\IBVSinst{Department of Physics and Astronomy,                 
University of British Columbia, 
6224 Agricultural Road, Vancouver, BC, V6T~1Z1, Canada.
E-mail: matthews@astro.ubc.ca}

\IBVSinst{Department of Mathematics, Physics \& Geology,       
Cape Breton University, 1250~Grand Lake Road, Sydney, NS, B1P~6L2, Canada.
E-mail: ccamerongeneral@gmail.com}

\IBVSinst{Institute for Computational Astrophysics,            
Department of Astronomy and Physics, Saint Mary’s University, 
Halifax NS, B3H~3C3, Canada.
E-mail: guenther@ap.stmarys.ca}

\IBVSinst{University of Vienna, Institute for Astronomy,      
T\"{u}rkenschanzstrasse 17, A-1180 Vienna, Austria.
E-mail: rainer.kuschnig@univie.ac.at}

\IBVSinst{D\'{e}partement de physique,  
Universit\'{e} de Montr\'{e}al, C.P.~6128, Succ.~Centre-Ville, 
Montr\'{e}al, QC, H3C~3J7, Canada.
E-mail: moffat@astro.umontreal.ca}

\IBVSinst{NASA Ames Research Center, Moffett Field, CA, 94035, USA.   
E-mail: jasonfrowe@gmail.com}

\IBVSinst{Harvard-Smithsonian Center for Astrophysics,        
60 Garden Street, Cambridge, MA 02138, USA.
E-mail: sasselov@cfa.harvard.edu}

\IBVSinst{University of Vienna, Institute for Astronomy,    
T\"{u}rkenschanzstrasse 17, A-1180 Vienna, Austria.
E-mail: werner.weiss@univie.ac.at}


\IBVSkey{W~UMa type binaries, O-C eclipse timing}  

\SIMBADobj{AW UMa}

\IBVSabs{MOST observations were obtained to search for photometric non-radial}
\IBVSabs{oscillations; none was detected with an upper limit of 0.0001 in}
\IBVSabs{relative amplitude. A single, precise moment of the primary eclipse}
\IBVSabs{confirms the progressive shortening of the orbital period.}

\IBVSfoot{Based on data from the MOST satellite, a
Canadian Space Agency mission, jointly operated by Microsatellite
Systems Canada Inc. (MSCI; formerly Dynacon Inc.), the University
of Toronto Institute for Aerospace Studies and the University of
British Columbia, with the assistance of the University of Vienna.}

\bigskip

Since its discovery by Paczy\'nski (1964), AW~UMa (HD~99946) has played 
a special role in efforts to understand structure of W~UMa-type binaries. 
This particular importance was realized
after it had been shown by Mochnacki \& Doughty (1972) that model light curves 
computed following the contact model of Lucy (1968a, 1968b) imply 
a very small mass-ratio of $q \simeq 0.080 \pm 0.005$. 
The agreement with the model for such a large disparity 
of masses has since been interpreted as a very strong support for the model. 
While the small mass ratio has been photometrically confirmed many times 
following the investigation of Mochnacki \& Doughty, 
new spectroscopic observations of Pribulla \& Rucinski (2008)
revealed that AW~UMa is a complex semi-detached binary with a significantly 
larger mass ratio ($q \ge 0.10$). 
These observations showed complicated flows in the system, with the undersized 
primary and a tiny secondary embedded in what may be interpreted as
an accretion disk. 
One of the spectroscopically detectable features was the presence of 
inhomogeneities on the primary component which could possibly be 
non-radial oscillations. Such oscillations would not be excluded 
on an F2 main-sequence star which is located exactly in the middle of 
the $\delta$~Sct instability strip. 

\IBVSfig{9.0cm}{6079-f1.eps}{The mean light curve of AW~UMa 
obtained from averaging all MOST observations in orbital phase is very well
defined with errors smaller than the thickness of the line.}
\IBVSfigKey{6079-f1.eps}{AW UMa}{light curve}

Continuous MOST observations with effective exposure times of one minute
were obtained between 25 February and 9 March 2013 resulting in
14,020 individual observations 
with a median error per point of 0.0005 mag. The mean, phased light curve 
obtained from averaging of 27 orbital cycles 
does not show any asymmetries which could be associated with spots or any
other peculiarities (Figure~1). However, such a light curve
is not convenient for modelling attempts because MOST uses 
one filter with a wide spectral band. Thus, we searched only
for variability in the {\it deviations\/} from the mean light curve. 
A periodogram analysis for coherent oscillations and a wavelet analysis for
localized oscillatory trains reveal no features which could not be
explained as vestiges of the orbital binary (2.28 c/d, 4.56 c/d) 
and orbital satellite (1.0 c/d, 2.0 c/d and the explicitly removed
14.3 c/d) periodicities, their
harmonics and combinations. Thus, no unexplained features larger than 
0.0001 mag appear in the variability spectrum of the residuals from
the average light curve (Figure~2).


\IBVSfig{9.0cm}{6079-f2.eps}{The periodogram of 
deviations from the average light curve of AW~UMa. 
The frequencies are expressed in cycles per day while the vertical axis gives 
the amplitudes in units of 0.001 of the mean flux.
}
\IBVSfigKey{6079-f2.eps}{AW UMa}{periodogram}


The MOST observations gave the mean moments of eclipses:\\
Primary $HJD = 2,456,354.6668 \pm 0.0002$,\\
Secondary $HJD = 2,456,354.8866 \pm 0.0002$.\\
The $O-C$ diagram for AW~UMa is shown in Figure~3.
The eclipse timing data for the figure, covering 
the range $HJD\,2,438,045 - 2,455,669$,
have been taken from the compilation in the Web page of
the Czech Amateur Observers: \newline
{\it http://var.astro.cz/ocgate/ocgate.php?star=AW+UMa\&submit=Submit\&lang=en  }
\newline
where the assumed ephemeris was: $HJD = 2,452,311.81 + 0.438726 \times E$.



\IBVSfig{9.0cm}{6079-f3.eps}{Deviations from the linear 
ephemeris of the Czech Amateur Observers for the
primary (filled circles) and secondary eclipses (open circles) with
visual observations marked by small symbols. The last symbol to
the right is for the MOST observations.
The short line for $E > 3000$ represents the current linear trend.}
\IBVSfigKey{6079-f3.eps}{AW UMa}{O-C diagram}

It has been known that AW~UMa shortens its orbital period. However,
the rate of the period change is not constant so that the eclipse moments 
cannot be fitted by a quadratic function. This is demonstrated in
Figure~3 where the quadratic fit versus the epoch $E$ is shown. 
The fit parameters are: 
$a_0 = +0.007 \pm 0.006$, $a_1 = (-8.6 \pm 1.7) \times 10^{-7}$, 
$a_2 = (-1.12 \pm 0.07) \times 10^{-10}$, where the 
quadratic term implies the e-folding time scale 
of $\simeq 2.4 \times 10^8$ years. It appears that
short segments better represent the observed $O-C$ curve. 
Because we needed a new linear ephemeris for the 2011 extensive 
spectroscopic observations of AW~UMa at CFHT, we fitted a linear ephemeris 
for epochs $E > 3000$:
$HJD = 2,455,632.9660(18) + 0.43872420(25) \times E$ where the mean errors,
in parentheses are in units of the last decimal places.  

Acknowledgements:
D.B.G., J.M.M., A.F.J.M., and S.M.R. are supported by NSERC (Canada),
R.K. and W.W.W. are supported by the Austrian Space Agency 
and the Austrian Science Fund.
CC was supported by the Canadian Space Agency.



\references

Lucy L. B., 1968a, {\it ApJ}, {\bf 151}, 1123

Lucy L. B., 1968b, {\it ApJ}, {\bf 153}, 877

Mochnacki S. W., \& Doughty N. A., 1972, {\it MNRAS}, {\bf 156}, 51

Paczy\'nski, B., 1964, {\it AJ}, {\bf 69}, 124

Pribulla, T. \& Rucinski, S.M., 2008, {\it MNRAS}, {\bf 386}, 377


\endreferences

\IBVSCorrectedVersion{Mon Jan 27 10:03:55 CEST 2014}

\IBVSerratum{6079}



The last paragraph of the paper contains major errors which
propagated into Fig.3. The corrected text and figure are given below.


It has been known that AW~UMa shortens its orbital period. Figure~3 shows
the quadratic fit versus the epoch $E$ with the fit parameters: 
$a_0 = +0.0109(4)$, $a_1 = -4.5(6) \times 10^{-7}$, 
$a_2 = -1.16(4) \times 10^{-10}$, where the mean errors,
in parentheses, are in units of the last decimal places.
The quadratic term implies the e-folding time scale for the period 
change of $2.3 \times 10^6$ years. 
The rate of the period change may vary in time so that 
short segments may better represent the observed $O-C$ curve. 
Because we needed a new linear ephemeris for the 2011 extensive 
spectroscopic observations of AW~UMa at CFHT, 
we fitted a linear ephemeris for epochs $E > 3000$:
$HJD = 2,455,632.9660(18) + 0.43872420(25) * E$. 

\renewcommand\thefigure{\arabic{figure}a}  
\setcounter{figure}{2}
\IBVSfig{9.0cm}{6079-f3a.eps}{Deviations from the linear 
ephemeris of the Czech Amateur Observers for the
primary (filled circles) and secondary eclipses (open circles) with
visual observations marked by small symbols. The last symbol to
the right is for the MOST observations.
The short thick line for $E > 3000$ represents the current linear trend.}
\IBVSfigKey{6079-f3a.eps}{AW UMa}{O-C diagram}

Acknowledgement:
Thanks are due to Dr.\ Miloslav Zejda for pointing the substantial 
error in the paper.

\bigskip
The Author

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