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\begin{document}

\IBVShead{4517}{10 September 1997}

\IBVStitletl{New Light Curves and Period Study of the}{ Contact Binary W Ursae Majoris}

\IBVSauth{N.\ MORGAN, M.\ SAUER, E.\ GUINAN}

\IBVSinsto{Astronomy Department, Villanova University, Villanova, PA,
USA}

\IBVSabs{O-C diagram and recent lightcurves are published for}
\IBVSabs{the prototype star W UMa}

\SIMBADobj{W UMa}
\IBVStyp{ EW }
\IBVSkey{photometry}

\begintext

W Ursae Majoris (HD 83950, BD+56\deg1400, F8V+F8V) is a well known
eclipsing variable and serves as the prototype for W UMa-type contact
binaries.  Recent observations of this famous star were conducted at
the Villanova Observatory by T. Anselowitz, R. Mittal, M.  Sauer, and
R. Slevinsky on the nights of April 2, 3, 5, 15, and 16 UT, 1997.  A 38
cm Cassegrain reflector equipped with a photoelectric photometer and a
refrigerated EMI 9658 photocell were used.  Over 300 observations were
secured in intermediate-band H$_{\alpha}$w ($\lambda_{max}$ = 6600\AA,
FWHM = 272\AA) and Str\"{o}mgren y ($\lambda_{max} = 5500$\AA, FWHM =
250\AA) filters.  The observation sequence used was the usual {\it
sky-comp.-variable-comp.-sky} routine with an integration time of 20
seconds for each observation.  HD 83728 (K2, m$_{v} = +9.2$) served as
the comparison star.  Corrections for differential extinction were
applied but were always very small because of the close proximity of
the comparison and variable stars.

The compiled light curves for both filters are shown in Figure 1.  The
primary eclipse is about 0.03 mag fainter than the secondary in both
bandpasses.  Also, the maximum at 0.25 phase is about 0.02 mag brighter
in each bandpass than the corresponding maximum at 0.75 phase.  The
presence of starspots as described by Guinan \& Bradstreet (1988) is
the likely cause for these asymmetries in the light curve. A
representative model using system elements from Maceroni \& van't Veer
(1996) and Bradstreet's {\it Binary Maker 2.0} program (1993) is shown
above the observed light curves.  Times of secondary minimum were
obtained on the nights of April 5 and 16 UT, 1997.  The moments of
minimum were calculated using the method of Kwee \& van Woerden (1956)
for both the H$_\alpha$w and y filters, yielding averaged secondary
minimum timings of Min II = HJD 2450543.5667 and Min II = HJD
2450554.5776 for the two nights.

The above photometry of W UMa represents only the most recent
data taken of this star at Villanova.  W UMa has been observed
from the Villanova  Observatory since 1982.  Times of minimum for these
observations are presented for the first time in Table 1.  The large
number of eclipse timings provide an important  opportunity to study
the system's dynamical evolution over the last decade and a half.
Figure 2 shows the observed minus computed (O$-$C) values of the
Villanova data (solid circles) along with other primary minimum timings
found in the IBVS (open circles) since 1982. An ephemeris of
\begin{equation} 
JD\: Hel.\: Min.~I = 2444986.3624 + 0.33363808~\times~E
\end{equation} taken from Hamzaoglu {\it et al.}\ (1982) was used in
calculating the (O$-$C)'s.

%\begin{table}
%\caption{Times of Minimum, March 1982 to March 1995}
\begin{center} 
{\figcap Table 1: Times of Minimum, March 1982 to March 1995}
\vskip5mm
{\small
\begin{tabular}{cccc||cccc}  \hline\hline
Filter&Type&JD Hel. &Observer&Filter&Type&JD Hel. &Observer\\ & &
(+2440000)& & & & (+2440000)&\\ 
\hline 
& & & & & & &\\
 y&I&5042.7469&R.
Donahue&H$_{\alpha}$w &II&8311.5459&K. Miller\\[0.02in]
y&I&5407.7463&D. Speranzini&y&II&8311.5448&"\\[0.02in]
y&II&5407.9145&"&H$_{\alpha}$w &I&9013.6793&B. Abbott\\[0.02in]
H$_{\alpha}$w &I&5753.7242&C. Robinson&y&I&9013.6785&"\\[0.02in] y &I
&6131.7368 &J. Buckley &H$_{\alpha}$w &II &9017.8517 &"\\[0.02in]
H$_{\alpha}$w &I &6521.7569 &S. Carroll &y &II &9017.8511 &"\\[0.02in]
y &I &6521.7590 &" &H$_{\alpha}$w &I &9021.6850 &D. Griffith\\[0.02in]
H$_{\alpha}$w &I &6851.7228 &E. Bergin &y &I& 9021.6860 &"\\[0.02in]
H$_{\alpha}$w &II &6851.8886 &" &H$_{\alpha}$w &I &9398.6945 &T.
Mahler\\[0.02in] y &I &6851.7218 &" &H$_{\alpha}$w &I &9399.6955
&"\\[0.02in] y&II&6851.8887&"&H$_{\alpha}$w &II&9402.8660&"\\[0.02in]
H$_{\alpha}$w &I &7558.6958 &C. Baluta &y &II &9402.8658 &"\\[0.02in] y
&I &7558.6967 &" &H$_{\alpha}$w &I &9423.7183 &J. Marshall\\[0.02in] y
&I &7602.7357 &L. Ilaria &y &I &9423.7178 &"\\[0.02in] H$_{\alpha}$w &I
&7602.7353 &" &H$_{\alpha}$w &I &9465.7523&J. Maley\\[0.02in]
H$_{\alpha}$w &II &7608.5743 &" &y &I &9465.7536 &"\\[0.02in]
y&II&7608.5741&"&y&II&9475.5999&Q. Nguyen\\[0.02in] H$_{\alpha}$w
&II&7990.5871&T. Thrash&H$_{\alpha}$w &II&9478.6011&M. Alexander
\\[0.02in] y&II&7990.5876&"&y&II&9478.6016&"\\[0.02in] H$_{\alpha}$w
&I&8296.6971&K. Miller&Bn&II&9789.5462&N. Morgan\\[0.02in]
y&I&8296.6967&"&Bn&I&9789.7144&"\\ & & & & & & &\\
\hline \end{tabular}
}
\end{center}
%\end{table}



\IBVSfig{6.5cm}{4517-f1.eps}{Str\"omgren y and intermediate band 
H$_\alpha$w light curves of W UMa, obtained on the nights of April
2, 3, 4, 5, 15, and 16 UT, 1997. The representative model at top
was made using {\it Binary Maker 2.0} (Bradstreet 1993). Phases were
computed using Eq.\ 1.}

%\vskip 6.9cm
%\begin{center}
%{\figfon Figure 1.}
%{\figcap Str\"omgren y and intermediate band 
%H$_\alpha$w light curves of W UMa, obtained on the nights of April
%2, 3, 4, 5, 15, and 16 UT, 1997. The representative model at top
%was made using {\it Binary Maker 2.0} (Bradstreet 1993). Phases were
%computed using Eq.\ 1.} 
%\end{center}


Figure 2 shows both a quadratic (dotted line) and linear (solid
line) least squares fit to the (O$-$C) data obtained between 1982
and 1997.  The quadratic (parabolic) fit exhibits a slight negative
concavity, indicating that the period of W UMa may have continuously
decreased during this time interval.  This deviation from a linear fit,
however, is very small and may not be physically meaningful.  The
linear fit to the (O$-$C) data assumes a constant period over the time
interval.  The slope of the linear regression \hbox{($-0.0254\times10^{-4}$}
days) represents the change in the system's period as given in Eq.\ 1
required to eliminate (O$-$C) residuals along the linear fit.  Correcting
the P$_{1} = 0\fday33363808$ period from Hamzaoglu {\it et al.}, the
system's new observed period becomes P$_{2} = 0\fday33363554$.  This
apparent change in period corresponds to a shortening of the period by
0.22 seconds.
\vfil

\IBVSfig{6.5cm}{4517-f2.eps}{(O$-$C) analysis for W UMa from 1982 
through 1997.
The parabolic fit (dotted line) shows a slight negative concavity. The
displayed equation describes the linear (solid line) least-squares fit
to the (O$-$C) residuals.}

\IBVSfig{6.5cm}{4517-f3.eps}{(O$-$C) analysis for W UMa from 1903 through 
1997. The curve prior to 1982 is reproduced from Hamzaoglu {\it et al.}
(1982). It is evident from the overall behavior of the (O$-$C) values that
W UMa has undergone a series of period changes throughout this century.}

%\vskip 6.9cm
%\begin{center}
%{\figfon Figure 2.}
%{\figcap (O$-$C) analysis for W UMa from 1982 through 1997.
%The parabolic fit (dotted line) shows a slight negative concavity. The
%displayed equation describes the linear (solid line) least-squares fit
%to the (O$-$C) residuals.}
%\vskip6.5cm
%{\figfon Figure 3.}
%{\figcap (O$-$C) analysis for W UMa from 1903 through 
%1997. The curve prior to 1982 is reproduced from Hamzaoglu {\it et al.}
%(1982). It is evident from the overall behavior of the (O$-$C) values that
%W UMa has undergone a series of period changes throughout this century.}
%\end{center}

When combined with the eclipse timings presented above, a new
working ephemeris of \begin{equation} JD\: Hel.\: Min.~I =
2450554.7444 + 0.33363554~\times~E \end{equation}
can be presented for the system.  This linear ephemeris
should be accurate enough to predict eclipse timings for the next few
years.

A complete set of visual and photometric data on W UMa has been
available since its discovery by M\"{u}ller \& Kempf (1903).
Hamzaoglu {\it et al.}\ (1982) have published a period study of W UMa
that covered over three quarters of a century, stretching from 1903
through 1982.  In Figure 3, we extend their (O$-$C) plot from 1982 up to
the present continuing to use the ephemeris of Eq.\ 1. The section of
the curve without data points is reproduced from Hamzaoglu {\it et al.}\
(1982).  Villanova timings (solid circles) and other minimum timings
presented in the IBVS (open circles) are shown.

Based on Figure 3, W UMa has apparently undergone a series of
significant period changes since 1903.  Exactly when these
period changes took place, however, is uncertain because of the
sometimes ambiguous nature of  (O$-$C) analysis. The (O$-$C) curve can be
viewed as a series of parabolas of alternating concavity ({\it i.e.},
epochs of continuous increases or decreases in period) or it can be
viewed as a series of straight lines with alternating slopes ({\it
i.e.}, abrupt changes in the apparent period).  Each interpretation
yields different times and rates of mass transfer/loss and magnetic
breaking effects. The uncertainties in eclipse timings make it
difficult to decide which interpretation is physically correct.

Regardless of when these period changes occurred, there still
remains the question of how.  One of the more attractive
explanations to describe this phenomenon is of mass transfer between
the components of the system, causing period changes on the basis of
angular momentum redistribution.  A favored mechanism to drive the mass
flows comes from activity-related phenomena, whose characteristic
time-scales match the relatively rapid period changes evidenced in W
UMa-type systems (Rucinski 1993).  Guinan \& Bradstreet (1988) argue
that the presence of large starspots, as betrayed from light curve
asymmetries, as well as the strong chromospheric and coronal X-ray
emissions typical of W UMa-type binaries, indicate the presence of a
strong, dynamo-related magnetic activity for the system.  These intense
magnetic fields may very well control the mass flows and magnetic
breaking effects responsible for the observed period changes.

Further photoelectric photometry of W UMa will continue at the
Villanova Observatory.  For this research, we utilized the
SIMBAD database, operated by CDS, Strasbourg, France.  This research
was supported in part by NSF grant AST-9315365, which we gratefully
acknowledge.  \bigskip


\references

Bradstreet, D.\ H.\ 1993, Binary Maker 2.0 (Contact Software:
Norristown, PA)

Guinan, E.\ F., \& Bradstreet, D.\ H.\ 1988, in {\it Formation and
Evolution of Low Mass Stars}, ed(s)., A.\ K.\ Dupree \& M.\ T.\ Lago
(Kluwer: Dordrecht) p.\ 345

Hamzaoglu, E., Keskin, V., \& Eker, T.\ 1982, IBVS, No.\ 2102

Kwee, K.\ K., \& van Woerden, H.\ 1956, Bull.\ Astron.\ Neth.\ 12, 327

Maceroni, C., \& van't Veer, F.\ 1996, A\&A 311, 523

M\"{u}ller, G., \& Kempf, P.\ 1903, ApJ 17, 201

Rucinski, S.\ M.\ 1993, in {\it The Realm of Interacting Binary Stars},
ed(s)., J. Sahade, G.\ E.\ McCluskey, Jr.\ \& Y.\ Kondo (Kluwer:
Dordrecht), 111

\endreferences

\end{document}







