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\IBVShead{4480}{26 May 1997}
\centerline{\ft ALDEBARAN: DISCOVERY OF SMALL AMPLITUDE}
\centerline{\ft  LIGHT VARIATIONS}
\vskip 6mm

\begintext

     Aldebaran (Alpha Tau = HD 29139) is one of the nearest and brightest
  red giant stars.  It is a standard spectroscopic star with a spectral
  type of K5 III; its average visual magnitude is about +0.87 mag with
  mean values of
  B$-$V = +1.52 and U$-$B = +0.90.  Aldebaran has a relatively well
  determined parallax of 0.048 $\pm$ 0.005 arcseconds that should improve
  after the Hipparcos parallax is published.  The star also has relatively
  high space motions with respect to the sun, indicating that it is an old,
  evolved disk star.  

      Because of its brightness and accessibility from both ground-based 
  and orbiting observatories, it has been a favorite target of numerous      
  studies.  It is listed in the Bright Star Catalog (Hoffleit, 1982) and
  SIMBAD as a variable star and Petit (1982) classifies it as an 
  Lb-type irregular star. In 
  the literature the visual magnitude range is from V $\cong$ +0.78 to  
  +0.93; most of these visual magnitude measurements are from surveys. 
  It should be noted that reported variability for bright stars such as
  Aldebaran can sometimes have systematic errors due to saturation 
  effects of the detectors and the lack of nearby appropriate comparison
  and check stars.  Hence some of the early visual magnitude values of
  Aldebaran should be treated with caution.

      The only concerted photometric study of Aldebaran was done by Krisciunas
  (1992). He obtained V-band photometry over 3 observing seasons (1987/88,
  1990/91, and 1991/92).  However, the photometry was conducted only 4 to 5
  nights per season and a total of only 13 nights of data were obtained.
  Krisciunas found no indication of variability of greater than 0.02 
  magnitude, and reported Aldebaran to be ``essentially constant" within
  the precision of his measurements.
  He found mean values of $\langle$V$\rangle$ = +0.876 $\pm$ 0.004 
  magnitude
  and $\langle$B$-$V$\rangle$ = +1.549 $\pm$ 0.026 magnitude.  The study 
  of Krisciunas does not support the relatively large $\approx$ 0.1
  light variations reported in the survey data, but there is not 
  sufficient coverage or precision to discern low amplitude brightness
  changes.  To understand and better quantify the photometric behavior
  of Aldebaran, we undertook a more intensive program of differential
  photometry of this famous, bright star.

     In August 1996, Aldebaran was added to the program of photometry of 
  cool giants and supergiants being carried out by us at Wasatonic
  Observatory and Villanova University Observatory.  The photometry
  reported here was conducted
  from August 1996 to March 1997 at the Wasatonic Observatory (Allentown,
  Pennsylvania) on 31 nights using an uncooled Optec photometer attached 
  to a 20-cm Schmidt-Cassegrain telescope.
  The detector employed was a silicon PIN-photodiode.  Differential
  photometry was conducted primarily with the V-band but on several
  nights the star was also observed with the Wing near-IR three filter
  intermediate band system to measure TiO (Wing, 1992).  The characteristics
  of the Wing three-color system are given in Table 1. The TiO index is 
  calculated according to Wing from:

\centerline{TiO-Index = A$-$B$-$0.13$\times$(B$-$C)}


\vfill \eject

\centerline{2}
\vskip 1mm


\begin{center}
Table 1. The Wing filter system
\end{center}

\begin{center}
\begin{tabular}{|l|c|c|c|c|c|}\hline
Filter&Region&Central&Bandpass&Measurements\\
      &Measured&Wavelength&(FWHM)&        \\ \hline 
A&TiO$\gamma$(0,0) Band&7190 A&110 A&TiO-Index\\
B&IR Continuum&7540 A&110 A&B-C Color Index\\
C&IR Continuum&10400 A&420 A&B-C Color Index\\ \hline
\end{tabular}
\end{center}

\begin{center}
Table 2. Photometric data
\end{center}

\begin{center}
\begin{tabular}{|l|c|c|}\hline
JD 2450+&Visual magnitude\\ \hline 
324.847&+0.871\\
356.800&+0.868\\
365.881&+0.877\\
380.850&+0.882\\
402.630&+0.870\\
418.649&+0.872\\
426.553&+0.867\\
438.583&+0.865\\
455.605&+0.877\\
470.526&+0.882\\
477.658&+0.880\\
483.546&+0.877\\
504.549&+0.873\\
517.591&+0.875\\
531.520&+0.876\\ \hline
\end{tabular}
\end{center}

\vskip 2mm

\noindent  where A, B, and C are standardized magnitudes measured with these filters
  A near-IR color index is also formed from these observations, and is 
  useful for determining the temperature of cool stars.  This color index is
  defined as:

\centerline{IR Color Index = B$-$C}

\noindent  where B and C are the magnitudes measured at 7540 A and 10,400 A,
  respectively, which are regions clear of molecular absorption.

     The comparison star was $\epsilon$ Tau (HD\,28305; V = 3.50, B$-$V = 1.04,
  G9.5 III), which is itself a wing IR standard star, and the check star 
  used was $\pi$ Tau (HD 28100; V = 4.69, B$-$V = 0.98, G7 IIIa). Three
  ten-second integrations were made for each observation using the usual
  sky-comparison-variable-comparison-sky sequence.  Atmospheric
  extinction and conversion to heliocentric Julian Day number was
  done during data reduction.  Corrections for the V-band observations
  to the standard UBV system was also done; IR magnitudes were
  standardized using magnitude values supplied by Wing (1979).

    Nightly and weekly means were computed from the V-data and these are
  plotted against
  heliocentric Julian Day in Figure 1, and tabulated in Table 2.  As can 
  be seen, the light variations observed over the 6-month period are
  relatively small.  Systematic trends in the data and spline-fits were 
  applied to see
  if any regularities in the light variations could be found. As shown in
  Figure 1, Aldebaran appears to vary on a time-scale of about 85-95 days;
  the full light variation is 0.018 magnitude.  To check this period 
  analytically,\vfill \eject

\centerline{3}
\vskip 1mm
\epsfysize=5.5cm
\centerline{\epsffile{4480-f1.ps}}
\normalsize
\begin{center}
Figure 1. Aldebaran visual light curve; calendar dates are mid-month.
The sine curve shown was generated using a 92-day period and varying semiamplitude
\end{center}
\large
\epsfysize=5.2cm
\centerline{\epsffile{4480-f2.ps}}
\normalsize
\begin{center}
Figure 2. Aldebaran DFT; note peak intensity at frequency 0.01095 (period=91.3~days)
\end{center}
\large
\epsfysize=6.2cm
\centerline{\epsffile{4480-f3.ps}}
\normalsize
\begin{center}
Figure 3. TiO-index - spectral type calibration, indicating Aldebaran as an M0 star
\end{center}
\large


 \noindent the observations were subjected to a formal period search using
  a Discrete Fourier Transform (DFT) of Sinnot (1988).  Figure 2 shows the
  results
  of the DFT; a peak frequency of 0.01095/days was found, corresponding to
  an approximate 92 day period.  This period is close to the photometric
  period found by inspection.  A sine curve of decreasing semi-amplitude,
  from 0\fm 005 to 0\fm 0015, was generated using the 92-day period.  This fit is shown in Figure 1. The agreement with the observations is reasonably
good.

 \vfill \eject

\centerline{4}
\vskip 1mm
     It is not certain if this period is stable with time and if there are
  any long-term changes in brightness.  The mean brightness observed by
  us of $\langle$V$\rangle$ = +0.873 magnitude is in
  good agreement with the $\langle$V$\rangle$ found earlier by Krisciunas;
  this indicates that the star does not have significant long-term
  brightness changes over the time scale of at least several years.




     Based on the apparent observed period and varying amplitude, it
  appears that Aldebaran has photometric characteristics similar to
  the so-called Small Amplitude Red Variables (SARVs). SARVs are 
  M-giants which pulsate with small light amplitudes and have periods
  of up to 200 days and visual amplitudes of up to 2.5 magnitudes
  (Percy, 1989). If so classified, Aldebaran
  would have the smallest observed amplitude of this class of stars.

     Although we did not attempt to obtain light curves using the Wing IR 
  filters, we did observe the star on four nights with this filter set. From  
  these observations we determined the TiO-index and the near-IR color index
  to be +0.282 $\pm$ 0.012 and $-$0.227 $\pm$ 0.009 magnitude, respectively. 
  From over
  20 cool standard stars observed with the Wing filters (Wing, 1978) a
  TiO-index vs. spectral type was calibrated for K and M-type stars.  Part
  of this calibration is seen in Figure 3, where Aldebaran's TiO-index
  indicates it is of spectral type M0-III, which is not the usual K5 III
  value associated with this star.
  Additionally the U$-$B and B$-$V colors are more suitable
  for a M0 III star rather than a K5 III star.     

     More observations using the Wing filters are needed to further quantify
  the spectral type of Aldebaran and also to search for outer atmospheric
  TiO variations. Continued photometry is also planned to ascertain
  period stability and amplitude changes.

     The authors wish to thank Dr. Emilia Belserene for her assistance in
  translating the DFT program from BASIC to FORTRAN.  We also than Dr. 
  Robert Wing for providing standard star IR data.  For this research we
  utilized the
  SIMBAD database, operated by CDS, Strasbourg, France.  This work was 
  supported in part by NSF grant AST-9315365, which we gratefully 
  acknowledge.

\vskip .5cm

\beginauthors
\authorsrightcol
Rick WASATONIC
Edward F. GUINAN
Dept. of Astronomy and Astrophysics
Villanova University
Villanova, PA. 19085
\endauthors

\vskip -5mm

\references

     Hoffleit, D., 1982, Yale Bright Star Catalog, 4th Edition, Yale
       University Observatory

     Krisciunas, K., 1992, {\it Inf. Bull. Var. Stars}, No.\,3729

     Percy, J., 1989, {\it PASP}, {\bf 101}, 893

     Petit, M., 1982, Variable Stars, John Wiley \& Sons, Ltd.

     Sinnot, R.W., 1988, {\it Sky and Telescope}, Vol.\,{\bf 76}, No.\,3,                   p.\,288

    
        Wing, R.F., 1978, Spectral Classification and Color Temperatures 
       For 280 Bright Stars In The Range K4 - M8, Astronomy Dept.,
       Ohio State University

       Wing, R.F., 1979, Standard Stars for Eight-Color Photometry,
       Dudley Obs. Report 14, A.G. Davis Philip, ed.


      Wing, R.F., 1992, {\it Journal of the American Association of Variable
      Star Observers}, {\bf 21}, 42, (No.\,1)
% Original:       Star Observers}, Vol.\,{\bf 21}, No.\,1, p.\,42
% Corrected: Tue Jul 29 14:21:39 MET DST 2003

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