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\IBVShead{5402}{10 April 2003}

\IBVStitletl{Discovery of a Bright Eclipsing Binary}{ in the Pleiades Cluster}

\IBVSauth{TORRES, G.}

\IBVSinsto{Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Mail Stop 20, Cambridge, MA 02138, USA; e-mail: gtorres@cfa.harvard.edu}

\SIMBADobjAliasJ2000{HD 23642}{HIP 17704}{03 47 29.45 +24 17 18.0}
\IBVStyp{ EWA }
\IBVSkey{photometry}
\IBVSabs{The bright star HD 23642 in the Pleiades cluster has been found to be}
\IBVSabs{an eclipsing binary with shallow eclipses. The discovery is based on}
\IBVSabs{an improved spectroscopic period and the Hipparcos Epoch Photometry.}


\begintext

The bright ($V=6.83$) Pleiades cluster member HD~23642 (Hertzsprung
designation H~III-1431, HIP 17704, 03\hr47\mm29\fsec45 / 
+24\deg17\arcm18\farcs0, J2000,SpT A0V) was the first double-lined 
spectroscopic binary to be
discovered in that cluster, by Pearce (1957) and Abt (1958). The
period is 2.46 days and the orbit is essentially circular. Abt
considered eclipses possible, but this was never confirmed. The
availability of 66 accurate photometric measurements (typical error
0.008 mag) from the Hipparcos mission (ESA, 1997) over a 3-year
interval prompted us to investigate this suggestion, and we indeed
found strong indications of a dip in brightness at about the expected
phase for a secondary eclipse. 

Because of the time elapsed since the spectroscopic measurements by
Pearce (interval 1943-1949) and Abt (Nov-Dec 1957), and the limited
precision of the orbital periods derived independently by the two
investigators, we combined the measurements to obtain a more accurate
ephemeris suitable for predicting eclipses some 35 years into the
future, at the mean epoch of the Hipparcos observations. Weights were
assigned to the photographic radial velocities according to the
information in the original sources, and a velocity offset was allowed
for (but was found to be insignificant).  The result, Min~I~(HJD)~$ =
2,\!434,\!127.6203 (80) + 2.4611240 (90) \times E$, leads to the
folded plot of the Hipparcos data seen in Figure~\ref{fig:fig1}a.  The
5 faintest measurements cluster very close to the predicted time of
secondary eclipse, and are essentially within the uncertainty in that
prediction indicated by the horizontal error bars. 

\IBVSfig{10.5cm}{5402-f1.eps}{Hipparcos epoch photometry for HD~23642. (a)
Data folded with the ephemeris derived from the radial velocities of
Pearce (1957) and Abt (1958). The eclipse phases are indicated with
dotted lines. The formal uncertainty in the phases is indicated by the
horizontal error bars. (b) Data folded with the improved ephemeris
derived by adding our new velocities. The phase uncertainty is
negligible on the scale of this figure.\label{fig:fig1}\vspace*{-3cm}}
%%\IBVSfigKey{5402-f1.eps}{HD 23642}{light curve}

In order to improve the period further, we obtained additional
high-resolution echelle spectra of HD~23642 with the 1.5-m Wyeth
reflector at the Oak Ridge Observatory (Harvard, Massachusetts). The
four single-order spectra cover 45~\AA\ centered at 5187~\AA, and have
a resolving power of $\lambda/\Delta\lambda = 35,\!000$.  We derived
radial velocities for both components using the two-dimensional
cross-correlation technique TODCOR (Zucker \& Mazeh 1994). Typical
uncertainties are 1--1.5 km~s$^{-1}$, far superior to the velocities
from the older sources. Merging these velocities (weighted
appropriately) with those of Pearce and Abt yields a period that is
more than an order of magnitude more precise: Min~I~(HJD)~$ =
2,\!436,\!096.5204 (44) + 2.46113329 (66) \times E$. The spectroscopic
orbit is shown in Figure~\ref{fig:fig2}, and the elements are given in
Table~1. The preliminary light ratio we obtain, at the mean wavelength
of our observations, is $l_B/l_A = 0.31 \pm 0.03$, or $\Delta m = 1.3$
mag. 

\IBVSfig{10.5cm}{5402-f2.eps}{Radial velocity curve of HD~23642, with
the observations by Pearce (1957; open circles), Abt (1958; open
squares), and our new observations (filled symbols). The solid line is
for the primary, and the dashed line for the secondary. The
center-of-mass velocity is indicated by the dotted
line.\label{fig:fig2}}
%%\IBVSfigKey{5402-f2.eps}{HD 23642}{radial velocity curve}

The Hipparcos data folded with this new period are shown in
Figure~\ref{fig:fig1}b.  The apparent dip in brightness is seen to
shift even closer to phase 0.5, which is unlikely to be a coincidence.
The eclipsing nature of the binary is thus confirmed. Further support
for this is given by the large minimum masses (Table~1), in particular
for the primary, which is consistent with the values expected from the
A0V spectral type. 

\vskip 4mm

\centerline{Table 1. Combined spectroscopic solution for HD~23642.}
\vskip 3mm
\begin{center}
\begin{tabular}{lc}
\hline
Parameter & Value \\
\hline
$P$ (days)                    &  2.46113329~$\pm$~0.00000066 \\
$\gamma$ (km~s$^{-1}$)        &  $+$6.1~$\pm$~1.7\phantom{$+$} \\
$K_A$ (km~s$^{-1}$)           &  97.40~$\pm$~0.84\phantom{5} \\
$K_B$ (km~s$^{-1}$)           &  140.47~$\pm$~0.85\phantom{16} \\
$e$                           &  0 (adopted) \\
$T_{\rm max}$ (HJD)           &  2,436,095.9051~$\pm$~0.0040\phantom{2,450,45} \\
\noalign{\vskip 10pt}
$a_A \sin i$ ($10^6$~km)      &  3.296~$\pm$~0.030 \\
$a_B \sin i$ ($10^6$~km)      &  4.754~$\pm$~0.030 \\
$a \sin i$ (R$_{\odot}$)      &  11.566~$\pm$~0.061\phantom{1} \\
$M_A \sin^3 i$ (M$_{\odot}$)  &  2.027~$\pm$~0.032 \\
$M_B \sin^3 i$ (M$_{\odot}$)  &  1.405~$\pm$~0.026 \\
$q\equiv M_B/M_A$             &  0.6934~$\pm$~0.0077 \\
\noalign{\vskip 10pt}
$\Delta RV$ (Pearce $-$ Abt)  (km~s$^{-1}$)        &  $+$1.9~$\pm$~1.9\phantom{$+$} \\
$\Delta RV$ (Pearce $-$ new)  (km~s$^{-1}$)        &  $+$0.8~$\pm$~1.8\phantom{$+$} \\
N$_A$, N$_B$ (Pearce)         &  21, 19 \\
N$_A$, N$_B$ (Abt)            &  15, 12 \\
N$_A$, N$_B$ (new)            &  4, 4 \\
$\sigma_A$, $\sigma_B$ (Pearce) (km~s$^{-1}$)      &  8.9, 20.8 \\
$\sigma_A$, $\sigma_B$ (Abt) (km~s$^{-1}$)         &  3.1, 17.8 \\
$\sigma_A$, $\sigma_B$ (new) (km~s$^{-1}$)         &  1.68, 1.01 \\
\hline
\end{tabular}
\end{center}

\vskip 4mm

As seen in Figure~\ref{fig:fig1}b, there are no Hipparcos observations
near phase 0.0, and thus the primary eclipse was missed.  A very
tentative fit to the secondary eclipse performed with the light curve
program EBOP along with estimates of the components' properties is
shown in Figure~\ref{fig:fig3}.  While the secondary eclipse has a
depth of $\sim$0.03~mag, the primary is expected to be approximately
0.07~mag deep. The eclipses are thus grazing ($i \approx 78^{\circ}$),
and the system is well detached. 

HD~23642 appears to be the first eclipsing binary to be confirmed in
the Pleiades cluster\footnote{A report by Prosser \& Stauffer (1993)
about an anomalous dimming in the light of the Pleiades member
H~III-263, possibly due to eclipses, has yet to be confirmed.}, and it
is quite remarkable that such a bright star has not been inspected
carefully before, particularly since it is known to be chemically
peculiar (spectral classification A0Vp(Si)+Am; Abt \& Levato 1978).
Further spectroscopic and photometric observations of HD~23642 are
underway to improve the orbits.  Eventually this system may well yield
the first dynamically determined stellar masses in the Pleiades, and
may even have a bearing on the issue of its distance. 
	
\clearpage

\IBVSfig{13cm}{5402-f3.eps}{Tentative EBOP light curve superimposed
on the Hipparcos epoch photometry of HD~23642.\label{fig:fig3}}
%%\IBVSfigKey{5402-f3.eps}{HD 23642}{light curve}

{\it Acknowledgements:} This research has made use of the SIMBAD database,
operated at CDS, Strasbourg, France, and of NASA's Astrophysics Data
System Abstract Service. The author acknowledges partial support for
this work by NASA's SIM project (JPL grant 1240033).

\references

Abt, H.\ A., 1958, {\it ApJ}, {\bf 128}, 139 
%%\BIBCODE{1958ApJ...128..139A}

Abt, H.\ A., \& Levato, H., 1978, {\it PASP}, {\bf 90}, 201
%%\BIBCODE{1978PASP...90..201A}

ESA 1997, {\it The Hipparcos and Tycho Catalogues}, ESA SP-1200

Pearce, J.\ A., 1957, {\it Publ. Dom. Astr.\ Obs.}, {\bf 10}, 435
%%\BIBCODE{1957PDAO...10..435P}

Prosser, C.\ F., \& Stauffer, J.\ R., 1993, {\it IBVS}, 3826
%%\BIBCODE{1993IBVS.3826....1P}

Zucker, S., \& Mazeh, T., 1994, {\it ApJ}, {\bf 420}, 806
%%\BIBCODE{1994ApJ...420..806Z}

\endreferences

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