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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1981

HR 4511 - A probable Cepheid with a supergiant-like hot companion

IUE observations have shown HR 4511 to have an early B-type companion with a strong stellar wind. Fitting the energy distribution of the system from far-UV to middle-IR yields a difference of about 3.2 mag at the V passband. Since HR 4511 is in the small cluster Stock 14 and has M(V) = -7.8, M(V) for the secondary is about -4.6, which is only slightly more luminous than the known BO.5 III and B1 stars at the turnoff point. This implies an inconsistency between the deduced luminosity of the secondary and the nature of its UV spectrum, which mimics stars of considerably greater luminosity. It is suggested that the secondary may have been observed in a brief stage between core and shell H burning.

Parsons, S. B.↗
NASA NTRS1981

Ultraviolet interstellar extinction toward 1367 stars observed by ANS

Five-band UV photometry is used to investigate the character of interstellar extinction toward 1367 stars of spectral type B3 and earlier, with observations producing a galactic average of 5.11, 4.78, 6.52, 4.10 and 1.95 at 1550, 1800, 2200, 2500, and 3300 A, respectively. Differences in the strengths of the UV extinction features of these stars appear to be independent, since objects with either strong or weak 2200 A extinction can have strong, normal or weak far-UV extinction. Further investigation reveals that some of the deviant stars are embedded in localized regions whose average extinction curves diverge dramatically from the galactic mean. The anomalous extinction of these regions attests to localized inhomogeneities in the UV characteristics of interstellar dust. Because the evaluation of UV extinction toward a particular star would be more accurate if based on the average curve of the region immediately surrounding that star rather than that of the entire Galaxy, it is recommended that the galactic average presented be used with caution to correct astronomical data of individual objects.

Meyer, D. M.↗
NASA NTRS1981

Ultraviolet interstellar extinction toward stars in the Orion Nebula and toward HD 147889

The International Ultraviolet Explorer (IUE) was used to obtain low resolution absolute ultraviolet energy distributions for the four Trapezium stars Theta(1) Orionis A, B, C, and D, Theta(2) Orionis A and B, HD 147889, and several unreddened standard stars. The observations for Theta(1) Orionis B reveal a peculiar extended object. The measurements for the rest of the stars were used to derive UV extinction curves. The derived curves are very different from the average galactic curve. The curves for the Orion stars all exhibit smaller than normal middle and far-UV extinctions. Although a 2175 A feature is present, its strength, when normalized to E(B-V), is smaller than in the average galactic curve. In contrast, the curve for HD 147889 has a pronounced 2175 A feature followed by smaller than normal extinction for an inverse wavelength between 5 and 7 per micron.

Bohlin, R. C.↗
NASA NTRS1982

Vacuum deposition of iridium on large astronomical mirrors for use in the far UV

An iridium coating has been deposited by electron-beam evaporation on a 0.91-m mirror which serves as the telescope primary of a sounding rocket instrument for far-UV spectrometry. The evaporation was carried out by applying 8 kV at 400 mA to the electron gun. Zone refined Ir of 99.99% purity was used, and the electron beam was electromagnetically swept over the surface of the evaporant. Under these conditions, deposition rates of 0.55 A/sec were achieved. The reflectance distribution achieved at a wavelength of 584 A was extremely uniform; the mean reflectance was 21.2% with a standard deviation of only 0.3%. This represents a substantial improvement over Al + MgF2 and Al + LiF coatings for applications involving multiple reflections and weak signals, as might be expected in a high-resolution spectrograph studying distant celestial objects.

Herzig, H.↗
NASA NTRS1982

The ultraviolet excess of Seyfert 1 galaxies and quasars

A sharp rise from 4000 to 3650 A in the excess flux was obtained when a power-law determined in the red and infrared was subtracted from IR-optical-UV composite spectra of eight Seyfert 1 galaxies and quasars to measure UV excess. The excess is produced by Balmer continuum emission which, relative to H-alpha, is greater than the prediction of Case B recombination by a factor of 1.5-2.5. An additional component from 5000 A to the far-UV was found when the spectra were fitted with combinations of power-law and hydrogen recombination continua. The component is well described by a blackbody at a single temperature ranging from 20,000 to 30,000 K. All of the observed spectra were producible with combinations of power-law, recombination and blackbody emission, and all of the objects in the sample were found to have a power-law component with a slope of -1.1 + or - 0.1.

Malkan, M. A.↗
NASA NTRS1982

Outer atmospheres of cool stars. X - HR 1099 at quadrature

IUE high-dispersion, far-UV spectra of the active-chromosphere RS CVn binary HR 1099 are reported. The emission features produced by such high-temperature species as C II and C IV are found to be very bright, to exhibit structure, change significantly in the one-week interval separating the two exposures, and generally follow the radial velocity motion of the K subgiant primary, while the less massive G dwarf secondary appears only weakly in the composite spectrum. It is concluded that chromospheric and transition region emission in RS CVn binaries is a stellar rather than system phenomenon, and the structure evident in some of the emission line shapes is interpreted as a patchy brightness distribution on and above the K star surface that is spread out in velocity by the rapid rotation.

Ayres, T. R.↗
NASA NTRS1982

Ultraviolet absorption by highly ionized atoms in the Orion Nebula

The International Ultraviolet Explorer was used to obtain high-resolution, far-UV spectra of theta 1 A, theta 1 C, theta 1 D, and theta 2 A Orionis. The interstellar absorption lines in these spectra are discussed with an emphasis on the high-ionization lines of C IV and Si IV. Theta 2 A Ori has interstellar C IV and Si IV absorption of moderate strength at the velocity found for normal H II region ions. Theta 1 C Ori has very strong interstellar C IV and Si IV absorption at velocities blueshifted by about 25 km/s from that found for the normal H II region ions. The possible origin of the high-ionization lines by three processes is considered: X-ray ionization, collisional ionization, and UV photoionization. It is concluded that the C IV and Si IV ions toward theta 2 A and theta 1 C Ori are likely produced by UV photoionization of surrounding nebular gas. In the case of theta 1 C Ori, the velocity shift of the high-ionization lines may be produced through the acceleration of high-density globules in the core of the nebula by the stellar wind of theta 1 C Ori.

Franco, J.↗
NASA NTRS1982

Ultraviolet spectrophotometry of 2A 1822-371 - A bulge on the accretion disk

It is suggested that the 5.57-hour modulation of the X-ray source 2A 1822-371 is caused by the combined effects of (1) an occultation of the emitting region by a companion star, and (2) a bulge on the accretion disk surrounding the X-ray source. It is speculated that the changing aspect of the X-ray-heated inner face of the bulge with orbital phase may also contribute to the modulation at UV and optical wavelengths. The bulge's position angle suggests it to have been the result of turbulence caused by the impact of a gas stream transferring matter from the companion, and 2A 1822-371 is held to provide the most direct indication extant of such a structure in an accretion disk. It is speculated that comparison of high-time resolution UV observations with optical and X-ray light curves will allow further deductions as to the size, structure and location of the far-UV emitting region.

Mason, K. O.↗
NASA NTRS1982

The scattering phase function of interstellar grains - The case of the reflection nebula NGC 7023

IUE observations of the reflection nebula NGC 7023 and the illuminating star, HD 200775, in the spectral range from 1300 to 3100 A are combined with ground-based measurements of the nebular brightness distribution at 3500, 4100, 4700, and 5500 A to determine the scattering properties of the nebular dust grains. Total nebular fluxes are derived from existing data obtained by satellite and ground-based observations. The relevance of far-IR fluxes from the nebula is considered, and models are derived that are applicable to NGC 7023 both with respect to the ratio of nebular to stellar fluxes and with respect to the nebular surface brightness distribution. It is shown that the average grain albedo in the UV is about 0.54 and that the albedo increases to a level of about 0.6 at 1400 A after reaching a minimum of approximately 0.4 near 2200 A. The results suggest that isotropically scattering particles of high albedo make a significant contribution to interstellar scattering in the far-UV.

Witt, A. N.↗
NASA NTRS1981

Further developments of electrographic image detectors

This paper reports the current status of a program to develop far- and middle-ultraviolet-sensitive electrographic detectors. These include large-format, semitransparent-photocathode detectors and opaque-photocathode electrographic Schmidt cameras. Modifications of a previously-demonstrated far-UV large-format detector to make it suitable for possible Shuttle/Spacelab space astronomy investigations have been implemented and tested. Middle-ultraviolet-sensitive large-format and Schmidt electrographic detectors are now in the laboratory development stage. Cesium telluride photocathodes are used to provide sensitivity in the 1700-3100 A spectral range, and techniques for processing these photocathodes are discussed. The preparation and use of polyimide barrier membranes, for protecting the photocathodes from deterioration by film outgassing, are described. A new opaque-photocathode vacuum-ultraviolet-sensitive electrographic detector has been demonstrated, which is based on an oblique-focusing electron optical system similar to that used by Princeton with electron-bombarded CCD arrays. Test results and potential applications of this detector are described.

Carruthers, G. R.↗
NASA NTRS1982

International Ultraviolet Explorer observations of the R Aquarii jet

Ultraviolet spectra were obtained with the International Ultraviolet Explorer of the newly discovered optical-radio jet feature in the symbiotic variable R Aquarii. The far-UV continuum of the jet is characterized by strong continuum which rises with decreasing wavelength in the 1200-2000 A wavelength range and is considerably different in appearance from the relatively flat continuum exhibited by ionized nebulosity in the central star. Prominent Si III semi-forbidden lines and Si II emission lines seen in the central region are virtually absent in the jet. This could reflect the depletion of silicon in the feature, the result of grain formation in material that has been ejected by the central star. Consistent with this interpretation is the overall excitation of the jet that suggests it is cooler than the nebulosity that engulfs the central UV object.

Michalitsianos, A. G.↗
NASA NTRS1983

HD 207739 - A strange composite star

This star, classified F8 IIe + B:, has a very unusual ultraviolet spectrum, with abnormally strong and numerous absorption features in the far-UV and exceptionally strong Mg II emission. There is some resemblance to shell and pre-main-sequence B stars, but it more closely matches the strange spectra of the eclipsing systems VV Cep and SX Cas, and it probably has considerable circumstellar material at fairly high temperature. HD 207739 is probably an interacting binary and needs to be monitored for light and velocity variations.

Parsons, S. B.↗
NASA NTRS1982

Microchannel intensified electrography

Although electrography is an attractive electronic imaging technique, it suffers at low light levels from limitations imposed by microphotometer and emulsion grain noise. The use of microchannel intensification allows individual photoelectron events to be unambiguously detected and measured, with little loss of achievable resolution. A series of quantitative studies have been conducted to determine the gains in detectivity and SNR yielded by the use of this technique, through microphotometry and computer analysis of images identical or similar to laboratory light sources, by means of both unintensified and microchannel-intensified electromagnetic Schmidt cameras. Applications of the technique to far-UV wide field imaging and nebular spectrograph experiments are described.

Heckathorn, H. M.↗
NASA NTRS1983

The broad-band X-ray spectrum of Cygnus X-2

Cygnus X-2 was observed with the broad-band X-ray spectroscopy experiment, HEAO 1 A-2, in the energy range 0.4-18 keV for four intervals of approximately 31 s over the course of 5 days in 1977. The spectra can be adequately represented by single-temperature thermal bremmstrahlung continua with temperatures ranging from 3.7 x 10 to the 7th K to 6.4 x 10 to the 7th K. An examination of the spectra and the spectra-luminosity relationship effectively rules out one degenerate dwarf model for the X-ray emission. The far-UV continuum emission could be dominated by this continuum component during X-ray high states, an effect which would be detected in optical UV line observations. A Comptonized X-ray cloud around a neutron star remains a viable model for the observed X-ray spectra.

Pravdo, S. H.↗
NASA NTRS1983

Observationally determined Fe II oscillator strengths

Absorption oscillator strengths for 21 Fe II resonance lines, have been determined using a curve-of-growth analysis of interstellar data from the Copernicus and International Ultraviolet Explorer (IUE) satellites. In addition to slight changes in strengths of the far-UV lines, new f-values are reported for wavelength 1608.45, a prominent line in interstellar and quasar absorption spectra, and for wavelength 2260.08, a weak, newly identified linen in IUE interstellar spectra. An upper limit on the strength of the undetected line at 2366.867 A (UV multiplet 2) is set. Using revised oscillator strengths, Fe II column densities toward 13 OB stars are derived. The interstellar depletions, (Fe/H), relative to solar values range between factors of 10 and 120.

Van Steenberg, M.↗
NASA NTRS1983

Far ultraviolet observations of the expanding shell in Eridanus

After correcting for hot stars in the instrument field of view, 1350-1750 A wavelength band observations of a thin strip of the sky near the south galactic pole have yielded count rates which vary smoothly from a minimum of about 55/cs near l = 30 deg, b = -60 deg to a maximum of about 740/cs in the direction of l = 199 deg, b = -49 deg. These values correspond to specific intensities of 600-8000 photons/sq cm per sec per sr per A, assuming a flat incident spectrum in this wavelength range. The magnitude and spatial variation of the observed signal are incompatible with any known source of terrestrial far-UV airglow, zodiacal light, or with the expected emission of high latitude reflection nebulosities. Attention is given to alternative sources that, together with the reflection nebulae, may explain the high intensities in this region.

Paresce, F.↗
NASA NTRS1983

Far-ultraviolet emission lines in U Cephei - Evidence for a hot, turbulent circumstellar envelope

The numerous emission lines in the far-UV spectrum which have been found in the Algol-type eclipsing binary U Cephei are similar to those observed in the W Serpentis stars, although they are weaker and appear only during the primary eclipse. The line-emitting region is studied in both emission and absorption.It surrounds the hotter component and extends to a distance of several of its radii. A model is proposed in which the emission originates in a highly turbulent circumstellar region, at an electron temperature of about 10,000 K. The energy needed for ionization is derived from collisions, and is probably extracted from the accretion energy.

Plavec, M. J.↗
NASA NTRS1984

A Far Ultraviolet Imaging Spectrograph for Shuttle

The development of the Far Ultraviolet Imaging Spectrograph (FUVIS) Shuttle sortie missions, is described. Objectives of the experiment are to obtain spatially-resolved far-ultraviolet spectra of extraterrestrial sources, including emission-line and reflection nebulae, diffuse background radiation, extragalactic objects, and comets. The use of fast focal ratio (f/1) Schmidt optics and an opaque CsI photocathode which affords high quantum efficiency in the far-UV provides the maximum possible diffuse source sensitivity. Measured emission line intensities of 5 Rayleighs (or continua of intensity 1 R/A) in 300 sec exposures are expected. The development includes a dedicated pointing platform and a low light level television camera for payload specialist use in target acquisition and guiding.

Carruthers, G. R.↗