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\IBVShead{5617}{22 March 2005}

\IBVStitletl{XY P\lowercase{ic}: a Detached Binary Misclassified}{ as a W UM\lowercase{a} System}


\IBVSauth{Dall, T. H.}

\IBVSinsto{European Southern Observatory, Casilla 19001, Santiago 19, Chile}

\GCVSobj{XY Pic}
\SIMBADobjAlias{XY Pic}{HD 38873}
\IBVStyp{ DSCT }
\IBVSkey{spectroscopy}

\IBVSabs{XY Pic was recently included in a list of genuine EW (W UMa type}
\IBVSabs{binaries) variables. We show from spectroscopy that XY Pic is not a}
\IBVSabs{W UMa star, but rather a delta Scuti}
\IBVSabs{variable, possibly member of a wide binary system.}


\begintext

\vspace*{-1cm}

Recently Selam (2004) published a list of 64 genuine W UMa-type binaries observed by \emph{HIPPARCOS}. These had been extracted from a larger
sample based on theoretical fitting of their light curves, using the simplified light curve synthesis by Rucinski (1993),
yielding the fundamental system parameters.  During the selection process, detached and semi-detached systems were excluded.

We here report on observations of one of the supposedly genuine W UMa or EW-type systems, XY~Pic\ (HD~38873), which we
show is not an EW system. The \emph{HIPPARCOS}\ light curve
is shown in Fig.~1 (taken from the on-line \emph{HIPPARCOS}\ catalogue; ESA 1997). The light curve resembles somewhat an EW system,
although with some asymmetry. In the GCVS (e.g. Kazarovets et al.~2003) it is also listed as an EW type variable.

\IBVSfig{7.0cm}{5617-f1.eps}{The HIPPARCOS light curve of XY Pic from ESA (1997).}
\IBVSfigKey{5617-f1.eps}{XY Pic}{light curve}

We observed XY~Pic\ on February 5 and 7, 2005 using the FEROS spectrograph at the ESO/MPI-2.20m telescope at La Silla, Chile.
Standard data reduction was performed with MIDAS
including bias and flatfield correction, order extraction and wavelength calibration.
The spectra have a FWHM resolution of 0.15~\AA\ (R$\sim$48000) and cover the 
range 3800--9000~\AA .  

In Fig.~2 we show a part of the three spectra, labeled by modified Julian date, and with the rest-wavelength of the
prominent Ca\,I line at 6162.173~\AA\ indicated.  As can be seen, the star is not a fast rotator as expected for an EW system,
and also the period $P=0.2972608$~d$^{-1}$ is incompatible with the observed spectral line shift. The star may indeed be a binary,
as indicated by the shift of the lower spectrum, but with a period much larger than typical EW-type periods.


\IBVSfig{11.5cm}{5617-f2.eps}{A part of the three spectra of XY Pic, including the 6162.173 Ca\,I line (dashed line).}
\IBVSfigKey{5617-f2.eps}{XY Pic}{spectrum}


\begin{table}[!hb]
\caption{\label{tab:results}The atmospheric parameters and the derived abundances for XY~Pic.  Errors on abundances
are rms around the mean for all lines used. 
$n$ is the number of lines of each element. Abundances are given relative to solar values, i.e. 
A(M) = [M/H] = $\log N_{\rm M}/N_{\rm H} - (\log N_{\rm M}/N_{\rm H})_\odot$.
}
\begin{center}
\begin{tabular}{ll}
 \noalign{\smallskip}
\hline
\hline
 $T_{\rm eff}$ &  $6900 \pm 100$~K \\
 $\log g$      &  $3.80 \pm 0.15$ \\
 $\xi_t$       &  $1.6 \pm 0.2$~km\,s$^{-1}$ \\
 \noalign{\smallskip}
A(Fe) & $-0.05(10)$, $n=236$ \\
 \noalign{\smallskip}
A(Ca) & $+0.04(6)$, $n=12$  \\
A(Cr) & $+0.18(18)$, $n=38$ \\
A(Ni) & $-0.17(11)$, $n=47$ \\
A(Ti) & $+0.10(11)$, $n=24$ \\
 \noalign{\smallskip}
\hline
\hline
\end{tabular}
\end{center}
\end{table}


Using the procedures described in Dall et al.~(2005a, 2005b) we performed an abundance analysis following the fitting of an
atmospheric Kurucz (1993) model using 236 Fe\,I and Fe\,II lines. The results are listed in Table~\ref{tab:results}.
Using the list of global stellar parameters by Gray (1992) we find that the atmospheric parameters correspond to 
a spectral type of F0III (SIMBAD quotes F0III/IV).

We propose that XY~Pic\ may be a wide binary with an unseen lower-mass companion, and that the \emph{HIPPARCOS}\ light curve can
be explained by $\delta$~Scuti type pulsation, which would be consistent with the observed period, the amplitude of the
variation and with the spectral type.

This example demonstrates that even the most careful light curve analysis is not enough to 
determine the true variability nature of an object, and that spectroscopy is needed to secure a
positive identification.

\vskip 1cm

\references

Dall, T.H., Schmidtobreick, L., Santos, N.C., Israelian, G., 2005a, {\it A\&A}, submitted

Dall, T.H., Bruntt, H., Strassmeier, K.G., 2005b, {\it A\&A}, in preparation

ESA 1997, The Hipparcos and Tycho Catalogues, ESA SP-1200

Gray, D.F., 1992, {\it The observation and analysis of stellar photospheres} (CUP, Cambridge) 
%%\BIBCODE{1992oasp.book.....G}

Kazarovets, E.V, Kireeva, N.N., Samus, N.N., Durlevich, O.V., 2003, {\it IBVS}, {\bf 5422} 
%%\BIBCODE{2003IBVS.5422....1K}

Kurucz, R., 1993, {\it Kurucz CD-ROM No.~13} (Cambridge, Mass.: SAO)

Rucinski, S.M., 1993, {\it PASP}, {\bf 105}, 1433 
%%\BIBCODE{1993PASP..105.1433R}

Selam, S.O., 2004, {\it A\&A}, {\bf 416}, 1097 
%%\BIBCODE{2004A&A...416.1097S}

\endreferences



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