COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                      Number 3052

                                         Konkoly Observatory
                                         Budapest
                                         27 July 1987

                                         HU ISSN 0374-0676


          VARIABLE SHELL STRENGTH OF PLEIONE (BU Tau)

  The peculiar shell star Pleione (28 Tau, B 8 V) is the most interesting
Be star. This star has been the subject of a number of investigations, 
possessing the very long recorded spectroscopic history. Since 1880, this 
fascinating object has undergone through different phases (Be --> B -->
Shell --> Be --> Shell). This star showed the Be phase from 1888
to 1903 (Pickering, 1889-90; Merrill, 1953). The observations from 1905
to 1936 revealed it to be a normal B star (Frost, 1906; Merrill, 1953). From
1938 to 1954 this star experienced a strong shell phase with maximum shell
intensity during 1945-46 (McLaughlin, 1938; Mohler, 1938; Struve and Swing,
1943; Merrill, 1953).
  From 1954 a new Be phase started for Pleione which persisted till 1972,
with maximum intensity during 1960-1963 (Delplace and Hurbert, 1973; Morgan
et al., 1973). The last shell phase of Pleione, which is still persisting,
started in 1972 (Gulliver, 1973). Since 1972 the shell strength, as derived
from lines, was found continuously increasing (Hirata and Kogure, 1976, 1977,
1978; Katahira and Hirata, 1984).
  This shell star possesses unusual large Balmer jump implying strong near-
ultraviolet deficiency. Goraya (1985) analysed the behaviour of its Balmer
jump from 1975 to 1981. In the present study we have combined our new 
spectrophotometric measurement of the continuum (lambdalambda 13200-5500 A) with earlier 
observations (Goraya, 1984). We have investigated the variation of the shell
strength as derived from the flux deficiency at the Balmer continuum in the
near-ultraviolet region.

  The present observations were made during 2 November, 1982 in the wavelength
range lambdalambda 3200-5500 A. We used a Hilger and Watts scanner at the 
cassegrain focus (f/13) of 104-cm reflector of Uttar Pradesh State Observatory,
Nainital. An exit slit admitting 50 A of the spectrum was used. The 
photomultiplier EMI 9658B and standard dc techniques were employed for detecting
and recording the signal.

  The instrumentation and observational procedures are described by Goraya
(1985). Along with Pleione, the comparison star 18 Tau (B 8 V) and the standard
star alpha Leo were also observed. The observations of Pleione and 18 Tau were
reduced to standard magnitudes with the help of the standard stars; absolute
calibration given by Taylor (1984). The standard deviation of the measurements
is +/- 0.03 mag.
  The differential magnitudes (Delta m) of Pleione (Pleione-18 Tau) were computed
and are compared with earlier measurements as shown in Figure 1.


                            [FIGURE 1]

          Figure 1: Variation of Balmer jump in Pleione


It is obvious that since 1975 there is a gradual increase in differential
magnitudes of Pleione. The highest value reached during 1981, followed by a
rapidly decreased value during 1982. This shows that the shell strength of
Pleione increased gradually from 1975 to 1981 followed by a sharp decrease
onwards. To understand the variation of the shell strength better, we have
also plotted the differential magnitudes of Pleione at two wavelengths i.e.
Delta m 3300 and Delta m 3600, as a function of time (cf. Figure 2).


                              [FIGURE 2]

    Figure 2: Variation of differential magnitudes of Pleione


It can be inferred from this figure also that Pleione gained regular increase
in shell strength since 1975 with maximum intensity during 1981, after which
the strength started decreasing faster.


                        P.S. GORAYA AND N.S. TUR

                    Department of Astronomy and Space Sciences,
                    Punjabi University, Patiala - 147002
               			INDIA


                              B.S. RAUTELA
                        U.P. State Observatory, Manora Peak,
                          Nainital - 263129
	         		INDIA


References:

Delplace, M. and Hubert, H.: 1973, C.R. Acad.Sci, Paris, Ser. B, 277, 575. [BIBCODE 1973CRASB.277..575D ]
Frost, E.B.: 1906, Astrophys. J., 23, 268. [BIBCODE 1906ApJ....23..264F ]
Goraya, P.S.: 1984, Astrophys. Space Sci., 105, 323. [BIBCODE 1984Ap&SS.105..323G ]
Goraya, P.S.: 1985, Spectrophotometric Studies of Be Stars, Ph.D. Thesis
   Punjabi University, Patiala, India.
Gulliver, A.F.: 1973, IAU Cir. No. 2491. [BIBCODE 1973IAUC.2491R...1G ]
Hirata, R. and Kogure, T.: 1976, Publ. Astron. Soc. Japan, 28, 509. [BIBCODE 1976PASJ...28..509H ]
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McLaughlin, D.S.: 1938, Astrophys. J., 88, 622. [BIBCODE 1938ApJ....88..622M ]
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Morgan, W.W., White, R.A. and Tapacott, J.W.: 1973, Astron. J., 78, 302. [BIBCODE 1973AJ.....78..302M ]
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