ceetanrpmnmoen
=
ete Ree eae chee he ee at
on
te
aye
he
within 0.5 mag. Unfortunately, all pub-
discover whether there are “strong line”
and “weak line” B ‘stars such as are
lished reports of the flights covering the
range longward from 2000 A are rather
brief, so that it is difficult to judge
the quality of the data; nevertheless it
seems that the discrepancies can be
resolved if we allow for the effects of
interstellar reddening and the errors
in the observations—if we make the
comparison with the predictions from
line-blanketed models.
Scans of seven stars between 1700
and 4000 A were made with 50-A
resolution by use of a rocket-borne
known among the A, F, and stars.
The “strong line” or “weak line” characteristic is assigned chiefly according
to the apparent strengths of the lines
from the metals.
Should the ultraviolet spectra of
stars be observed with photometers of
narrow or wide band-pass, the effect
of all the lines lying in the passband
will reduce the measured intensity from
ent. Elst (6) has estimated the lineblocking for each 100 A between 1900
and 3000 A in the spectrum of one
model atmosphere; the results appear
in Fig. 5, the blocking being expressed
as a magnitude difference Am. Clear-
0.03
“
Li
2500
_
what it would be if no lines were pres-
.
3000
MA)
Fig. 5. Line blocking, in the region 1900
to 3000 A, in the spectrum of a model
atmosphere of type B1.5,
spectrometer
(J4).
At
wavelengths
greater than 2600 A the shapes of the
observed spectra closely resembled those
predicted from model stellar atmospheres, but at shorter wavelengths the
ultraviolet spectrum of a B star will
not appear smooth. In the spectral region below 1900 A where the wide,
deep resonance lines occur, the photometer response will vary even more
irregularly with central wavelength and
width of the passband, as a result of
line blocking.
models, and the effects of interstellar
reddening.
It is certain that line blanketing
plays an important role in establishing
the emitted spectrum at wavelengths
less than 1600 A; this fact is borne
fluxes decreased rapidly. This sudden
decline does not accord with normal
theories of stellar spectra. Later observations with the same type of spectrometer seem to give reasonable agreement with the models, suggesting that
the earlier abrupt decline in flux may
have been instrumental in origin.
The first ultraviolet line spectra of
stars other than the sun were obtained
out by the very recent results for 96
stars having spectral types from O7
(75) by use of Aerobee rockets; ob-
The Observations
jective-grating spectrographs were used
to A2 obtained
with
ly, to a narrow-band photometer, the
Broad-band photoelectric observations of O and B stars in the ultraviolet
have been made by several groups.
Much interest was raised when it was
reported that bright nebulosities in the
direction of Spica and other hot stars
had been detected by rocket-borne instruments recording in a wavelength
band from 1225 to 1350 A. However,
subsequent flights found nothing unusual, so that the suspected nebulosity
is now considered probably spurious
(7).
Chubb et al. (8) have measured ab-
solute fluxes from many hot stars in
bands 60-, 70-, and 200-A wide at
1115, 1314, and 1427 A, respectively;
relative to the fluxes at 5560 A, determined from ground-based obser-
vations, the ultraviolet was fainter than
expected, according to wunblanketed
models, by factors of 2 to 10. Morton
(4) has shown that part of the dis-
crepancy can be removed by inclusion
of the strong absorption lines falling
within the detector bandwidths in the
calculation of the predicted fluxes. The
differences are further reduced because
the latest blanketed models predict lower fluxes in the relevant ultraviolet spec-
tral regions and higherfluxes near 5560
A than unblanketed models predict. The
remaining discrepancies are no greater
8 DECEMBER 1967
than may be expected from the possible systematic errors in the observations, the omission of weak lines in the
(9) with a satellite-
borne photometer. After correction for
interstellar reddening, the observed ratio of ultraviolet to visual fluxes is consistently below the prediction by unblanketed models, but in reasonable
agreement with the Mihalas-Morton
blanketed B1.5V model.
A rocket (10) scanned 22 early-type
southern stars at 1900 A with detectors
of 400-A half-width; the results indicated that the B stars were only 50
to 33 percent as bright at this wavelength as was expected from unblanketed models, but unfortunately there
was
no
absolute calibration of the
flight photomultipliers. Other (77) rock-
et-borne photometers have found fluxes at 2120 A, in a passband with
half-width of 188 A, that were about
0.75 times those expected from models
lacking lines. A few observations at
2200 and 2600 A have been reported
(12); no comparisons have been made
with model atmospheres, but these
data were used for estimation of the
effects of interstellar reddening at these
wavelengths. Fluxes from some 100
stars in bands of 400-A half-width,
centered at 2100, 2500, and 2800 A,
have been measured (73); when com-
pared with the visual fluxes, these ul-
traviolet observations agree with the
predictions
by
unblanketed
models
f/2
Schmidt
dispersion
direction
cameras
having
fields of view of 10 degrees. An active
gyro system with gas jets oriented the
rocket toward the desired target. The
+ %-degree limit-cycle jitter in the
was
reduced
to
+ 16 seconds of are by pivoting the
spectrographs in the direction of the
dispersion and by attaching to them a
large gyro rotor gimbaled about a
perpendicular axis. Torques in the dispersion direction resulted in precession
of the gyro, with the end result that
fine stabilization in one coordinate was
achieved by a purely mechanical system.
On 2 June 1965 spectra of the mainsequence stars § Scorpii, BO, and x
Scorpii, Bl, were photographed with a
camera having a calcium fluoride corrector
that
transmitted
rocket
motion
wavelengths
longer than 1265 A (Fig. 6); wavelength resolution was about 1 A. No
fine stabilization was provided normal
to the dispersion, so that the residual
widened
the
spectra,
causing them to overlap in part; such
widening increases confidence in the
identification of features. The spectra of
$§ and z Scorpii are much as anticipated—continuous emission, with absorption lines.
The zero-order images of 48 and 49
Librae permit one to determine a wave1277