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At least 55 records · Page 3
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NASA NTRS1993

Hopkins Ultraviolet Telescope observations of far-ultraviolet scattering in NGC 7023 - The dust albedo

We have obtained the first sub-Ly-alpha spectroscopic observations of the reflection nebula NGC 7023 and its illuminating star, HD 200775, using the Hopkins UV Telescope during the Astro-1 mission in December, 1990. The ratio of the nebular to stellar flux is virtually flat between 1100 and 1860 A, indicating that sigma(a), the cross section for absorption, must rise sharply with decreasing wavelength. Independent of any model, this means that much of the far-UV rise in the extinction curve is due to an increase in absorption rather than scattering. If, in addition, we assume a spherical geometry, we derive an albedo of 0.5 at 1100 A with somewhat higher values at longer wavelengths. If the geometry is not spherical, lower values of the albedo may be obtained.

Murthy, Jayant↗
NASA NTRS2001

Far Ultraviolet Spectroscopic Explorer Observations of the Seyfert 1.5 Galaxy NGC 5548 in a Low State

We present far-ultraviolet spectra of the Seyfert 1.5 galaxy NGC 5548 obtained in 2000 June with the Far Ultraviolet Spectroscopic Explorer (FUSE). Our data span the observed wavelength range 915-1185 A at a resolution of approximately 20 km s(exp -1). The spectrum shows a weak continuum and emission from O VI (lambda)(lambda)1032, 1038, C III (lambda)977, and He II (lambda)1085. The FUSE data were obtained when the AGN (Active Galactic Nuclei) was in a low state, which has revealed strong, narrow O VI emission lines. We also resolve intrinsic, associated absorption lines of O VI and the Lyman series. Several distinct kinematic components are present, spanning a velocity range of approximately 0 to -1300 km s(exp -1) relative to systemic, with kinematic structure similar to that seen in previous observations of longer wavelength ultraviolet (UV) lines. We explore the relationships between the far-UV (ultraviolet) absorbers and those seen previously in the UV and X-rays. We find that the high-velocity UV absorption component is consistent with being low-ionization, contrary to some previous claims, and is consistent with its non-detection in high-resolution X-ray spectra. The intermediate velocity absorbers, at -300 to -400 km s(exp -1), show H I and O VI column densities consistent with having contributions from both a high-ionization X-ray absorber and a low-ionization UV absorber. No single far-UV absorbing component can be solely identified with the X-ray absorber.

Brotherton, M. S.↗
NASA NTRS1972

Possibility that the far ultraviolet excess in M31 is due to main sequence stars

The far ultraviolet excess in the central region of M31, observed by OAO-2, could be due to young main sequence stars. More than enough such stars are present in the model for the M31 inner disk population derived by Tinsley and Spinrad (1971) to match line- and color-indices at longer wavelengths. If the far ultraviolet radiation of typical galaxies arises from young stars, the theoretical ultraviolet background is enhanced greatly by evolutionary effects. For evolution at the rate of Tinsley and Spinrad's model for M31, or of Arnett's (1971) linear model for our galaxy, the enhancement is a factor 2.5 to 14, depending on the Hubble constant and the spectrum at wavelengths below 1700 A.

Tinsley, B. M.↗
NASA NTRS1977

Far-ultraviolet studies. II - Galactic-latitude dependence of the 1530 A interstellar radiation field

A 0.62-sq cm Geiger counter, sensitive between 1425 and 1640 A, was used to map the far-ultraviolet brightness of about half the sky, providing an experimental measurement of the far-ultraviolet interstellar radiation field. At 1530 A, the energy density is approximately 7.4 by 10 to the -17th power erg/cu cm per A. Comparison with integrations of star catalogs calibrated to the ultraviolet shows, as expected, that the bulk of the radiation comes directly from B- and A-type stars. The galactic-latitude dependence of the radiation is analyzed in an unsuccessful attempt to set limits on the absorbing and scattering properties of the interstellar grains in the far-ultraviolet. Excess radiation observed at the galactic pole is probably residual airglow from above the rocket altitude.

Henry, R. C.↗
NASA NTRS1972

Apollo 16 far-ultraviolet camera/spectrograph - Earth observations.

A far-ultraviolet camera/spectrograph experiment was operated on the lunar surface during the Apollo 16 mission. Among the data obtained were images and spectra of the terrestrial atmosphere and geocorona in the wavelength range below 1600 angstroms. These gave the spatial distributions and relative intensities of emissions due to atomic hydrogen, atomic oxygen, molecular nitrogen, and other species - some observed spectrographically for the first time.

Carruthers, G. R.↗
NASA NTRS1973

The far-ultraviolet spectrum of Jupiter.

A far-ultraviolet spectrum of Jupiter, observed from a sounding rocket, is presented. The H I 1216-A disk brightness was 4.4 kR. Other characteristics of the 1180- to 1900-A spectrum were Rayleigh scattering of solar radiation longward of 1600 A and emission between 1250 and 1500 A and near 1600 A.

Rottman, G. J.↗
NASA NTRS1982

High dispersion far ultraviolet spectra of cool stars

Recent far ultraviolet high dispersion spectra of two cool supergiant stars, Beta Dra (G2 Ib) and Alpha Ori (M2 Iab) are examined in the context of current questions regarding stellar chromospheres, coronae and mass loss. These stars show very different outer atmosphere structure. Beta Dra has a geometrically thin transition region with bright emission lines of 100,000 K plasma that are red-shifted, indicating downflow in magnetic flux tubes. By contrast, Alpha Ori has a cool extended chromosphere and circumstellar envelope with large mass loss.

Stencel, R. E.↗
NASA NTRS1981

Observations of the far ultraviolet nightglow

The far UV emission in the 1350-1700 A band observable at all latitudes and solar zenith angles up to 165 deg looking down from 225 km is discussed with reference to the Apollo-Soyuz data and results reported by Huffman et al. (1980). It is suggested that the much higher intensities of nighttime far UV radiation reported by Huffman et al. must be due either to an impulsive short-lived source or to a secular increase since 1975 of one or more of the parameters affecting the unknown emission mechanism.

Paresce, F.↗
NASA NTRS1976

A far-ultraviolet photometer for planetary surface analysis

The measurement of local variations in the far-ultraviolet albedo is explored as a means of detecting changes in the refractive index of rocks and dust on the surface of atmosphereless planets and satellites. Far-ultraviolet spectrophotometric measurements of the lunar surface, which were obtained on the Apollo 17 orbital mission, are presented to demonstrate that significant albedo variations occur in the spectral range 120 to 170 nm. These data also confirm the hypothesis that the albedo variations represent refractive-index differences in the surface materials. A three-band photometer is described which, when put in orbit around a solar system object, is capable of providing refractive-index maps with a sensitivity of one part in the second decimal place and with kilometer resolution. Comparative surface-composition and surface-history analyses based on such maps are discussed.

Henry, R. C.↗
NASA NTRS1977

Apollo-16 far-ultraviolet spectra in the Large Magellanic Cloud

Spectra in the wavelength ranges from 900 to 1600 A and 1050 to 1600 A of some OB associations in the Large Magellanic Cloud were obtained from the lunar surface by the Apollo-16 far-ultraviolet camera/spectrograph on April 22, 1972. The observed spectral distributions appear consistent with a stellar model having an effective temperature of 30,000 K, reddened by E(B-V) = 0.3, and characterized by the average far-ultraviolet extinction curve of Bless and Savage (1972). However, the absolute intensity of the far-ultraviolet spectrum of the associations NGC 2050 and 2055 seems somewhat too bright in comparison with ground-based photometry.

Carruthers, G. R.↗
NASA NTRS1973

Apollo 16 far-ultraviolet camera/spectrograph - Instrument and operations.

A far-ultraviolet camera/spectrograph experiment was designed and constructed for studies of the terrestrial upper atmosphere and geocorona, the interplanetary medium, and celestial objects from the lunar surface. The experiment was successfully operated during the Apollo 16 mission 21-23 April 1972. Discussed are the design and operating principles of the instrument, the actual events and operations during the Apollo 16 mission, and also anomalies encountered and suggested improvements for future experiments of this type. This experiment demonstrated the utility of the electronographic technique in space astronomy, as well as the great potential of the lunar surface as a base for astronomical observations.

Carruthers, G. R.↗
NASA NTRS1989

Far ultraviolet observations of hot DA white dwarfs

Far ultraviolet (FUV) fluxes have been used for determining the effective temperatures of a number of DA white dwarfs hotter than 20,000 K. The analysis consisted of comparing the observed FUV fluxes with model fluxes scaled to the V-band flux. After suitable corrections were performed for the time-dependent sensitivity degradation of the IUE, it was found that the available flux calibrations for the IUE were insufficiently accurate for precise temperature determinations. Accordingly, seven white dwarfs for which accurate, independent temperature determinations were made from line profile analyses were used to improve the accuracy of the IUE flux calibration. The correction to the original calibration was as great as 20 percent in individual 5-A wavelength bins, while the average over the IUE wavelength range was 5 percent. Both the IUE flux correction and the temperatures obtained for the hot white dwarfs are presented.

Finley, David↗
NASA NTRS1978

S201 catalog of far-ultraviolet objects

A catalog of star images was compiled from images obtained by an NRL Far-Ultraviolet Camera/Spectrograph operated from 21 to 23 April 1972 on the lunar surface during the Apollo-16 mission. These images were scanned on a microdensitometer, and the output recorded on magnetic tapes. The catalog is divided into 11 parts, covering ten fields in the sky (the Sagittarius field being covered by two parts), and each part is headed by a constellation name and the field center coordinates. The errors in position of the detected images are less than about 3 arc-min. Correlations are given with star numbers in the Smithsonian Astrophysical Observatory catalog. Values are given of the peak density and the density volume. The text includes a discussion of the photometry, corrections thereto due to threshold and saturation effects, and its comparison with theoretical expectation, stellar model atmospheres, and a generalized far-ultraviolet interstellar extinction law. The S201 catalog is also available on a single reel of seven-track magnetic tape.

Page, T.↗
NASA NTRS1978

Far-ultraviolet studies. III - A search for light scattered at large angles by dust

The Apollo 17 far-ultraviolet spectrometer was used during trans-earth coast to measure the brightness of two regions at moderate galactic latitudes. The signal obtained can be entirely accounted for by light from stars in the field of view, with no contribution from galactic-plane starlight scattered off interstellar dust. It is concluded that the interstellar grains are extremely poor large-angle scatterers in the far-ultraviolet.

Henry, R. C.↗
NASA NTRS1973

Far ultraviolet spectral radiance calibrations at NBS.

The range of NBS radiometric calibration services has been extended into the far ultraviolet region of the spectrum where a dc high-power hydrogen wall-stabilized arc is used as a primary standard of spectral radiance. A capability in the range from 130 to 360 nm (overlapping conventional tungsten strip lamp radiometry) is presently available with estimated uncertainties between 5 and 10%, depending upon wavelength. The status of radiometric source standards in the far ultraviolet is briefly reviewed, and the hydrogen arc and NBS calibration facility are described in detail. The use of commercially available mercury Krefft-Rossler lamps and deuterium arc lamps as transfer or secondary standards is discussed, and the spectra of these lamps calibrated with the hydrogen arc standard are presented.

Ott, W. R.↗