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\IBVShead{4095}{3 October 1994}                      % Use it with dummy or empty arguments

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\centerline{\ft A NEW Be PHASE OF PLEIONE}
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The well known Be-shell star Pleione (BU Tauri, HD\,23862, B7-8IV-Ve, 
v{\it sin}i=320 km/s) passed through several B, Be and Be-shell 
phases since 1888. The last Be-shell phase began in 1972 (Delplace and Hubert,
1973; Hirata and Kogure, 1976), reached a maximum around 1982 and entered a
decrease in 1984 (Ballereau et al., 1988). In 1987-88, emission lines were 
strong, shell lines almost completely disappeared (with the exception
of FeII multiplet No.\,42), while the photospheric component of Balmer lines,
weak and shallow in 1982, became deeper and wider (Ballereau et al., 1994).
The disappearance of the shell lines goes along with the increase of their 
negative RVs (higher terms of the Balmer lines and metallic ions), which 
indicates that the surrounding envelope of gas gradually expands in space 
over several years. Recent observations of H$\alpha$ emission component made
by Menchenkova and Luthardt (1993) confirm the gradual increase of its equivalent
width until 1991-92 and the faintness of the central reversal.

On Aug. 9, 1994, we obtained the H$\beta$ and FeII $\lambda$4924\AA\ line profiles
of Pleione, with the AURELIE receptor attached to the coud\'e focus of the
1.52m telescope of Haute-Provence Observatory (Figure 1). The parameters of the 
spectrum are as follows:
\vskip 1mm
\hspace*{3cm}Resolution=$\lambda$/$\Delta$$\lambda$=16400

\hspace*{3cm}Signal to noise ratio=S/N=200

\hspace*{3cm}Recorded spectral wavelength range=$\lambda$$\lambda$4806-4952\AA
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The spectroscopic parameters measured on the H$\beta$ and FeII emission
profiles (radial velocities (RV) in km/s, intensities (I) in fraction
of stellar continuum intensity) give the following results:
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\underline{H$\beta$}
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\noindent Violet peak: RV=$-$140, I=1.13; central reversal: $-$79, 1.09; red 
peak: +15, 1.44; V/R=0.78; emission peak separation, $\Delta$e=155 km/s.
RV and total width of the emission line at the junction with the photospheric 
profile: $-$91, 617; at the intensity level 1.00: $-$53, 328; at the 
intensity level 1.10: $-$36, 254.

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\underline{FeII$\lambda$4924\AA}
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\noindent Violet peak: RV=$-$220, I=1.02; central reversal: $-$86, 0.99; red 
peak: +28, 1.11; V/R=0.92; $\Delta$e=248km/s.
\break

From comparison of the present data with those obtained these last years,
several remarks can be emphasized:
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\noindent 1. The decrease and vanishing of shell lines on Balmer series and
metallic ion spectra, simultaneously with the strengthening of the emission
components observed since 1984 until the nineties are monotonic phenomena, which
translate the gradual spatial development of the surrounding emitting/absorbing
envelope of Pleione.
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\centerline{2}
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\centerline{Figure 1. Normalized intensity tracing of the H$\beta$ and FeII$\lambda$4924\AA}
\centerline{lines of Pleione, on Aug. 9, 1994. The vertical bars give the rest wavelengths}
\centerline{of each line.}

\vskip 3mm
\noindent 2. The progressively negative RVs of emission/shell-absorption lines of
the optical spectrum from 1984 indicate the onset of a new instability which
leads to a gradual expansion of the envelope, as well in the equatorial plane,
then, as probably, vertically on both sides of this plane. In 1994, the RVs of
the absorption-like remaining depression in the H$\beta$ and FeII lines (Figure
1, respectively $-$79 and $-$86 km/s) suggest a rather rapid expansion, 
while before 1984, during the ``quiet and strong shell phase'', they were small 
with periodic variations (Ballereau et al., 1988).
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\noindent 3. Measurements of peak separation ($\Delta$e) on H$\beta$ obtained
by ourselves since the seventies, and used in Huang's (1972) formula in the case of
a Keplerian rotational motion in the envelope, permit to obtain the outer
radius of the H$\beta$ emitting envelope: R(H$\beta$)/R$_{\star}$=2.0 (1973); 
5.5 (1981); 10.0 (1987-88); 17.0 (1994). The 1992 H$\alpha$ emission profile of 
Menchenko\-va and Luthard (1993) has $\Delta$e=2.77\AA, which gives 
R(H$_\alpha$)/R$_{\star}$=25.6. All these data confirm the expanding movement of
the envelope of Pleione. 

\vskip 2mm
\noindent 4. The fading of the Balmer photospheric profiles during the ``quiet
and strong shell phase'', before 1984, and their gradual strengthening after,
show that at first the envelope is probably compact, cool and optically thick,
while its spectrum dominates the one of the underlying B8 star. It looks the one
of an A-F star. The expansion of the envelope on both sides of the equatorial 
plane makes the central star more visible, which can also explain the 
reappearance of the Balmer photospheric spectrum of Pleione.
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\noindent 5. The equivalent width of the emission in the H$\beta$ line increases
progressively: W(H$\beta$)$_{em}$=\ 0.42\AA (1981); 1.80\AA (1987-88); 2.70\AA
(1994).
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\centerline{3}
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\noindent 6. The peak separation of the FeII emission line in 1994 (248 
km/s) gives an outer radius of the FeII emitting disc of 6.7 R$_{\star}$. 
This indicates that this disc is nearer to the stellar surface than the 
H$\beta$ emitting disc, as generally observed in Be star envelopes.
\vskip 2mm
\noindent 7. All these changes in the structure of the Pleione shell claim in
favour of the phase-change model proposed by Kogure (1989), where the envelope
responsible for a shell phase should be a compact thin/thick disc-like one,
which has to expand to transform itself into an extended spheroidal/doughnut 
shape to produce  a spectrum characteristic of a Be phase. The same type of 
model was also proposed by Zorec and Briot (1991) to explain the photometric 
variations during the phase changes of Be stars.
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% Call for macros \authors - \authorsrightcol - \endauthors
\beginauthors
\authorsrightcol
D. BALLEREAU
J. CHAUVILLE
Observatoire de Paris
section de Meudon
5 place Jules Janssen
92195 Meudon principal cedex
France
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J. ZOREC
Institut d'astrophysique de Paris
98$^{bis}$ boulevard Arago
75014 Paris
France
\endauthors

% Call for macro \references
\references

Ballereau, D., Chauville, J., Mekkas, A., 1988, {\it A}\&{\it AS}, {\bf 75}, 139

Ballereau, D., Chauville, J., Zorec, J., 1994, International Conference 
{\it``Circumstellar Matter 1994''} held at Edinburgh (Scotland), Aug. 29th-Sep.
2nd, 1994

Delplace, A.-M., Hubert, H., 1973, {\it C. R. Acad. Sc. Paris}, {\bf 277},
S\'erie B. p. 575

Hirata, R., Kogure, T., 1976, {\it PASJ}, {\bf 28}, 509

Huang, S.-S., 1972, {\it ApJ}, {\bf 171}, 549

Kogure, T., 1989, {\it ASS}, {\bf 163}, 7

Menchenkova, E. V., Luthardt, R., 1993, {\it IBVS}, No.\,3961

Zorec, J., Briot, D., 1991, {\it A}\&{\it A}, {\bf 245}, 150

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