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\IBVShead{6012}{7 February 2012}

\IBVStitle{Light curve modeling of short-period W~UM\lowercase{a}-type stars}

\IBVSauth{Pilecki, B.$^{1,2}$; St\c epie\'n, K.$^1$}

\IBVSinst{Warsaw University Observatory, Warsaw, Poland. 
e-mail: pilecki@astrouw.edu.pl; kst@astrouw.edu.pl}
\IBVSinst{Universidad de Concepci\'on, Departamento de Astronomia, Casilla 160-C, Concepci\'on, Chile. \\
e-mail: bpilecki@astro-udec.cl}

\IBVStyp{Binaries: eclipsing}
\IBVSkey{photometry}

\IBVSabs{The results of light curve modeling of 15 W UMa-type stars with}
\IBVSabs{periods shorter than 0.3 d, detected within the ASAS program, are}
\IBVSabs{presented. All stars belong to W-type and have moderate mass ratios,}
\IBVSabs{in agreement with the predictions of theoretical evolutionary models.}


\begintext

We report the results of the curve modeling of a selected set of W UMa-type
variables. They all belong to the low mass contact binaries (LMCB) which
have orbital periods shorter than about 0.3 d, the total mass lower than
about 1.4 M$\odot$ mass, moderate mass ratios and radii indicating that both
components are on the main sequence (St\c epie\'n and Gazeas 2012). As a rule,
they belong to W-type variables, where the more massive component is cooler
than its companion. According to Rucinski (2007) LMCBs make a majority of
all W UMa-type stars existing in the solar neighbourhood, yet the knowledge
of their properties is very limited. Accurate values of the binary
parameters, based on the precise photometric and spectroscopic
observations, are known for a mere dozen LMCBs (Gazeas and St\c epie\'n
2008). The reason for the shortage of good data is, of course, the
intrinsic faintness of these stars.

Evolutionary models of cool contact binaries have been obtained by St\c
epie\'n (2006, 2011) under the assumption that they originate from detached
binaries with the initial periods close to 2 d and angular momentum loss
(AML) by the magnetized winds occurs during the whole evolution. The
results indicate that LMCB are old MS objects with a characteristic age of
8-9 Gyr, but their lifetime in contact takes only about 10\% of their total
age. The orbit evolution of a contact binary is determined by the mass
transfer from the presently less massive component to its companion and AML
by the wind. In LMCBs the AML dominates, so the orbit contracts quickly and the
binary overflows its outer critical surface, which leads to coalescence of
both components. The mass ratio never reaches the extreme value of
0.1 or less.

All the variables presented in this paper have been discovered during the
ASAS survey (Pojma\'nski 2002). They were selected for the further
investigation because they were bright and there were high quality, two-color
light curves available for them.
The light curve modeling of our stars was a part of a larger project
(Pilecki 2009) in which almost 3000 stars were modeled using Monte Carlo
approach and the Wilson-Devinney code (Wilson and Devinney 1971, 1973) in order
to better undestand the nature of contact binaries in particular and the evolution
of compact binaries in general.

In this project light curves for systems that exhibit period change were cleared
from this influence, we also subtracted all long-term brightness variation.
Both actions were necessary because of the long time span of collected data.
To make analysis of this amount of stars feasible we have varied the smallest
possible subset of parameters including temperature ratio, modified potentials
of both components and inclination. The temperature of the hotter component was
based on the $V-I$ index and was set fixed. Keeping in mind that the photometric
mass ratios are quite inaccurate (except for binaries with total eclipses) we
have used a set of 12 predefined values spread logarithmically from 0.09 to 4.0 --
the step in $q$-value roughly corresponding with the average accuracy that
was expected for these data.
The light curves were solved for this set and a best solution was then accepted.

Nevertheless we have crosschecked our values of $q$ for about 50 stars
with those already established
in the literature and found that they seldom differ significantly for short-period
compact systems presented here, which is rather expected as for such
systems mass ratio manifests itself in the shapes of components and thus in the
shapes of the light curves. This is not true for well resolved systems.
Our results show also that the $q$-values are indeed concentrated around the
value 0.5, the property noticed already by Rucinski (2010).
Because of the performed Monte Carlo analysis we were able to determine values
and their reliable errors for all the other parameters.

Table 1 presents the data for the investigated
stars. The consecutive columns give ASAS designation (with the
coordinates for 2000), ephemeris, apparent $V$ magnitude, $V$-amplitude of the
light curve, $V-I$ index, temperature of the hotter (but less massive)
component, temperature ratio, orbit inclination $i$ and mass ratio $q=M_1/M_2$.
For the temperature ratio and the orbit inclination $1-\sigma$ errors in
the last two digits are quoted in the parentheses.

Spectroscopic observations of these stars are urgently needed. They would
permit calculations of the absolute parameters of the binaries and checking
their values against model calculations. In particular, it will be possible
to determine the progenitor binaries and evolutionary paths of LMCB.

\begin{table*}
\caption{The data for the new, short-period W~UMa-type variables}
\center
\small
\begin{tabular}{llrlccccc}
\hline
     ASAS  &  Ephemeris &    $V_{\rm{app}}$ & $\Delta V$ & $V-I$ &  $T_2$ &
     $T_1/T_2$ &  $i$ & $q$\\
   design. &   HJD($T_0$)=2450000+ & mag&  mag&  mag&   K &&        deg &  \\
\hline
033959+0314.5 &1920.623+0.282709$\times E$ & 11.33 & 0.87 &0.99 & 4830 &0.964 (12) &84.5 (1.4) & 0.71\\
050837+0512.3 &2496.963+0.266349$\times E$ & 11.06 & 0.66 &1.09 & 4550 &0.955 (16) &73.8 (1.2) & 0.71\\
050852+0249.3 &1952.830+0.295706$\times E$ & 11.25 & 0.7 & 0.95 & 4950 &0.940 (12) &83.5 (4.3) & 0.35\\
052452-2809.2 &1868.922+0.275778$\times E$ & 10.61 & 0.37 &0.76 & 5510 &0.964 (20) &68.5 (1.4) & 0.25\\
061531+1935.4 &2621.829+0.287842$\times E$ & 11.06 & 0.59 &0.83 & 5300 &0.982 (16) &81.3 (4.7) & 0.35\\
085710+1856.8 &2622.863+0.291015$\times E$ & 11.23 & 0.61 &0.91 & 5060 &0.957 (15) &78.2 (2.1) & 0.35\\
095048-6723.3 &1869.068+0.276944$\times E$ & 11.13 & 0.74 &1.12 & 4480 &0.977 (09) &84.3 (3.0) & 0.50\\
120036-3915.6 &1871.020+0.292670$\times E$ & 10.45 & 0.5 & 0.90 & 5100 &0.974 (09) &80.8 (1.9) & 0.25\\
143751-3850.8 &1903.125+0.292395$\times E$ & 11.35 & 0.79 &1.14 & 4460 &0.949 (12) &78.1 (1.5) & 0.50\\
144243-7418.7 &1903.86+0.294768$\times E$   & 8.25 & 0.45 &0.77 & 5460 &0.951 (11) &72.1 (1.2) & 0.25\\
155227-5500.6 &1926.952+0.297646$\times E$ & 11.45 & 0.33 &0.93 & 5020 &0.977 (35) &55.8 (5.4) & 0.71\\
155906-6317.8 &1920.924+0.266766$\times E$ & 10.47 & 0.71 &1.03 & 4710 &0.949 (07) &78.4 (0.8) & 0.50\\
174655+0249.9 &2159.601+0.299953$\times E$   &9.57 & 0.25 &0.95 & 4950 &0.926 (30) &55.9 (1.8) & 0.35\\
195350-5003.5 &1874.706+0.286828$\times E$ & 11.44 & 0.95 &1.10 & 4520 &0.935 (10) &83.9 (1.2) & 0.71\\
212915+1604.9 &2754.934+0.282956$\times E$ & 11.33 & 0.61 &1.03 & 4720 &0.981 (12) &88.3 (3.6) & 0.35\\
\hline
\end{tabular}
\end{table*}

{\bf Acknowledgements.}
This research was supported by the grant N203 304335 from the Polish Ministry of Science and Higher Education. Support from the BASAL Centro de Astrofisica y Tecnologias Afines (CATA) PFB-06/2007 and TEAM subsidies of the Foundation for Polish Science (FNP) is also acknowledged.

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