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\input ibvs2.sty

\begin{document}
\IBVShead{4688}{24 March 1999}

\IBVStitle{{\Large $\tau$}$^1$ Hydrae:  NOT A {\Large $\gamma$} DORADUS VARIABLE}

\IBVSauth{G. W. HENRY$^1$, S. M. Henry$^1$, A. B. Kaye$^2$}

\IBVSinst{
Center of Excellence in Information Systems, Tennessee State 
University, 330 10th Ave.\ North, Nashville, Tennessee  37203, U.S.A., 
e-mail: henry@schwab.tsuniv.edu, henrysm@galileo.tsuniv.edu}

\IBVSinst{
Applied Theoretical and Computational Physics Division, Los Alamos
National Laboratory, X--TA, \\ MS~B-220, Los Alamos, New Mexico, U.S.A.,
email: kaye@lanl.gov}

\SIMBADobj{tau1 Hya}
\IBVStyp{ F6V }
\IBVSkey{photometry}

\begintext

The F6~V star $\tau^{1}$~Hydrae (= 31~Hya = HR~3759 = HD~81997~A) is
listed as a candidate $\gamma$~Doradus variable by Krisciunas \&
Handler (1995) and Handler \& Krisciunas (1997).  The $\gamma$~Dor
stars have spectral types near F0 and low-amplitude photometric
variability with periods between about 0.5 and 3 days.  The inclusion
of $\tau^{1}$~Hya in the candidate list was based on photometric
variability reported by Lockwood, Skiff, \& Thompson (1993).  A more
complete analysis of its photometric variability is given by Lockwood,
Skiff, \& Radick (1997, hereafter LSR).  They reported $\tau^{1}$~Hya
to be slightly variable from night to night (at greater than 95\%
significance) within five of their twelve observing seasons but did not
have sufficient observations to search for periodicity.  The average
night-to-night rms variation for their twelve seasons was 0.0021~mag in
the combined Str\"{o}mgren $(b+y)/2$ wavelength band.  They also noted
significant long-term variability of 0.01~mag.
  
Duquennoy \& Mayor (1991) discovered $\tau^{1}$~Hya to be a
single-lined spectroscopic binary with a period of 7.7~yr.  No
photometric variability was detected by Perryman et al.\ (1997) in the
{\it HIPPARCOS} catalogue, which gives $V=4.59$, $B-V=0.411$, and
$\pi=58.48$~mas.  However, the {\it HIPPARCOS} color index is probably
in error and should be 0.461; the catalogue cites ground-based
photometry as the source of the color index, and the General Catalogue
of Photometric Data lists 11 determinations of $B-V$ with a mean of
0.461 (Mermilliod, Hauck, \& Mermilliod 1997).  Thus, the apparent
magnitude, color index, and parallax are consistent with a spectral
classification of F6~V.

We have observed $\tau^{1}$~Hya for six seasons with a 0.75~m automatic
photoelectric telescope (APT).  Our Str\"{o}mgren $b$ and $y$
observations were made differentially with respect to the comparison
star HD~81342 ($V=6.88$, F5), the same one used by LSR.  HD~81997~B,
located 66 arcseconds from $\tau^{1}$~Hya, was excluded from the
diaphragm during our measurements.  Each differential magnitude was
corrected for extinction and transformed to the Str\"{o}mgren system.
We also combined our $b$ and $y$ observations into a single $(b+y)/2$
band to increase precision.  Further details on the reductions can be
found in Henry (1995).

The analysis of our APT photometry is summarized in Table~1.  Column~1
gives the year, column~2 the Julian Date range, column~3 the number of
observations, and column~4 the seasonal-mean differential magnitudes in
the Str\"{o}mgren $(b+y)/2$ band.  Column~5 lists the night-to-night
variability ($\sigma_{\rm short}$) for each observing season, computed as
the standard deviation of a single observation from the seasonal mean.
The final column gives the uncertainty in the seasonal means
($\sigma_{\rm mean}$), computed as the standard deviation from Column~5
divided by the square root of the number of observations from
Column~3.

\begin{table}[t!]
\caption{Photometric Analysis of $\tau^{1}$~Hya}
\begin{center}
%{\normalsize Table 1: Photometric Analysis of $\tau^{1}$~Hya}
%\vskip 2mm
\begin{tabular}{cccccc}
\hline
Year & Date Range & $n$ & Seasonal Mean & $\sigma_{\rm short}$ & $\sigma_{\rm mean}$ \\
 & HJD${} - 2400000$ & & (mag) & (mag) & (mag) \\
\hline
1993 & 49094--49121 &  8 & $-$2.3170 & 0.0013 & 0.0005 \\ 
1994 & 49295--49482 & 78 & $-$2.3162 & 0.0011 & 0.0001 \\ 
1995 & 49662--49840 & 74 & $-$2.3163 & 0.0015 & 0.0002 \\ 
1996 & 50030--50213 & 46 & $-$2.3157 & 0.0013 & 0.0002 \\ 
1997 & 50405--50552 & 30 & $-$2.3154 & 0.0014 & 0.0002 \\ 
1998 & 50761--50944 & 47 & $-$2.3144 & 0.0018 & 0.0003 \\ 
\hline
\end{tabular}
\end{center}
\end{table}

Our observations overlap those of LSR for the 1993--1995 seasons and are 
considerably more numerous; within these three seasons, LSR reported 
variability in 1995.  The mean of our six seasons of night-to-night 
variability is only 0.0014~mag.  This level of variation is just what we 
observe for the most constant stars and so represents the precision 
limit of this telescope (Henry~1995).  Furthermore, periodogram analysis of 
our six seasons of observations, taken separately and also combined into a 
single data set, show no significant periodicities in the range 0.1 to 
10 days.  Thus, we can find no evidence at all for short-term photometric 
variability in $\tau^{1}$~Hya.  This precludes its classification as a 
$\gamma$~Dor star.

Our seasonal means, however, {\it do} confirm the long-term variability 
observed by LSR, although the shorter time span of our observations gives a 
smaller range of only 0.002 or 0.003~mag.  Given the location of the star in 
the HR diagram, these slow variations are probably the result of a 
magnetic-activity cycle (see, e.g., Baliunas et al.\ 1998).
  
Astronomy with automated telescopes at Tennessee State University is
supported through NASA grant NCC5-228 and NSF grant HRD-9706268.  Kaye
works under the auspices of the U.\ S.\ Department of Energy at the
Los Alamos National Laboratory under contract No.\,W-7405-Eng-36.

\references

Baliunas, S. L., Donahue, R. A., Soon, W., \& Henry, G. W. 1998, 
ASP Conf. Ser., 154, 153, in The 10th Cambridge Workshop on Cool Stars, 
Stellar Systems, and the Sun, ed.\ R.~A.~Donahue \& J. A. Bookbinder (San
Francisco: ASP)
%Changed: Fri Jun  3 09:38:47 MEST 2005

Duquennoy, A., \& Mayor, M. 1991, {\it A\&A}, {\bf 248}, 485

Handler, G., \& Krisciunas, K. 1997, {\it DSSN}, {\bf 11}, 3

Henry, G. W. 1995, ASP Conf. Ser., 79, 44, in Robotic Telescopes:  
Current Capabilities, Present Developments, and Future Prospects for 
Automated Astronomy, eds.\ G.~W. Henry and J. A. Eaton (ASP:  San
Francisco)
%Changed: Fri Jun  3 09:38:47 MEST 2005

Krisciunas, K., \& Handler, G. 1995, IBVS, No.\,4195

Lockwood, G. W., Skiff, B. A., \& Radick, R. R. 1997, {\it ApJ}, {\bf 485}, 789

Lockwood, G. W., Skiff, B. A., \& Thompson, D. T. 1993, in Stellar
Photometry---Current Techniques and Future Developments, IAU Colloq.\
136, eds.\ C. J. Butler \& I. Elliott (Cambridge University Press:
Cambridge), 99

Mermilliod, J.-C., Hauck, B., \& Mermilliod, M. 1997, {\it A\&AS}, {\bf 124}, 349

Perryman, M. A. C., et al.\ 1997, The Hipparcos and Tycho Catalogues (ESA SP-1200: The Netherlands)

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