The study of Be stars in Japan was initiated by Miyamoto Shotaro in the 1940s
(Chap. 4) and was taken over by Kogure Tomokazu on the theoretical radiation field
of extended stellar atmospheres in the late 1950s. Spectroscopic observations of Be
stars began in 1966 with the Coudé focus of the 188-cm reflector at the OAO. The
first study was conducted on some Be pole-on stars, while Kogure remained at the
Paris Observatory, Meudon, by making use of spectral plates obtained at the OAO
and the Haute Provence Observatory. Based on Miyamoto’s revised theory, Kogure
analyzed the profile of Hβ emission and derived the disk and ring parameters for
some pole-on stars (Kogure 1969).
B and Be stars often show changes in type, such as from B to Be, Be to Be-shell,
and vice versa, known as a phase change. Hirata Ryuko and Kogure started observations of Be-shell stars, in particular Pleione (28 Tauri, B8 IVe), which showed
remarkable phase changes in the twentieth century as follows: Be (1905–1918) ! B
(1918–1938) ! Be-shell (1938–1956) ! Be (1956–1971) ! Be-shell
(1971–1988) ! Be (1988–2005). Hirata and Kogure observed the whole period of
the second shell phase in 1871–1988, and they analyzed the formation of shell lines
(Hirata and Kogure 1984; Hirata 1995).
The phase change of Pleione was caused by a change in the optical depth of the
disk, not by a change in inclination angle, as shown in Fig. 7.21. With the increase in
optical depth, shell absorption lines in the Balmer series are produced up to higher
members. In strong shell stars, shell lines become visible up to H30 or even H40.
Hirata and Kogure analyzed the central depth of shell absorption lines and derived
the optical depth at Hα lines, τ (Hα), and the fractional area, β, of a shell-lineforming disk relative to the stellar surface. The variations in τ (Hα) and β in the early
period of the shell phase of Pleione are illustrated in Fig. 7.22, where β is composed
of two parts: β 1 , the fractional area of an optically thicker region (τ (Hα) ! 2000),
and β 2 , that of an optically thinner region (τ (Hα) ~ 600). The optical depth τ (Hα)
remained at a high level, 10
3
, throughout the period (Fig. 7.22).
From this analysis the structure of Pleione’s envelope was described as follows.
The Be star phase in 1955–1971 was characterized by an optically thin (τ(Hα) < 100)
but vertically expanded disk (vertical height ! stellar radius). The shell phase started
with the formation of an envelope, optically thick (τ(Hα) ~4000), but vertically thin
near the equatorial plane (the disk area relative to the stellar photosphere was around
0.1 in the initial phase). The vertical height of the absorbing disk gradually increased,
as seen in Fig. 7.22, and finally covered the whole photosphere. In contrast, the optical
depth τ(Hα) gradually decreased and shell absorption lines disappeared in around
1988, and Pleione changed to the ordinary Be star in the 1990s.
In 1996, Katahira Jun’ichi, Hirata Ryuko, and their group performed a period
analysis of the radial velocities of the shell lines of Pleione and found the star to be of
a binary nature with a period of 218 days (Katahira et al. 1996). Based on polarimetric observations and considering the effects of binary interaction, Hirata proposed the precession model for Pleione (Hirata 2007). He supposed that the longterm spectroscopic variations could be explained in terms of the disk precession
(precession angle 59
with a period of 81 years). He argued that the maximum shell
phase occurred at the edge-on view of the disk.
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7 Postwar Development of Astrophysics, 1946–2000 (Part II: Astrophysics)
(Chap. 4) and was taken over by Kogure Tomokazu on the theoretical radiation field
of extended stellar atmospheres in the late 1950s. Spectroscopic observations of Be
stars began in 1966 with the Coudé focus of the 188-cm reflector at the OAO. The
first study was conducted on some Be pole-on stars, while Kogure remained at the
Paris Observatory, Meudon, by making use of spectral plates obtained at the OAO
and the Haute Provence Observatory. Based on Miyamoto’s revised theory, Kogure
analyzed the profile of Hβ emission and derived the disk and ring parameters for
some pole-on stars (Kogure 1969).
B and Be stars often show changes in type, such as from B to Be, Be to Be-shell,
and vice versa, known as a phase change. Hirata Ryuko and Kogure started observations of Be-shell stars, in particular Pleione (28 Tauri, B8 IVe), which showed
remarkable phase changes in the twentieth century as follows: Be (1905–1918) ! B
(1918–1938) ! Be-shell (1938–1956) ! Be (1956–1971) ! Be-shell
(1971–1988) ! Be (1988–2005). Hirata and Kogure observed the whole period of
the second shell phase in 1871–1988, and they analyzed the formation of shell lines
(Hirata and Kogure 1984; Hirata 1995).
The phase change of Pleione was caused by a change in the optical depth of the
disk, not by a change in inclination angle, as shown in Fig. 7.21. With the increase in
optical depth, shell absorption lines in the Balmer series are produced up to higher
members. In strong shell stars, shell lines become visible up to H30 or even H40.
Hirata and Kogure analyzed the central depth of shell absorption lines and derived
the optical depth at Hα lines, τ (Hα), and the fractional area, β, of a shell-lineforming disk relative to the stellar surface. The variations in τ (Hα) and β in the early
period of the shell phase of Pleione are illustrated in Fig. 7.22, where β is composed
of two parts: β 1 , the fractional area of an optically thicker region (τ (Hα) ! 2000),
and β 2 , that of an optically thinner region (τ (Hα) ~ 600). The optical depth τ (Hα)
remained at a high level, 10
3
, throughout the period (Fig. 7.22).
From this analysis the structure of Pleione’s envelope was described as follows.
The Be star phase in 1955–1971 was characterized by an optically thin (τ(Hα) < 100)
but vertically expanded disk (vertical height ! stellar radius). The shell phase started
with the formation of an envelope, optically thick (τ(Hα) ~4000), but vertically thin
near the equatorial plane (the disk area relative to the stellar photosphere was around
0.1 in the initial phase). The vertical height of the absorbing disk gradually increased,
as seen in Fig. 7.22, and finally covered the whole photosphere. In contrast, the optical
depth τ(Hα) gradually decreased and shell absorption lines disappeared in around
1988, and Pleione changed to the ordinary Be star in the 1990s.
In 1996, Katahira Jun’ichi, Hirata Ryuko, and their group performed a period
analysis of the radial velocities of the shell lines of Pleione and found the star to be of
a binary nature with a period of 218 days (Katahira et al. 1996). Based on polarimetric observations and considering the effects of binary interaction, Hirata proposed the precession model for Pleione (Hirata 2007). He supposed that the longterm spectroscopic variations could be explained in terms of the disk precession
(precession angle 59
with a period of 81 years). He argued that the maximum shell
phase occurred at the edge-on view of the disk.
202
7 Postwar Development of Astrophysics, 1946–2000 (Part II: Astrophysics)