\documentstyle[twoside, epsf]{article}
\input ibvs2.sty
\begin{document}

\IBVShead{5575}{12 November 2004}

\IBVStitle{Detection of the Rotational Period of HD~179949?}

\IBVSauth{Wolf, M.$^{1}$; Harmanec, P.$^{1,2}$}

\IBVSinst{Astronomical Institute, Charles University Prague,
 V Hole\v{s}ovi\v{c}k\'ach 2, CZ-180 00 Praha 8,
 Czech Republic, \\
  e-mail: {\tt wolf@mbox.cesnet.cz}}

\IBVSinst{Astronomical Institute, Academy of Sciences,
  CZ-251 65  Ond\v{r}ejov, Czech Republic}

\SIMBADobjAlias{HD 179949}{HR 7291}

\IBVSabs{HD 179949 has brightness variation  }
\IBVSabs{with a period of 7.06549 days.      }

\IBVStyp{    }

\IBVSkey{photometry}
\IBVSkey{stars: rotation}

\begintext
\vspace*{-0.7cm}

HD~179949 (HR 7291, HIP 94645, Spectral class F8-F9) was reported
to be a 51~Peg-type star having a planetary companion with a minimum mass
of $0.84 \pm 0.05$ M$_{\rm Jupiter}$ orbiting in a slightly eccentric
orbit $e = 0.05$ with the period of 3.093 days (Tinney et al. 2001).

Recently, Shkolnik (2004) confirmed the planetary orbit. She detected
sinusoidal radial-velocity variations of HD~179949 with the 3.093-d
period and a full amplitude of about 250 ms$^{-1}$. She also reported
the modulation of the strength of the
Ca~II~K emission of HD~179949 with the orbital period of the planetary
companion and interpreted it as a convincing case of planet-induced
activity. She pointed out the need for determination of the
rotational period of HD~179949.

We attempted to detect the light variations of HD~179949.
Our new photoelectric $UBV$ observations were obtained with
the modular photometer utilizing a Hamamatsu EA1516
photomultiplier attached to the 0.5-m telescope at the Sutherland site
of the South African Astronomical Observatory (SAAO) during
two weeks in April/May 2004.
The photoelectric measurements were carried out by MW through the
$UBV$ filters of the Johnson's photometric system with 10-second
integration time. All observations were carefully reduced to the
Cousins E-region standard system (Menzies et al. 1989)
and corrected for differential extinction using the
reduction program HEC~22 rel. 14 (Harmanec \& Horn 1998).
The standard errors of these measurements were about 0\fmm009,
0\fmm007 and 0\fmm006
in $U, B$ and $V$ filters, respectively.
The nearby star 42~Sgr (also HR 7292, HIP 94643, Sp. K0), the constancy
of which was verified from its Hipparcos $H_p$ photometry
(Perryman et al. 1997) served as the only comparison star. (Since the
observations were carried out along with another observing program,
we regrettably did not observe any check star.)
Extinction and its mild time variability during each the night were
taken into account, although the effect for differential photometry
was small in this particular case. We derived the following very accurate
mean all-sky values of $UBV$ magnitudes and colour indices of the comparison
42~Sgr

\begin{center}
$V$ = 4\fmm860 $\pm$ 0\fmm013,
  $B-V$ = 0\fmm566, $U-B$ = 0\fmm317
\end{center}

\noindent and of HD~179949:

\begin{center}
$V$ = 6\fmm237 $\pm$ 0\fmm012,
  $B-V$ = 0\fmm550, $U-B$ = 0\fmm061\,.
\end{center}

\noindent
The differential photometry of HD~179949 relative to 42~Sgr gives
exactly the same values
as the above quoted all-sky values, which indicates that the reduction
to the standard system was reliable.

\IBVSfig{8cm}{5575-f1.eps}{Normal points of SAAO $V$ photometry of HD 179949.}
\IBVSfigKey{5575-f1.eps}{HD 179949}{light curve}

\IBVSfig{8cm}{5575-f2.eps}{Individual SAAO ($\bullet$) and Hipparcos $V$ ($\circ$)
observations of HD 179949 plotted vs. phase of the 7.06549-d period.}
\IBVSfigKey{5575-f2.eps}{HD 179949}{light curve}

\vskip -3mm

We also transformed the $H_p$ Hipparcos photometry (Perryman et al. 1997)
to Johnson $V$ magnitudes using the colour indices derived by us
and the transformation formula derived by Harmanec (1998)
and obtained $V = 6\fmm243 \pm 0\fmm007$,
in an excellent agreement
with the SAAO result. Having such accurate determination, we used the
Hipparcos parallax and the above $V$ magnitude estimates, bolometric
correction after Code et al. (1976) and Popper's (1980) $T_{\rm eff}$
scale to obtain the stellar radius:

$R = 1.25 \pm 0.03 R_{\odot}$  for F8 (SIMBAD database), or

$R = 1.30 \pm 0.03 R_{\odot}$  for F9 (Groot et al. 1996).

\noindent
According to our experience, the $V$ band photometry is the most accurate
(lower extinction, small colour transformation terms, flat energy distribution)
for detection of any brightness changes. The SAAO $V$ photometry
(normal points consisting of 3-5 individual observations for each night)
is presented in Fig. 1. It seems to show
smooth variation with a possible time scale of about 7 days. We also
carried out a Stellingwerf (1978) period analysis of Hipparcos photometry
from 20 days down to 0.5 days (with the standard (5,2) bin/cover structure)
to find out independently a significant period of 7.07-day. Combining both
data sets and allowing for the small zero point shift, we found finally
a period of $7.06549 \pm 0.00061$ days from a sinusoidal fit to the
combined data set. The phase plot in Fig.~2 is based on all SAAO and
transformed Hipparcos {\sl individual} $V$ observations.

With the above derived radius and identifying the 7.065-day period
with the stellar rotational period, one obtains the equatorial rotational
velocity of HD~179949 of about 9.3 km/s.
Groot et al. (1996) derived an accurate value of the projected
rotational velocity of HD~179949: $v \sin i = 6.3$ km/s.
This would imply an inclination of rotational axis of HD~179949
of about 45\deg. This is a very plausible value since
such intermediate inclinations are usually most favourable to
the detection of rotation-associated phenomena, both in the spectra
and in photometry.


We therefore tentatively conclude that the 7.06549-d period is
indeed the rotational period of the star. No doubt, however,
that more photometric observations in excellent observing
conditions are needed to verify our result. The same is actually
true also for the Ca~II flux observations of Shkolnik (2004)
although the phase dependence on the planetary period seems
quite impressive. She claims that the PDM Stellingwerf period
search detected a period near 3 days as the best one in her
Ca~II~K data but does not mention which bin/cover structure
she used. We found that using different bin/cover structures
like (5,2), (4,3) or (4,4) we detected different possible
periods as the best ones, never a period near 3 days. A very
good phase diagram was found, for instance, for a period
of 1.42079 days but this can be a fortuitous coincidence only.
Plotting the Ca~II flux vs. phase of the 7.065-d period does not give
a convincingly looking phase diagram.

\bigskip
{\bf Acknowledgements} This investigation was supported by
from the research plan J13/98: 113200004 of Ministry of Education,
Youth and Sports. The research of MW was also supported by
the Grant Agency of the Czech Republic, grant 205/04/2063 and
by allocation of SAAO observing time.
MW thanks the staff at SAAO for their warm hospitality
and help with the equipment. We also thank to Drs. Evgenya Shkolnik
and Gordon A.H. Walker for calling our attention to HD~179949
and for the allowance to see their results prior to publication.
This research has made use of the SIMBAD database, operated at CDS,
Strasbourg, France, and of NASA's Astrophysics Data System
Bibliographic Services.

\vfill\eject

\references
Code, A.~D., Bless, R.~C., Davis, J., Brown, R.~H.,
 1976, {\it ApJ}, {\bf 203}, 417

Groot, P.J., Piters, A.J.M., van Paradijs, J., 1996, {\it A\&AS}, {\bf 118}, 545

Harmanec, P., Horn, J., 1998, {\it JAD}, {\bf 4}, 5 
(Journal of Astronomical Data; Publ. by C. Sterken, Vrije Universiteit Brussel)

Harmanec, P., 1998, {\it A\&A}, {\bf 335}, 173

Menzies J.W., Cousins A.W., Banfield R.M., Laing J.D., 1989,
{\it SAAO Circ.}, {\bf 13}, 1

Perryman, M.A.C, H\o g, E., Kovalevsky, J., Lindegren, L., \& Turon, C.,
1997, ESA SP-1200 {\it The Hipparcos and Tycho Catalogues}

Popper, D.M., 1980, {\it ARA\&A}, {\bf 18}, 115

Shkolnik , E., 2004, PhD Thesis {\sl Chromospheric Activity Induced by
 Short-Period Planets: A Search for Modulation of Ca~II~H~\&~K Emission},
 Dept. of Physics and Astronomy, The University of British Columbia,
 Vancouver, Canada, March 2, 2004, 105 pp.

Stellingwerf, R.F., 1978,  {\it ApJ}, {\bf 224}, 953

Tinney, C.G., Butler, R.P., Marcy, G.W., Jones, H.R.A.,
Penny, A.J., Vogt, S.S., Apps, K., Henry, G.W., 2001, {\it ApJ}, {\bf 551}, 507

\endreferences

\end{document}

