a
SII 1194 SII 1201
CI 1175
90+
(3 lines)
:
(2 lines}
{5 lines)
80}
T
TT
T
T
‘
i
T
1
“a
100
4
.
70}
:
1206.52
60;
50r
Theoretical
.
from model
4
40+
spectrum
30+
7
approximate type
B15¥V
oot
iO;
1180
1170
1160
1150
1220
1210
1200
1190
1240(A)
1230
Fig. 3. Predicted strong stellar lines in the neighborhood of 1200 A.
those of Mg II at 2800 A. Calculations
suggest that these lines will be quite
strong in B-type spectra although less
in the outermost layers of the models.
Thus study of the observed profiles
and displacements of the strong resonance lines should yield information
about the electron temperature and
pressure and the velocity field in the
outermost parts of the stellar atmosphere, especially whene an improved
theory of line-formation is used. Mor-
strong than the resonance lines of C
IV and Si IV. This region contains a
few strong lines of Si III and C III
and very many lines of the second and
third spectra of the metals. Estimates
of the strength and profiles of the lat-
pear in Fig. 4. The lines from the
third spectra of the metals should be
easily observed, with a spectral purity
of the order of 0.5 to 1 A.
The line profiles shown in Fig. 4
have been calculated with a so-called
microturbulence equal to three times
the thermal Doppler broadening. Observation has shown that the spectral
lines of the standard “sharp line” stars
are not really sharp, and that predicted
profiles most easily can be madetofit
the observed profiles by introduction
of the arbitrary parameter microturbulence. The adopted motions are small,
the root-mean-square velocity of an
atom or ion being about 12 kilometers
per second instead of 3 kilometers per
second as is characteristic of heavier
atoms at the temperatures in B1.5
atmospheres.
There are many ways in which such
a velocity distribution could arise in
a stellar atmosphere. Trial computations have shown that the adopted velocity field, which is used in descrip-
tion of the shape of the line-absorplines of the second spectra will be tion coefficient, has far more influence
weak, but that the lines of the third
tinuous spectrum from O and stars
on the shape and strength of lines
(Fig. 1) at wavelengths between 911.6
spectra of the metals will be deep and
predicted at wavelengths longer than
and 1500 A; rather the spectrum of as wide as 1 A. The spectra of B
2000 A than have differences in the
stars between 1900 and 3000 A are temperature-pressure structure of the
main-sequence stars will be cut up by
many deep absorption lines—some of expected to look rather like spectra - models due to the inclusion or omiswhich are shown schematically in -Of F stars in the spectral region obsion of line blanketing in the process
Fig. 2.
served with ground-based instruments;
of construction of models.
many weak lines and a moderate numThe wavelength region between 1900
Observation and interpretation of the
and 3000 A has been studied at Utrecht.
ber of moderately strong, deep lines
line spectra of B stars in the wavelength
The only resonance lines expected are
will be present; typical line profiles apregion between 1900 and 3000 A
should lead to new and improved estimates of the abundances of the metals
(Fe, Mn, Cr, Ti) in the atmospheres
-05
+05
-05
+05
-05
+05
-05
+05 A
re
d
tT
T
|
OD
t
T
T
i
er
i
]
I
of B stars. These estimates should be
100
—~
100
quite reliable because the line spec90+
\
+ 90 trum in the region between 1900 and
3000 A is formed through exactly the
sol
| 80
same parts of the atmosphere as is
70
Cri
Fell
Fell
NiI
|
P
283563
275329
2739.54
221648 70 the part of the spectrum observed with
ground-based instruments. (This state60~ 60
Model B13
ment appears obviously true from the
50+
B15
50
relative value of the continuous-absorption coefficient in the region between
100
100
1900 and 6800 A.) Thus good models
90
|
|
-| 90
for the relevant layers of the atmo80}.
+ BO
sphere can be established and controlled by normal ground-based obser7Or
| 70
Til
Cri
MnIl
Fell
vation.
6oL
2516.01
211753
_| 2068.96
2103.80 | 6,
The reason the abundances of the
metals in B stars cannot be estimated
50+
+ 50
easily from the parts of the spectrum
40
4 40
that are observed by ground-based instruments is that too few lines of suf30+
+ 30
ter lines (5, 6) have shown that the
ton’s calculations show that it is quite
unreal to expect observation of a con-
1
-05
{
t
+05
!
!
-05
al
t
+05
!
I
-05'
!
t
+05
l
ft
-05
i
+05 A
Fig. 4. Predicted lines from the second and third spectra of the metals in the spectral
region 1900 to 3000 A.
1276
ficient strength occur tn the accessible
spectral region to permit successful applications of the usual methods of spectrum analysis. It will be interesting to
SCIENCE, VOL. [58