After this work, Araki and Milne both switched their focus from the topic. Araki
turned his interest to the physics of stellar atmospheres, and Milne moved to Oxford
University and began work on relativistic cosmology.
4.2.3 Extended Atmospheres of Stars
In 1937–1942, Araki, collaborating with Kurihara Michinori (栗原道徳,
1903–1978), carried out theoretical studies on the formation of emission lines in
extended atmospheres (or envelopes) in a state of expansion, as seen in Wolf-Rayet
(WR), Novae, and P Cygni type stars.
The nature of WR stars was first investigated in 1929 by C. S. Beals (1899–1979)
in the Dominion Astrophysical Observatory, Canada (Beals 1929). He paid special
attention to the spectral line profiles of these stars. Their profiles are characterized by
a strong and broad emission with an absorption edge on the violet side. Beals
proposed for the first time a theory that the emission lines of these stars are formed
in spherically expanding envelopes, whereas the dark violet edges represent the
absorption of stellar radiation in the envelope lying between the star and observer.
He also applied this theory to the P Cygni stars, which show similar line profiles
(Beals 1934a, b).
Araki and Kurihara considered the emission-line profiles formed in an
acceleratively expanding envelope (Araki and Kurihara 1937). Compared to planetary nebulae, stellar envelopes are characterized by higher electron densities that
require the solution of radiative transfer in parallel with the equations of recombination processes.
To avoid solving such cumbersome processes, Araki and Kurihara introduced a
new concept of optically effective boundary in the case of acceleratively expanding
envelopes. Suppose a frequency ν in an emission line, and the radiation of this
frequency is emitted from a limited region having an equal Doppler velocity inside
the envelope. Araki and Kurihara called this region the optically effective boundary.
By introducing this concept, they were able to calculate emission-line profiles. For a
spherical envelope, they assumed that the expanding velocity V (r) at radius r is
given as a power function of index s as
V r
ð Þ ¼ V
Ã
r
R
Ã
s
ð4:2Þ
where V* denotes the outflow velocity at the stellar surface and R* the stellar radius.
They calculated emission-line profiles for some cases of s, and the results are
shown in Fig. 4.12, where the envelope size is assumed to be sufficiently large
compared to the stellar radius.
Araki and Kurihara applied these theoretical profiles to the variation of the
spectrum of Nova Aquilae 1918 observed by Beals, as shown in Fig. 4.13, where
the profile of the Hβ line is observed in two epochs of August 23 (lower profile) and
92
4 Astronomy in Early Showa. II. Kyoto 1926–1945
turned his interest to the physics of stellar atmospheres, and Milne moved to Oxford
University and began work on relativistic cosmology.
4.2.3 Extended Atmospheres of Stars
In 1937–1942, Araki, collaborating with Kurihara Michinori (栗原道徳,
1903–1978), carried out theoretical studies on the formation of emission lines in
extended atmospheres (or envelopes) in a state of expansion, as seen in Wolf-Rayet
(WR), Novae, and P Cygni type stars.
The nature of WR stars was first investigated in 1929 by C. S. Beals (1899–1979)
in the Dominion Astrophysical Observatory, Canada (Beals 1929). He paid special
attention to the spectral line profiles of these stars. Their profiles are characterized by
a strong and broad emission with an absorption edge on the violet side. Beals
proposed for the first time a theory that the emission lines of these stars are formed
in spherically expanding envelopes, whereas the dark violet edges represent the
absorption of stellar radiation in the envelope lying between the star and observer.
He also applied this theory to the P Cygni stars, which show similar line profiles
(Beals 1934a, b).
Araki and Kurihara considered the emission-line profiles formed in an
acceleratively expanding envelope (Araki and Kurihara 1937). Compared to planetary nebulae, stellar envelopes are characterized by higher electron densities that
require the solution of radiative transfer in parallel with the equations of recombination processes.
To avoid solving such cumbersome processes, Araki and Kurihara introduced a
new concept of optically effective boundary in the case of acceleratively expanding
envelopes. Suppose a frequency ν in an emission line, and the radiation of this
frequency is emitted from a limited region having an equal Doppler velocity inside
the envelope. Araki and Kurihara called this region the optically effective boundary.
By introducing this concept, they were able to calculate emission-line profiles. For a
spherical envelope, they assumed that the expanding velocity V (r) at radius r is
given as a power function of index s as
V r
ð Þ ¼ V
Ã
r
R
Ã
s
ð4:2Þ
where V* denotes the outflow velocity at the stellar surface and R* the stellar radius.
They calculated emission-line profiles for some cases of s, and the results are
shown in Fig. 4.12, where the envelope size is assumed to be sufficiently large
compared to the stellar radius.
Araki and Kurihara applied these theoretical profiles to the variation of the
spectrum of Nova Aquilae 1918 observed by Beals, as shown in Fig. 4.13, where
the profile of the Hβ line is observed in two epochs of August 23 (lower profile) and
92
4 Astronomy in Early Showa. II. Kyoto 1926–1945