\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}

\IBVShead{4692}{7 April 1999}

\IBVStitle{A transit of the planet 51 P\lowercase{eg} B?}

\IBVSauth{K. Krisciunas}

\IBVSinsto{Department of Astronomy, University of Washington, Box 351580,
Seattle, WA 98195, USA, \\ e-mail: kevin@astro.washington.edu}

\SIMBADobj{51 Peg}
\IBVStyp{ EB }
\IBVSkey{Photometry}
\IBVSabs{We present some V-band photometry of 51 Peg, obtained in}
\IBVSabs{September of 1995, which is consistent with a transit of its}
\IBVSabs{planet across the star.}

\begintext

``We see it [a probable new planet] as Columbus saw America from the
shores of Spain.  Its movements have been felt, trembling along the
far-reaching line of our analysis, with a certainty hardly inferior to
that of ocular demonstration.'' $-$ John Herschel, in a speech to the
British [Astronomical] Association, 10 September 1846.\IBVSfm

\IBVSfoot{Quoted by Spencer Jones (1956).  Of course, Herschel was
referring to Neptune, which in fact was first seen by J. G. Galle and
H. L. d'Arrest in Berlin 13 days later.}

Mayor \& Queloz (1995) first announced that the solar-type star 51 Peg
has a Jovian-mass planet with a 4.23 day orbital period.  Their
observations were confirmed by Marcy et al.\ (1997).  Since this
exciting discovery many people naturally have wondered if transits of
the planet could be observed across the disk of the parent star.
Guinan (1995) and, later, Henry et al.\ (1997) reported null events.
They could not find evidence of transits.  However, Henry et al. show
only one, possibly two, averages-of-three data points in the transit
window.  Under the assumption that the planet has a zero-eccentricity
orbit, has a size equal to 0.8 times that of Jupiter, and assuming that
the star has a size 1.3 times that of the Sun and a mass of 1.0
M$_{\odot}$, we would expect a full transit to be 4.3 mmag deep, last
for 4 hours, and be centered at phase 0.25 (see Henry et al.\ 1997).
Any {\em reflection effect} at other phases, or an eclipse at phase
0.75, would change the light of the system less than 0.1 mmag
(Charbonneau, Jha, \& Noyes 1998).  Incidentally, if the planet is 0.8
times the size of Jupiter, there will be full transits if the orbital
inclination is within 6.3 degrees of $i = 90\deg$.

Guillot et al.\ (1996) modelled Jupiter-like planets and suggest that
because 51~Peg~A is 8.5 Gyr old (Edvardsson et al.\ 1993), 51~Peg~B may
actually be as large as 1.3 times that of Jupiter.  In that case a
transit of the planet could produce a dip in the light curve as deep as
11 mmag.  This, of course, assumes that a Jovian planet so close to its
parent star (0.051 AU) still has its thick atmosphere.

Zerbi and 16 coauthors (1999), of which I am one, recently published
the results of a multi-longitude photometric campaign on the $\gamma$
Dor-type star BS 8799.  The principal comparison star was HD~217715.
The recommended check star was HD~218261.  For reasons that I cannot
quite recall, I decided to use a different check star, 51 Peg.  This
was two months {\em before} Mayor \& Queloz announced the existence of
the planet 51~Peg~B\@.  From 1210 Johnson V-band and Str\"{o}mgren
$v$-band measures of HD~218261 vs. HD~217715 over the course of 40 days
Zerbi et al. conclude that both stars are constant.  This allows us to
investigate whether the 51 Peg vs.\ HD~217715 data show evidence of
transits of 51~Peg~B across its parent star.

\newpage

\IBVSfig{9cm}{4692-f1.ps}{
Differential measures of 51 Peg vs.\ HD~217715, obtained at Mauna Kea
from 31 August to 6 September 1995 UT\@.  The data are folded according
the ephemeris of Marcy et al.\ (1997).  Points without error bars are
individual differential measures.  Points with error bars are averages
of 3 or 4 measures.  A dip of 4.3 mmag is shown, corresponding to a
full transit of a planet with ${\rm R = 0.8\,R_{Jup}}$.}

\IBVSfig{9cm}{4692-f2.ps}{
Data from Fig.~1, but only near the time of ``inferior conjunction'' of 51~Peg~B.}

\newpage

Our observations were made with the University of Hawaii 0.6-m telescope
at Mauna Kea from 31 August to 6 September 1995 UT, using an Optec SSP-5
photometer.  The only night we observed near orbital phase 0.25 was 3
September 1995.  From data on 6 of 7 nights we obtained 123 differential
measurements under clear sky conditions with no tracking problems.  Each
``measurement'' of a star was the result of two 30 second integrations.
Each measurement of 51 Peg was bracketed by measurements of HD~217715. We
linearly interpolated the comparison star counts by time for the
calculation of the differential magnitude of 51 Peg, and made what
we believe were appropriate differential extinction corrections.  We
added $\varepsilon _v \times \Delta{\rm (B-V)} = -0.0253 \times 0.67 = 
-0.0170$ to all the data points to place them on the UBV system.  Any
systematic error in this color term has no bearing on the conclusions
of this paper.

In Figure 1 we show the data folded according to the ephemeris of Marcy
et al.\ (1997). We also show the predicted dip of 4.3 mmag centered at
orbital phase 0.25 and lasting 4 hours.  The dots in Fig.~1 are
individual differential measures, and the points with error bars are
averages of 3 or 4 differential measures.  From 111 differential
measures outside the transit window (phase 0.23 to 0.27) we find a mean
differential magnitude of $\Delta{\rm V} = -1.3740 \pm 0.0004$.  The
internal error of a typical individual differential value is $\pm$ 3.8
mmag.  Thus, an average of three measures should typically be accurate
to $\pm$ 2.2 mmag.

In Figure 2 we show only the data near phase 0.25, when transits of the
planet should occur, if in fact transits {\em do} occur. The first five
points plotted are averages of three individual differential measures,
and the last two points plotted are averages-of-four.  Clearly, there
is no 11 mmag dip.  Thus, if Guillot et al.\ (1996) are correct, that
51~Peg~B has ${\rm R = 1.3\,R_{Jup}}$, full transits do not
occur.  If we average the 12 individual measures that occur in the
transit window, we get $\Delta{\rm V} = -1.3714 \pm 0.0019$.  Our data,
taken at face value, indicate a dip of $2.6 \pm 1.9$ mmag.  The
probability of observing a 1.37-$\sigma$ {\em dip} at just the right
phase corresponds to the area under {\em one} tail of the unit-area
Gaussian distribution.  A random sample of points, with errors
distributed in a Gaussian manner, would show this dip with a
probability of 0.085.

Our data are consistent with there being no transit, but the data are
not inconsistent with a 4.3 mmag transit of a 0.8\,R$_{\rm Jup}$ planet
having been observed.  Clearly more data are warranted.  We should be
encouraged that ground-based or satellite observations will sooner or
later reveal transits of extra-solar planets.

\smallskip
{\it Acknowledgments\/}: I thank the University of Hawaii Institute for
Astronomy for telescope time; the Joint Astronomy Centre for observing
support; and Richard Crowe of UH Hilo for loan of their photometer.
The data of 4--6 September 1995 were obtained with the assistance of
Mavourneen Roberts.  I also thank Guillermo Gonzalez, Ed Guinan, Artie
Hatzes, and Greg Henry for useful comments and discussions.


\references

Charbonneau, D., Jha, S., and Noyes, R. W., 1998, {\it ApJ}, {\bf 507},
L153

Edvardsson, B., Anderson, J., Gustafsson, B., Lambert, D. L., Nissen,
P. E., and Tomkin, J., 1993, {\it A\&A}, {\bf 275}, 101

Guillot, T., Burrows, A., Hubbard, W. B., Lunine, J. I., and Saumon, D.,
1996, {\it ApJ}, {\bf 459}, L35

Guinan, E. F., 1995, IAU Circular, No.\,6261

Henry, G. W., Baliunas, S. L., Donahue, R. A., Soon, W. H., and Saar, S.
H., 1997, {\it ApJ}, {\bf 474}, 503

Marcy, G. W., Butler, R. P., Williams, E., Bildsten, L., Graham, J. R.,
Ghez, A. M., and Jernigan, J. G., 1997, {\it ApJ}, {\bf 481}, 926

Mayor, M., and Queloz, D., 1995, {\it Nature}, {\bf 378}, 355

Spencer Jones, H., 1956,  ``John Couch Adams and the discovery of
Neptune,'' in James Newman, ed., {\it The World of Mathematics},
New York: Simon and Schuster, vol.\ 2, pp.\ 823--839

Zerbi, F. M., et al., 1999, {\it Monthly Notices Royal Astr.\ Soc.}, 
{\bf 303}, 275

\end{document}
