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\begin{document}

\IBVShead{4591}{30 April 1998}

\IBVStitletl{On the Variability of S Stars}{ as Observed by the Hipparcos 
Satellite}

\IBVSauth{S.J.~ADELMAN, D.W.~MAHER}

\IBVSinsto{Department of Physics, The Citadel, 171 Moultrie Street, 
Charleston, SC 29409 USA}
\vskip -1.5mm
\IBVSinsto{e-mail: adelmans@citadel.edu, maherd@citadel.edu}

\SIMBADobj{T Cet, R And, W And, omicron1 Ori, NO Aur, R Gem, AA Cam, Y Lyn}
\SIMBADobj{HD63733, NQ Pup, RS Cnc, S UMa, ST Her, OP Her, HD170970}
\SIMBADobj{T Sgr, chi Cyg, AA Cyg, HD199799, HR8062, X Aqr}
\SIMBADobj{pi1 Gru, HR Peg, HD6409, HR363, BD Cam, HD35155}
\SIMBADobj{V613 Mon, NZ Gem, V Cnc, DE Leo, HD96360, HD165774}
\SIMBADobj{V1743 Cyg, HD191226, HD191589, HR4647, S Her, HD191630}
\SIMBADobj{HD310, HD29704, HD119667,HD150922, HD151011, HD215336}
\IBVStyp{ S }
\IBVSkey{photometry}
\IBVSabs{The Hipparcos photometry of S type stars shows that they are all}
\IBVSabs{variable.  The intrinsic S stars show a larger range of amplitudes} 
\IBVSabs{than do the extrinsic S stars.} 

\begintext

    The local part of our Galaxy contains two kinds of cool peculiar red giants
of spectral type S: the intrinsic S stars with high luminosity, Tc lines in 
the spectra, and chemical peculiarities which were produced by intrinsic 
nucleosynthetic processes and the extrinsic non-Tc binary S stars which have 
evolved from the barium stars (Iben \& Renzini 1983, McClure \& Woodsworth 
1990). Groenewegen (1993) examined Stephenson's (1984) catalog of S stars, 
Little et al. (1987), and Smith \& Lambert (1988, 1990) and found 27 stars 
without Tc and 36 stars with Tc.  Further he photometrically separated many of 
these stars into two groups.  Simultaneously Jorissen et al. (1993) performed a 
similar study on the correlation between Tc lines, binary, and infrared 
excesses.   

    Recently by use of Hipparcos parallaxes Van Eck et al. (1998) found the
intrinsic Tc-rich S stars are thermally pulsating AGB stars of low and 
intermediate masses while the extrinsic Tc-poor stars comprise mostly low-mass 
stars on either the red giant branch or the early AGB.  The later are binaries 
with an average mass of 1.6 $\pm$ 0.2 M$_{\odot}$.  Unfortunately Hipparcos 
parallaxes were not obtained for most of the large amplitude variables.  Their 
division of S-stars is similar to that of Groenewegen (1993) and Jorissen et 
al. (1993).  When a variability type is given for any large amplitude variable 
it is listed as a Mira. 

    The Hipparcos photometry (ESA 1997) provides data on many S stars with and
without Tc lines in their spectra.  Table 1 shows for each observed star the 
average magnitude, the standard error, and the amplitude (95th - 5th 
percentile magnitudes), any known periods from the GCVS (Kholopov et al. 1988),
 and the variability type (Stephenson 1984).  The periods from Hipparcos when 
given are very similar to these.  We also included Hipparcos periods for 
o$^{1}$ Ori, $\chi$ Cyg, and HR\,363.  Especially striking is that almost 
all 
large amplitude variables (those with amplitudes greater than 1 magnitude) are 
intrinsic S stars. Those stars with amplitudes of 2.24 mag or greater have 
periods longer than 220 days.  This is in accord with Little et al. (1987) who 
found that M star Mira variables tend to show Tc for periods greater than 300 
days.

    
\begin{table}

\caption{Hipparcos Photometry of Various S Stars}

	{\small
\begin{center}
\begin{tabular}{c@{\hspace{7mm}}c@{\hspace{8mm}}c@{\hspace{4mm}}c@{\hspace{1mm}}c@{\hspace{1mm}}c@{\hspace{1mm}}c@{\hspace{1mm}}c@{\hspace{1mm}}}
\hline
Name & HD  & HIP & Average & Standard & Ampl. & Period & Sp.\\
     &     &     & mag.    & error    & (mag.)    &  (days)&  Type \\ 
\hline
\end{tabular}
\vskip 2mm
Stars with Tc lines\\
\vskip 1mm
\begin{tabular}{l@{\hspace{5mm}}r@{\hspace{5mm}}r@{\hspace{3mm}}r@{\hspace{3mm}}r@{\hspace{3mm}}r@{\hspace{3mm}}c@{\hspace{3mm}}c@{\hspace{3mm}}}
T Cet  &     1760   &   1728 &  5.4391  & 0.0334 & 0.66 & 158.9  & M5S \\
R And  &     1967   &   1901 & 10.7054  & 0.0114 & 2.97 & 409.3  & S6,6e \\ 
W And  &    14028   &  10687 &  8.0759  & 0.2264 & 2.24 & 395.9  & S7/1e \\
o$^{1}$ Ori & 30959 &  22667 &  4.7029  & 0.0052 & 0.09 &  30.29 & S3/1 \\
NO Aur &    37536   &  26718 &  6.2431  & 0.0111 & 0.17 &        & M2S \\
R Gem  &    53791   &  34356 &  7.5292  & 0.4569 & 3.78 &  369.9  & S5/5 \\
AA Cam &    54587   &  35045 &  7.5787  & 0.0079 & 0.48 &         & M5S     \\
Y Lyn  &    58521   &  36288 &  6.8979  & 0.0051 & 0.75 &         & M6S    \\
       &    63733   &  38217 &  7.9816  & 0.0021 & 0.08 &         & S4/3  \\
NQ Pup &    64332   &  38502 &  7.5320  & 0.0155 & 0.22 &  120.   & S5/2    \\
RS Cnc &    78712   &  45058 &  5.4500  & 0.0267 & 0.93 &         & M6S \\ 
HR 4647 &  106198   &  59588 &  6.1429  & 0.0101 & 0.51 &         & M4S \\
S UMa  &   110813   &  62126 &  8.9095  & 0.1489 & 3.01 &  225.9  & S3/6e\\
ST Her &   142143   &  77619 &  6.9206  & 0.0262 & 0.71 &  148   & M6.5S\\ 
S Her  &   152276   &  82516 &  8.2809  & 0.2368 & 4.10 &  307.3  & M4Se\\
OP Her &   163990   &  87850 &  6.1059  & 0.0102 & 0.35 &  120.5  & M5S\\  
       &   170970   &  90723 &  7.4571  & 0.0027 & 0.11 &         & S3/1\\ 
T Sgr  &   180196   &  94706 & 10.8263  & 0.2237 & 3.97 &  394.7  & S5/6e\\
$\chi$ Cyg & 187796   &  97629 &  6.1696  & 0.1686 & 5.19 &  402.3  & 
S7/1.5e\\
AA Cyg &   190629   &  98856 &  8.1592  & 0.0376 & 1.01 &  212.7  & S6/3\\ 
       &   191630   &  99758 &  6.7577  & 0.0046 & 0.15 &         & S4,4\\
       &   199799   & 103476 &  7.2485  & 0.0168 & 0.27 &         & MS\\
HR 8062 &  200527   & 103828 &  6.1764  & 0.0046 & 0.18 &         & S4/1\\
X Aqr   &  211610   & 110146 & 10.1887  & 0.1796 & 3.66 &  311.6  & S6,3\\
$\pi^{1}$ Gru & 212087 & 110478 & 5.4957 & 0.0119 & 0.80 &         & 
S5,7:\\  
HR Peg  &  216672   & 113131 &  6.3460  & 0.0164 & 0.23 &   50   & S4/1\\
\hline
\end{tabular}
\vskip 2mm
Stars without Tc lines\\
\vskip 1mm
\begin{tabular}{lrrrrrcc}
          &    310  &     621 & 7.5704 & 0.0038 & 0.09 &        & S3.1\\ 
          &   6409  &    5091 & 7.4622 & 0.0043 & 0.10 &        & M2wkS\\
HR 363    &   7351  &    5772 & 6.3950 & 0.0045 & 0.09 &  7.494 & S3/2\\
BD Cam    &  22649  &   17296 & 5.0956 & 0.0035 & 0.10 &        & S4/2\\
          &  29704  &   21688 & 8.2551 & 0.0098 & 0.24 &        & S:\\   
          &  35155  &   25092 & 6.8824 & 0.0115 & 0.21 &        & S3/2\\
V613 Mon  &  49368  &   32627 & 7.7492 & 0.0041 & 0.16 &        & S3/2\\
NZ Gem    &  61913  &   37521 & 5.5874 & 0.0033 & 0.08 &        & M3S\\
V Cnc     &  70276  &   40977 & 9.2621 & 0.1521 & 2.59 & 272.1  & S3/6e\\
DE Leo    &  90254  &   51008 & 5.6925 & 0.0055 & 0.06 &        & M3S\\
          &  96360  &   54396 & 8.0759 & 0.0060 & 0.19 &        & M3Swk\\
          & 119667  &   67070 & 8.5417 & 0.0024 & 0.05 &        & M1Swk\\
          & 150922  &   81970 & 7.8030 & 0.0049 & 0.08 &        & M2S\\
          & 151011  &   82038 & 6.6890 & 0.0029 & 0.05 &        & Swk\\
          & 165774  &   88940 & 8.2111 & 0.0032 & 0.07 &        & S4,6\\
V1743 Cyg & 184786  &   96198 & 6.0075 & 0.0092 & 0.18 &        & M5S\\
          & 191226  &   99124 & 7.3741 & 0.0020 & 0.07 &        & M3S\\
          & 191589  &   99312 & 7.3682 & 0.0017 & 0.06 &        & S\\
          & 215336  &  112227 & 7.9295 & 0.0019 & 0.06 &        & Swk\\
GZ Peg    & 218634  &  114347 & 5.0336 & 0.0117 & 0.24 &  92.7  & M4S\\
\hline
\end{tabular}
\end{center}
}
\end{table}

 V Cnc is the only possible extrinsic S star with a large amplitude.  But 
it is uncertain whether it has Tc lines in its spectrum.  If it is an 
intrinsic S star, then the extrinsic binary S stars would be more unified in 
their class properties.  Their amplitudes would represent the typical 
pulsations from similar single stars with a mean amplitude of 0.11 $\pm$ 
0.07 
mag (19 stars).  Of the intrinsic S stars, 8 have similar amplitudes while 10 
have intermediate amplitudes.  Smith \& Lambert (1988) found four of these 
stars NO Aur, HD\,63733, ST Her, and HD\,170970 to have wavelengths of the 
$\lambda$ 4262 line just at the boundary between Tc-rich and Tc-poor 
stars. Three 
have amplitudes in the extrinsic S star range.  However a resolution of 0.18 
\AA\ is insufficient to cleanly separate the two types of S stars.  According to 
Jorissen et al. (1993) HD\,63733 and HD\,170970 are really extrinsic S stars 
being binary and lacking infrared excesses.  Ake \& Johnson (1988) discuss 
o$^{1}$ Ori which is another outlyer in the Tc-rich stars.  It has a white 
dwarf companion and intrinsic S star features.  Hence the other S stars in the 
intrinsic list with amplitudes like those of the binary stars should be 
studied carefully to make sure they are in the correct category.


     Adelman (1998) found that BD Cam was pulsating with a period of 24.76 days
with an amplitude of almost 0.20 magnitudes.  This was superimposed on the 
binary period of 596.21 $\pm$ 0.19 days found by Griffin (1984).  
HD\,35155 acts
in a somewhat similar manner with the intrinsic variations superimposed on an 
eclipsing behavior which is related to the orbital motion (Jorissen et al. 
1992, 1996).  As both are extrinsic binary S stars, we might expect that the 
other extrinsic S stars exhibit somewhat similar behavior.  In making 
comparisons between Hipparcos light curves, a difficulty is that the sampling 
depends on where the star is in the sky.  Many observations are closely 
bunched rather than sampled in a more random manner.  The Hipparcos light 
curves of these stars (except for V Cnc) look like random samples of pulsating 
stars with periods of order a month.  A complete demonstration requires 
photometry which samples the light of these stars every few days.  

    Of the intrinsic S stars, T Cet shows a systemic decline of about 0.8 mag. 
with pulsations superimposed.  The large amplitude intrinsic S stars tend to 
have fragmentary light curves for the most part with suggestions of pulsation. 
The smaller amplitude stars have light curves often resembling those of the 
extrinsic S stars.  But AA Cam shows a definite minimum as does HD\,63733.

    The intrinsic S stars show a range in amplitude variability with periods 
between 30 and 419 days. But many such stars still have periods to be 
determined.  It is possible that all the large and medium (0.3 - 1.0 mag.) 
amplitude variables are intrinsic S stars and the small amplitude variables are
extrinsic S stars.  Thus the conclusion of Little et al. (1987) that Miras with 
periods greater than 300 days tend to be what are known as intrinsic S stars 
is confirmed and extended.  

    We thank Dr. A. Jorissen for useful comments on the original manuscript.


\vskip 1cm

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\end{document}

