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\IBVShead{5155}{9 August 2001}

\IBVStitle{Detection of the Secondary minima in TX UM\lowercase{a}}

\IBVSauth{Kom\v{z}\'{i}k, R.$^1$; Ulla, A.$^2$; Pribulla, T.$^1$; Chochol, D.$^1$; Grygar, J.$^3$}

\IBVSinst{Astronomical Institute of the Slovak Academy of Sciences,
059\,60 Tatransk\'a Lomnica, Slovakia}

\IBVSinst{Universidade de Vigo, Departamento de F\'{\i}sica Aplicada,
\'Area de F\'{\i}sica da Terra, Astronom\'{\i}a e Astrof\'{\i}sica,
\\ \indent Facultade de Ciencias, Campus Lagoas-Marcosende, 36200 Vigo,
Spain}

\IBVSinst{Institute of Physics, Center for Particle Physics, Czech
Academy of Sciences, 182\,21 Prague, Czech Republic}

\SIMBADobjAlias{TX UMa}{HD 93033}
\IBVStyp{ EA } 
\IBVSkey{photometry}

\begintext

TX UMa (HD 93033) is a well known bright ($V_{\rm max} = 7.0$, $V_{\rm
min} = 8.7$) Algol-like eclipsing binary (B8V+F6IV) discovered
independently by R\"ugemer (1931) and Schneller (1931). However, the
photographic times of minima are available since 1903. The $O-C$
diagram (Kreiner et al., 2001) shows that the system exhibits rather
peculiar orbital period variability that is not easy to explain by a
single cause.

The observations of the system are complicated by its orbital period
($P \approx 3\fday063$) which prevents quick coverage of the whole
light curve. Due to the extreme shallowness of the secondary minimum
($\Delta V = 0.06$) and its long duration (9.7 h), the only available
reliable minimum is at HJD 2444616.7811 (Oh \& Chen, 1984).  Its
position relative to the primary minimum is important for the
discussion of possible apsidal motion in the system suggested by Pearce
(1940) and Payne-Gaposchkin (1941).

We present new primary minima times obtained between 1992 and 1998
($UBVR$), the observations of the secondary minima taken in 1994
($JHK$) and in 2001 ($VR$) and discuss the likelihood of proposed
apsidal motion eventually present in the system.

Photoelectric $UBVR$ observations of TX UMa were obtained in 1992-8
and 2001 at the Skalnat\'e Pleso (SP) and Star\'a Lesn\'a (SL)
observatories of the Astronomical Institute of the Slovak Academy of
Sciences. In both cases the 0.6-m Cassegrain telescope equipped with a
single-channel photoelectric photometer was used. The stars HD 92764 =
SAO 43442 ($V = 9.05$, $B = 9.27$, $U = 9.39$, sp.\ type A7) and HD
93213 = SAO 43467 ($V = 7.95$, $B = 8.44$, $U = 8.39$, sp.\ type F5)
served as a comparison and check star, respectively.

Standard data reduction, atmospheric extinction correction and
transformation to the $UBV$ international system were carried out.
Observations in the $R$ passband and observation of the secondary
minimum on February 15/16, 2001 were not transformed to the
international system.

Photoelectric $JHK$ observations of the secondary minimum in 1994 were
obtained with the CVF instrument on the 1.5-m Carlos S\'anchez IR
telescope at the Observatorio del Teide (TO) in Tenerife, operated by
the Instituto de Astrof\'\i sica de Canarias (IAC)\@. Standard data
reduction was performed using software available at the IAC.

Our new SP observations of TX UMa consist of 13 different primary
minima (presented in Table~1), giving 40 individual minima times for
$UBVR$ passbands.  The minima times were determined by parabolic fits
as well as by employing Kwee \& Van Woerden (1956) method. The times
obtained for our secondary minima as determined by the former method
are given in Table~2.

\begin{table}[htb]
\caption{New mean times of the primary photoelectric minima determined from the
$UBVR$ SP observations. The epochs were calculated using ephemeris (1). The errors
are given in parentheses}
\begin{center}
\begin{tabular}{ll|ll|ll}
\hline
 Epoch & JD$_{\rm hel}^{\rm mean}$ & Epoch & JD$_{\rm hel}^{\rm mean}$ & Epoch & JD$_{\rm hel}^{\rm mean}$ \\
\hline
 1038 & 2\,448\,643.4919(1) & 1152 & 2\,448\,992.710(1)  & 1513 & 2\,450\,098.5640(5)  \\
 1039 & 2\,448\,646.5555(4) & 1165 & 2\,449\,032.5337(4) & 1527 & 2\,450\,141.4450(4)  \\
 1053 & 2\,448\,689.4419(6) & 1194 & 2\,449\,121.3700(4) & 1636 & 2\,450\,475.3497(2)  \\
 1150 & 2\,448\,986.5840(1) & 1399 & 2\,449\,749.3480(7) & 1637 & 2\,450\,478.4137(7)  \\
      &                     &      &                     & 1795 & 2\,450\,962.4149(1)  \\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{8.cm}{5155-f1.eps}{$O-C$ diagram for TX UMa}

\begin{table}[htb]
\caption{New times of the photoelectric secondary minima. The epochs were calculated
using ephemeris~(1). The errors are given in parentheses}
\begin{center}
\begin{tabular}{llcll}
\hline
  Epoch  & JD$_{\rm hel}$& Filter & JD$_{\rm hel}^{\rm weighted~mean}$ & Obs. \\
\hline
  1297.5 & 2\,449\,438.40(5)   & $J$ &                     &  TO \\
  1297.5 & 2\,449\,438.42(3)   & $H$ &                     &  TO \\
  1297.5 & 2\,449\,438.42(3)   & $K$ &  2\,449\,438.417(20)&  TO \\[2.mm]
  2119.5 & 2\,451\,956.442(8)  & $V$ &                     &  SL \\
  2119.5 & 2\,451\,956.445(40) & $R$ &  2\,451\,956.442(8) &  SP \\
\hline
\end{tabular}
\end{center}
\end{table}

We combined our 40 primary minima times with other published 61
photoelectric and 27 photographic primary minima times (see e.g., Kreiner et
al., 2001).  The minima were weighted according to their standard errors
(see Kom\v{z}\'{\i}k, 1998). The least-square solution resulted in the
following ephemeris:
\begin{equation}
\begin{array}{r@{}r@{}l}
{\rm Min~I = HJD}~2\,445\,463.797 & {} + 3.063\,291 & {} \times E. \\
                     \pm 2 &     \pm 1 &          \\
\end{array}
\end{equation}

The corresponding $O-C$ diagram is presented in Fig.~1. It is clearly
seen, that 69 primary photoelectric minima times after 1992 can be
approximated well by a parabolic fit with the following ephemeris:
\begin{equation}
\begin{array}{r@{}r@{}r@{}l}
{\rm Min~I = HJD}~2\,445\,463.736 & {} + 3.063\,375 & {} \times E - 2.5 & {} \times 10^{-8} \times E^2. \\
                                 \pm 3 &       \pm 5 &           \pm 2&                    \\
\end{array}
\end{equation}

Our secondary minima observations performed on March 26/27, 1994 and
February 15/16, 2001 are displayed on Figs.~2 and 3, respectively. 
The phases were calculated using the ephemeris given in Eq.~(2).

\IBVSfig{7.5cm}{5155-f2.eps}{The secondary minimum in $J$, $H$, $K$
(full vertical line: minimum, dashed: standard errors)}

\IBVSfig{7.5cm}{5155-f3.eps}{The secondary minimum in $V$, $R$ (full
vertical line: minimum, dashed: standard errors)}

\vskip-11pt
The $O-C$ diagram (Fig.~1) shows that the orbital period variations of
this semi-detached binary are very complex. Long intervals of (almost)
constant period were suddenly interrupted by period jumps, while recent
times of minima suggest a continuous period decrease.

Plavec (1960) and Rovithis-Livaniou et al.\ (1998) found a 34 years
periodicity in the $O-C$ residuals of the primary minima. The authors
suggested apsidal motion as a likely explanation of the observed period
changes. According to Todoran \& Roman (1992) the observed period
changes could be caused by the apsidal motion superimposed on a
light-time effect or strong period variations due to mass exchange.

The reality and amplitude of the apsidal motion can be tested by the
shifts of the secondary minima with respect to phase 0.5, as determined
from the primary minima. Now we have at disposal three independent
minima times obtained in 1981 (Oh \& Chen, 1984), 1994 and 2001
(Table~1). The first one is shifted from phase 0.5 by +0\fday0085, the
second and third one by $-0\fday006$ and $-0\fday005$, respectively.

Thus, it is apparent that apsidal motion alone cannot explain the
observed orbital period changes. Other explanations such as light-time
effect, Applegate's mechanism and mass transfer are still viable.

Detailed photometric and spectroscopic analysis of all available data
necessary to decipher the causes of such behaviour will be presented in a
forthcoming paper.

\bigskip

\noindent
{\bf Acknowledgements:} This work was financially supported by the VEGA grant 
1157, J.G. was partially supported through project No.\ LN00A006 of the
Czech Ministry of education granted to the Center for Particle Physics in
Prague. A.U. is indebted to Dr.~Kidger for his assistance with the obtention
of the $JHK$ data employed in the present work and to the Spanish Ministerio
de Educaci\'on y Cultura for financial support under grant AYA2000-1691.


\references

Kom\v{z}\'{\i}k, R., 1998, PhD Thesis, Astronomical Institute of
the SAS, Tatransk\'a Lomnica

Kreiner, J.M., Kim, C.H., Nha, I.S., 2001, {\it An atlas of $O-C$ diagrams
of eclipsing binary stars}, Wydawnictvo Naukowe AP, Krakow

Kwee, K.K., van Woerden, H., 1956, {\it Bull.\ Astron.\ Inst.\ Nether.},
{\bf 12}, 327
% \BIBCODE{1956BAN....12..327K}

Oh, K.D., Chen, K.Y., 1984, {\it Astron.\ J.}, {\bf 89}, 126
% \BIBCODE{1984AJ.....89..126O}

Payne-Gaposchkin, C., 1941, {\it Harvard Bull.}, {\bf 916}, 2
% Corrected Mon Aug 16 15:51:17 MEST 2004
% Original Payne-Gaposchkin, C., 1941, {\it Harvard Bull.}, {\bf 916}, 2
\IBVSecomment{Error corrected 2004 Aug 16}

Pearce, J.A., 1940, {\it Publ.\ Astron.\ Soc.\ Pacific}, {\bf 52}, 287
% \BIBCODE{1940PASP...52..287P}

Plavec, M., 1960, {\it Bull.\ Astron.\ Inst.\ Czechosl.}, {\bf 11}, 148
% \BIBCODE{1960BAICz..11..148P}

Rovithis-Livaniou, H., Kranidiotis, N.A., Rovithis, P., 1998,
in: {\it Proceedings of the 29th Conference on Variable Star
Research}, J. Du\v{s}ek \& M. Zejda (eds.), Nicolaus Copernicus
Observatory, Brno, p.~175
% \BIBCODE{1998stel.conf..175R}

R\"{u}gemer, H., 1931, {\it Astron.\ Nachr.}, {\bf 242}, 177

Schneller, H., 1931, {\it Astron.\ Nachr.}, {\bf 242}, 180

Todoran, I., Roman, R., 1992, {\it IBVS}, No.\ 3819
% \BIBCODE{1992IBVS.3819....1T}

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