The envelope of Pleione in the new shell phase, 1972 - 1975.

The shell spectrum of Pleione has gradually developed during the new shell phase that began in 1972. A series of high-dispersion spectra (10 or 13 A in the blue region) was analyzed. First, the emission intensities of the Balmer lines were measured. The Ha emission was much weaker in 1973 as compared to the value in the pre-shell phase, and thereafter it steadily increased in parallel with the strengthening of the metallic and hydrogen shell lines. Second, the radial velocities, the central residual intensities, and the widths of the shell lines in the Balmer series were measured. No appreciable Balmer progression was found, and the mean radial velocity coincided with the stellar radial velocity within the error of measurement. A spectroscopic model was then derived on the basis of these measurements. It was shown that the absorption envelope, which was concentrated toward the equatorial plane, was stratified in two layers in the direction perpendicular to the equatorial plane. The first layer had a large optical thickness in the Ha line, (Ha) 5000, and was concentrated toward the equatorial plane, while the second layer had a smaller optical thickness, r(Ha) 200, and extended on both sides of the first layer. Both layers were slowly expanding with a velocity of 0.03 km 1 The total mass of the envelope increased from in 1973-1974 to 10-10M in 1975. Finally, the appearance of the broad absorption feature in the K line of Ca ii was interpreted as the formation of a rapidly rotating layer around the star's equator. Key words: Be star; Extended atmosphere; Pleione; Shell star; Spectral variation.