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\IBVShead{4697}{15 April 1999}

\IBVStitle{57 T\lowercase{au} = HD 27397 -- A Spectroscopic Binary}


\IBVSauth{A.\ B.~KAYE}

\IBVSinsto{Los Alamos National Laboratory; X-TA; MS B-220; Los Alamos,
New Mexico 87545; USA\\ e-mail: kaye@lanl.gov}

\SIMBADobj{HD 27397}
\IBVStyp{ F0 IV }
\IBVSkey{spectroscopic binaries}

\begintext

In a recent paper, Papar\'{o} et al.\ (1999) suggest that the
low-frequency variations observed in the bright F0 star 57 Tauri (= HD
27397 = HR 1351) may be due to either duplicity effects, $g$-mode
pulsations (similar to those found in $\gamma$ Doradus stars, see,
e.g., Kaye et al.\ 1999), or a combination of the two.  Their
multi-site photometry, although sufficient for finding the frequencies
associated with these phenomena, was not able to distinguish between them.

To this end, 139 high signal-to-noise ratio
spectra were obtained over fourteen nights at the Kitt Peak National
Observatory during December 1998.  The signal-to-noise ratio (SNR) is
estimated to be over 350.  Each spectrum covers the wavelength region
5840 to 6160 \aa ngstr\"{o}ms and were obtained using grating A, camera
5, and the long collimator.  An OG550 filter was used to block both
higher and lower orders.  Data were recorded on the F3KB CCD; these
spectra have a reciprocal dispersion of 0.105 \aa ngstr\"{o}ms per
pixel, resulting in a resolving power of approximately 35,400.  The
slit width was fixed at 250~$\mu$m, which corresponds to 1.81 seconds
of arc.  The projected slit image was 0.024~mm and covered 1.60
pixels.

Subsequent analysis based on the time series of first moments of
the Fe I $\lambda$6122.226 photospheric line indicates that 57 Tauri is
a spectroscopic binary.  Table 1 presents the preliminary orbital
elements of the system; the solution is plotted as a phased
radial-velocity curve in Figure 1.  The standard deviation of the
orbital fit (noted as $\sigma$ in Table~1) is higher than expected due
to the ongoing pulsations present in 57 Tauri (see Papar\'{o} et
al.\ 1999).
  
Additionally, since this binary has such a short period, at least 
some of the low-frequency variations observed by Papar\'{o} et al.\ (1999) 
are probably due to duplicity (i.e., geometric and proximity) effects.

\vskip 2mm

Acknowledgments:

This work was performed under the auspices of the U.\ S.\ Department of
Energy by the Los Alamos National Laboratory under contract
No.\ W-7405-Eng-36.  These data were obtained with the assistance of
Corinne Neuforge, Joyce Guzik and Tom Beach.

\newpage

\IBVSfig{11cm}{4697-f1.ps}{A phased radial-velocity curve of the primary star (57 Tauri) based on the first moments of the Fe~I $\lambda$6122.226 line.}
\vskip 1cm


\begin{table}[h]
\caption{Preliminary Orbital Elements of 57 Tauri = HD 27397}
\begin{center}
\begin{tabular}{cc}
\hline
Orbital Element &   Value   \\
\hline
$\gamma$ & 40.268 $\pm$ 0.203 km s$^{-1}$\\
$K_{1}$ & 29.906 $\pm$ 0.316  km s$^{-1}$\\
$e$ & 0.028 $\pm$ 0.010 \\
$\Omega$ & $140\fdeg 5 \pm 20\fdeg 4$ \\
HJD of $T_{{\rm periastron}} - 2451100$ & 64.968 $\pm$ 0.144 days \\
$P$ & 2.4860 $\pm$ 0.0017 days\\
Mass Function & 0.0069 $\pm$ 0.0002 $M_{\odot}$ \\
$a \sin i$ & 6831.0 $\pm$ 72.4 AU \\
$\sigma$ & 2.12  km s$^{-1}$\\
\hline
\end{tabular}
\end{center}
\end{table}




\references

Kaye, A.B., Handler, G., Krisciunas, K., Poretti, E., \& Zerbi, F.M., 1999, 
PASP, submitted

Papar\'{o}, M., et al., 1999, A\&A, submitted


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