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