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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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At least 181 records · Page 10
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NASA NTRS1981

The ultraviolet properties of dust in the Orion Nebula

The continuum in the Orion Nebula was observed with the IUE at 16 positions varying from 30 arcsec to 5 arcmin in angular separation from the central star Theta(1) Orionis C. The atomic continuum was estimated from the H-beta brightnesses. The remaining scattered starlight was analyzed by multiple-scattering models in which the main parameters are: (1) the albedo of the dust; (2) the scattering phase function parameter, the averaged cosine of the angle of scattering; and (3) the density of dust grains at small distances from the star. It is found that (a) dust must be depleted near the star; (b) the albedo is fairly well determined and is constant across the 2200 A extinction feature; and (c) grains are quite forward-throwing at 1300 A, as they have been found at 4600 A earlier. The scattering is more isotropic at 2400 A. However, the absolute value of the scattering phase function parameters at all wavelengths is quite uncertain because it depends sensitively upon the density of grains close to the star.

Mathis, J. S.↗
NASA NTRS1981

No evidence of rings around Neptune

The results of two observations of stellar occultations of Neptune to determine if the planet has a ring system are reported. The sightings were made from Mt. Stromlo, Mauna Kea, and Cerro Tololo, noting that an equatorial ring would subtend only two arcsec of view. An upper accretion limit was defined to set the region around Neptune where rings, rather than satellites, could form. The intensities of the starlight from the two selected stars were recorded by photometers on magnetic tape during the occultation period. One of the stars did not occult, but passed through the entire region where a ring system might be present. No definitive evidence for rings was found, although an optical depth for a Neptunian ring was calculated at 0.07, with a width of more than 5 km and a radius of 31,400 km.

Elliot, J. L.↗
NASA NTRS1982

Extreme ultraviolet observations from the Voyager 2 encounter with Saturn

Combined analysis of helium (584 A) airglow and the atmospheric occultations of the star delta Scorpii imply a vertical mixing parameter in Saturn's upper atmosphere of K (eddy diffusion coefficient) of approximately 8 x 10 to the 7th sq cm per second, an order of magnitude more vigorous than mixing in Jupiter's upper atmosphere. Atmospheric H2 band absorption of starlight yields a preliminary temperature of 400 K in the exosphere and a temperature near the homopause of 200 K. Certain auroral emissions can be fully explained in terms of electron impact on H2, and auroral morphology suggests a link between the aurora and the Saturn kilometric radiation. Absolute optical depths have been determined for the entire C ring and parts of the A and B rings. A new eccentric ringlet has been detected in the C ring. The extreme ultraviolet reflectance of the rings is fairly uniform at 3.5 to 5 percent. Collisions may control the distribution of H in Titan's H torus, which has a total vertical extent of about 14 Saturn radii normal to the orbit plane.

Sandel, B. R.↗
NASA NTRS1982

Detection of the underlying galaxy in the QSO 3C48

Observation of the quasar 3C48 reveals that the faint nebulosity to the north and south of it is dominated by starlight and that the stars have a redshift close to that determined from the broad emission lines in the quasar spectrum. This provides the first direct evidence that a quasar is associated with a galaxy and lies at a distance corresponding to its redshift. The observations were made with a combination of a spectrograph and a charge-coupled device as a detector and took two hours of telescope time. A continuum with absorption features was seen as indicative of a young, hot stellar population. While the redshift of the underlying galaxy is the same as that of the permitted lines in the quasar, it differs from the redshift of the forbidden lines by 500 km/s.

Boroson, T. A.↗
NASA NTRS1982

Galactic and zodiacal light surface brightness measurements with the Atmosphere Explorer satellites

Galactic and zodiacal light surface maps based on the Atmosphere Explorer-C, -D, and -E satellite data are presented at 7320, 6300, 5577, 5200, and 4278 A. A procedure used to generate these maps, which involves separation of the individual stars and diffuse starlight from the zodiacal light, is described in detail. The maps can be used in atmospheric emission studies to correct for galactic emissions which contaminate satellite as well as ground-based photometric observations. The zodiacal light maps show enhanced features which are important for understanding the nature of interplanetary dust.

Abreu, V. J.↗
NASA NTRS1982

Far-ultraviolet studies. VI - Further limits on diffuse galactic light scattered at large angles by dust

The Apollo 17 wide-field far-ultraviolet spectrometer was used to scan six large bands of the sky during trans-earth coast. After accounting for internal scattered light and the light expected from stars in the field of view, no significant residual flux remains. This imposes important constraints on the scattering properties of the interstellar dust grains: either the grains are extremely strongly forward scattering, or the albedo of the grains is low. The absence of a scattered light signal from dusty, moderate galactic latitude regions of the sky indicates that the high galactic latitude flux reported by Henry et al. (1977) and by Anderson et al. (1979) does not represent starlight backscattered by dust.

Anderson, R. C.↗
NASA NTRS1982

A study of the diffuse galactic gamma radiation

Assuming cosmic rays pervade the Galaxy, they necessarily produced high energy gamma-rays as they interact with the instellar matter and photons. The cosmic ray nucleon interactions five rise to gamma rays primarily through the decay of pi mesons, giving a unique spectrum with a maximum at approximately 68 MeV. Cosmic ray electrons produce gamma rays through bremsstrahlung, but with a markedly different energy spectral shape, one which decreases monotonically with energy. Cosmic ray electrons also interact with the interstellar starlight, optical and infrared photons, and the blackbody radiation through the Compton process. A model of galactic gamma ray production is discussed, and the predicted spatial distribution and energy spectra are presented. Considering the uncertainty in the point source contributions, the agreement between the theoretical predictions and the gamma ray data seems quite reasonable.

Fichtel, C. E.↗
NASA NTRS1983

Autonomous satellite navigation by stellar refraction

This paper describes an error analysis of an autonomous navigator using refraction measurements of starlight passing through the upper atmosphere. The analysis is based on a discrete linear Kalman filter. The filter generated steady-state values of navigator performance for a variety of test cases. Results of these simulations show that in low-earth orbit position-error standard deviations of less than 0.100 km may be obtained using only 40 star sightings per orbit.

Gounley, R.↗
NASA NTRS1984

The S201 far-ultraviolet imaging survey. III - A field in Sagittarius

Far-ultraviolet imagery of a 20 deg diameter field in Sagittarius, centered near (1950) R.A. 18 h 34 m, decl. -30 deg 25 arcmin, was obtained by the S201 far-ultraviolet camera during the Apollo 16 mission. In a 10-minute exposure covering the 1250-1600 A wavelength range, 1034 star images are detectable, with a limiting ultraviolet magnitude of about 10. Most of these objects are identified with early-type stars listed in the Smithsonian Astrophysical Observatory Star Catalog, the Catalog of Stellar Identifications, or both, but 203 objects remain unidentified or are identified with late-type stars. The photometric measurements appear to be in reasonable agreement with those of the International Ultraviolet Explorer for stars in common, and with expectations for A0 stars. A detailed photometric study was made of the Messier 8 region, and it is concluded that dust-scattered starlight contributes about half of the total radiation observed from the central region of M8.

Carruthers, G. R.↗
NASA NTRS1984

The underlying galaxies of X-ray-selected quasars

Deep red images of 24 X-ray selected quasars were obtained using the SIT Area Photometer on the Palomar 1.5 meter telescope. About half were also imaged through green and violet filters. The images of all 15 of the quasars with redshifts up to 0.4 certainly are extended and the two at z = 0.45 probably are extended. The remainder of higher redshift are unresolved, presumably because they are too distant. The resolved structures are 10-30 in. across, symmetric, and centered on the point-like quasar nuclei. For most of the quasars the nuclear light and the extended emission can be roughly separated. Evidence that the extended emission is starlight from a surrounding galaxy is presented, permitting several properties of the host galaxy to be deduced. Its colors, surface brightness, linear extent, ellipticity, and frequent association with groups of galaxies indicate that it is usually a normal spiral galaxy.

Malkan, M. A.↗
NASA NTRS1984

Photometric study of NGC 2023 in the 3500 A to 10000 A region - Confirmation of a near-IR emission process in reflection nebulae

A surface brightness study of the reflection nebula NGC 2023 covering the 3500-10,000 A wavelength region performed with uvby photoelectric photometry and BVRI imaging with a CCD detector is reported along with VRI photometry of a cluster of embedded red stars. The nebular radiation in the 3500-5500 A region is dust-scattered starlight originating in the star HD 37903. The embedded red stars are probably pre-main sequence stars. The nebular surface brightness in R and I exceeds that expected on the basis of a reasonable radiative transfer model by factors of two and more than three, respectively. The excess radiation is extended across the nebula in a manner similar to the scattered light. The extended red emission may be interpreted as the high-frequency extension of extended emission discovered by Sellgren, Werner, and Dinerstein (1983) in the 2-5 micron region in NGC 2023 and two other reflection nebulae.

Schild, R. E.↗
NASA NTRS1984

Comet Bowell (1980b) - Measurement of the optical thickness of the coma and particle albedo from a stellar occultation

Comet Bowell (1980b) was observed to pass within 0.25 + or 0.09 arc sec of a star (about 540 km at the comet), where the absorption of starlight by the dust coma was found to be 3 percent (+ or - 1 percent). The implied optical thickness of 0.03 differs greatly from other determinations and gives a mass of 3 x 10 to the 13th g for the coma within 10,000 km of the nucleus. Coupled with absolute continuum filter photometry, these results indicate a very low particle albedo consistent with fluffy carbonaceous material. This experiment indicates the need to observe nearly central occultations by several observers to measure the optical thickness profile of a comet. The advantages of using a charge-coupled device area photometer for such observations are discussed.

Larson, S. M.↗
NASA NTRS1984

Spectral components at visual and infrared wavelengths in active galactic nuclei

Aperture-dependent infrared photometry of active galactic nuclei are presented which illustrate the importance of eliminating starlight of the galaxy in order to obtain the intrinsic spectral distribution of the active nuclei. Separate components of emission are required to explain the infrared emission with a spectral index of alpha approx = 2 and the typical visual-ultraviolet continuum with alpha approx = 0.3 (where F(nu) varies as nu(sup-alpha). Present evidence does not allow unique determination of the appropriate mechanisms, but the characteristics of each are discussed.

Stein, W. A.↗
NASA NTRS1984

The Draco Nebula, a Molecular Cloud Associated with a High Velocity Cloud?

Extended and very faint bright nebulae are found in high galactic latitudes at the Palomar Observatory Sky Survey. Such a nebula, located in the constellation Draco and called Draco Nebula or Dracula, was found to be in detailed positional coincidence with a 21 cm emission line feature. Estimates of the minimum visual extinction from star counts ON and OFF Dracula and an estimated visual surface brightness indicate that Dracula fits the relation SBV = 24.2 - 2.5 log AV for dust clouds located above the galactic plane and reflecting the integrated starlight of the galactic disk. Hence Dracula is probably a reflection nebula. Indicators of molecular hydrogen in Dracula, molecules such as CO, were searched for by using a 2.5-m mm-telescope. Molecular hydrogen column densities were estimated. The dynamics of CO clumps was studied. Dracula has a close positional and possibly even astrophysical relationship to the high velocity cloud phenomenon.

Mebold, U.↗
NASA NTRS1984

The 1.5 - 3.5 Micron Spectroscopy of the Orion H2 Source

A synopsis of current research with near infrared airborne spectroscopy of the Orion molecular hydrogen emission line source is presented. It is suggested that (H2) is the most abundant molecular species in the interstellar medium and is of paramount importance to understand its behavior and distribution in that medium. The H2 has no permanent electric dipole moment which precludes dipole transitions. It is, however, detected in absorption against background starlight by UV electronic transitions and in the IR through electric quadrupole rotation/vibration and pure rotation emission lines. The physical parameters of this region, in the context of their relationships to other members of the complex family of Orion objects, molecular clouds, compact IR sources, H 2 regions, and shock fronts are described.

Davis, D. S.↗
NASA NTRS1984

Occultations of stars and radio sources by comets: Predictions and observing prospects

The results that might stem from observations of occultations of stars and radio sources by cometary comae and ion tails are outlined. Taking account of the current orbital accuracy of P/Halley, specific predictions of stellar appulses with that comet, including preliminary topocentric circumstances, are presented. A crude model of the absorption of starlight by grains in a dusty comet supports the notion that stellar extinction may be observable in practice. Some thoughts on observing techniques are set down, discussing the merits of a two-dimensional detector (a CCD camera) and a conventional aperture photometer. The need for accurate photometric observations with time resolution approx. 1 sec is stressed because the interval of greatest extinction will usually be brief. Finally, the efficacy of undertaking accurate relative astrometry of comets during predicted approaches to stars is explored.

Bowell, E.↗
NASA NTRS1984

Structure of the Uranian rings. I - Square-well model and particle-size constraints

A least squares fitting is added to the diffraction model for the Uranian rings and the model is then used to determine the relative optical depths of the rings at visible and near-IR wavelengths. Light transmission is assumed constant on a scale of a few radial km in the rings. The amount of starlight passing through the rings is calculated from occultations with account taken of a monolayer ring, diffraction effects, and multiple ring layers. A diffracted occultation profile is generated with consideration given to the angular diameter of the star, the impulse response of the detector, and atmospheric aberrations. Calculation procedures which remove noise are outlined and results are compared with the observational data base. The wavelength dependence of optical depth is formulated. It is shown that occultation data do not support the presence of a large fraction of submicron particles. No optical depth variations are projected for 0.88-2.2 microns wavelengths. Unchopped data are needed to verify the model predictions.

Elliot, J. L.↗
NASA NTRS1984

Multiple spacecraft Michelson stellar interferometer

Results of an orbital analysis and performance assessment of SAMSI (Spacecraft Array for Michelson Spatial Interferometry) are presented. The device considered includes two one-meter telescopes in orbits which are identical except for slightly different inclinations; the telescopes achieve separations as large as 10 km and relay starlight to a central station which has a one-meter optical delay line in one interferometer arm. It is shown that a 1000-km altitude, zero mean inclination orbit affords natural scanning of the 10-km baseline with departures from optical pathlength equality which are well within the corrective capacity of the optical delay line. Electric propulsion is completely adequate to provide the required spacecraft motions, principally those needed for repointing. Resolution of 0.00001 arcsec and magnitude limits of 15 to 20 are achievable.

Stachnik, R. V.↗