Engineering topics

Holberg, J. B.

Publications and source records attributed to Holberg, J. B..

At least 55 records · Page 3

NASA NTRS1985

Far-ultraviolet background observations at high galactic latitude. I - The Coma Cluster

A series of high-galactic-latitude observations made with the Voyager 2 ultraviolet spectrometer are used to place stringent upper limits on the far-ultraviolet emissions from the central regions of the Coma Cluster of galaxies. In particular, consideration of the 912-1150 A region yields lower limits upon the radiative decay lifetimes of any neutrino species with masses in the range 22.1-27.8 eV, assuming massless decay products. Limits on the radiative decay lifetimes of such neutrinos are found to vary from 2.4 x 10 to the 25th to 7.1 x 10 to the 24th s, respectively, as a function of neutrino rest mass. This represents a substantial improvement over previous measurements.

Holberg, J. B.↗

NASA NTRS1985

Studies of hot B subdwarfs. Part 2: Energy distributions of three bright sdB/sdOB stars in the 950-5500 angstrom range

Voyager ultraviolet spectrometer observations of the subdwarf B or OB stars HD 205805, UV 1758+36 and Feige 66 are presented. All three objects display the H I Layman series in absorption. These observations are combined with low dispersion IUE spectrophotometry and with Stroemgren photometry to construct virtually complete energy distributions, which extend over the range 950-5500 angstroms. Effective temperatures based on model atmosphere calculations for high gravity, hydrogen rich stars are determined. Our analyses yield T Sub e 28,200 + or - 1300 K for HD 205805, T sub e 31, 800 + or - 1100 K for UV 1758+36, and T sub e 35,700 + or - 1500 K for Feige 66. The importance of far ultraviolet observations below L sub alpha in reducing the uncertainties associated with the interstellar extinction and the degradation of the IUE sensitivity is emphasized.

Wesemael, F.↗

NASA NTRS1984

Voyager EUV and FUV Observations

The Voyager 1 and 2 ultraviolet spectrometers are sensitive over the wavelength range 500 to 1700 A. In the extreme ultraviolet (EUV), at wavelengths shortward of the Lyman limit (912 A), Voyager observations detected emission from three out of a sample of 11 nearly hot DA white dwarfs. These observations imply very low HI column densities in the directions of the three stars detected. In the far ultraviolet (FUV), at wavelengths between 912 and 1200 A, Voyager observations of O and B stars can be used to study interstellar reddening at the shortest wavelengths and to provide useful estimates of interstellar H2 column densities.

Holberg, J. B.↗

NASA NTRS1984

The eccentric Saturnian ringlets at 1.29 R(s) and 1.45 R(s)

The shapes and kinematics of the Titan and Maxwell ringlets of Saturn's C ring are studied. The ringlets are narrow and eccentric, with mean widths of about 25 km for Titan and abut 64 km for Maxwell, and with eccentricities of order 10 to the -4th. They have sharp edges on a scale of about 1 km, normal optical depths of roughly 0.5-2.0, normal optical depth of the surrounding region, and positive linear width-radius relations. The Titan ringlet is the only feature whose dynamics appears to be completely determined by an external satellite; the kinematics of the Maxwell ringlet are determined solely by Saturn's nonspherical gravity field. The comparatively large measured optical depths and large inferred mass extinction coefficients for the ringlets suggest an environment and particle size distribution different from the remainder of the C ring and presumably caused by the mechanism responsible for ring confinement.

Porco, C.↗

NASA NTRS1984

Saturn's nonaxisymmetric ring edges at 1.95 R(s) and 2.27 R(s)

The outer edges of Saturn's A and B rings, at 2.27 R(s) and 1.95 R(s), have been examined using data acquired by four Voyager experiments. The shapes and kinematics of these features are influenced by their proximity to strong low-order Lindblad resonances. The data for the A-ring edge are consistent with a seven-lobed radial distortion of amplitude 6.7 + or - 1.5 km which rotates with the mass-weighted mean angular velocity of the coorbital satellite system. The B-ring edge has essentially a double-lobed figure of radial amplitude 74 + or - 9 km which rotates with the mean motion of Mimas, though there is an indication that it is not completely described with a simple Saturn-centered ellipse. An upper limit of 10 m has been placed on the vertical thickness in the unperturbed region of the B ring.

Porco, C.↗

NASA NTRS1984

Far-ultraviolet spectrophotometry of two very hot O type subdwarfs

As a result of a spectroscopic survey of stars classified as nonemission OB+, Drilling (1983) has detected 12 new subluminous O stars. It was found that these stars are the hottest known O type subdwarfs. The effective temperatures of the stars are 60,000 K or higher. It has been possible to observe two of these stars with Voyager 1, taking into account LSE 21 and LS IV +10.9 deg. LSE 21 is one of the hottest of the new subdwarfs, with an effective temperature of at least 100,000 K. The optical spectrum indicates a hydrogen-rich atmosphere of high surface gravity. LX IV +10.9 deg is one of the cooler objects with an effective temperature of 65,000 K. The optical spectrum indicates an extremely helium-rich atmosphere and a somewhat lower surface gravity than LSE 21. The Voyager 1 observations confirm the temperature scale set up by Schoenberger and Drilling (1984) for the hottest O type subdwarfs.

Drilling, J. S.↗

NASA NTRS1984

Far-ultraviolet observations of BW Vulpeculae with Voyager 2

BW Vul has been the subject of numerous ground-based spectroscopic and photometric studies. However, observations in the ultraviolet have been limited. The Voyager 2 observations presented in the current study include the first observations of BW Vul shortward of 1200 A. Voyager 2 observations in the 912-1700 A region are employed to investigate the variations in the temperature, radius, and luminosity of BW Vul during a pulsation cycle. Attention is given to details regarding the observations, the light curves representing the variation of BW Vul at effective wavelengths of 1050 A and 1395 A, the UV spectra, comparisons with other stars, comparisons with models, and the amplitude-wavelength relation.

Barry, D. C.↗

NASA NTRS1984

The far-ultraviolet energy distribution of Sirius B from Voyager 2

Observations of Sirius obtained with the Voyager 2 ultraviolet spectrometer clearly reveal the presence of flux from the white dwarf Sirius B at wavelengths between 950 and 1100 A. These observations are in good agreement with all previous ultraviolet observations of Sirius B, and in particular with the IUE observations of Boehm-Vitense, Dettmann, and Kapranidis. A joint analysis of the Voyager 2 and IUE observations yields a temperature range of 26,000-28,000 K. A reexamination of current ultraviolet, visible, and X-ray observations produces good general agreement, but no single, mutually consistent, temperature for Sirius B. The Voyager 2 observations can be used to place a firm upper limit of 28,000 K on the temperature of Sirius B.

Holberg, J. B.↗

NASA NTRS1984

Saturn's rings - Structure, dynamics, and particle properties

The current state of knowledge of Saturn's rings is reviewed. A brief historical introduction is given, followed by a discussion of the radial profile of the rings, ring dynamics, features without azimuthal symmetry, ring particle size distribution, and the physical properties of ring particles. The direction for future analysis and observation is considered.

Esposito, L. W.↗

NASA NTRS1984

Saturn's rings - Properties and processes

In the present consideration of the structural and particle properties of Saturn's rings, emphasis is given to spacecraft observations and an attempt is made to relate observed properties to favored causative processes. While the ring particles are primarily icy, there is evidence for compositional variation on both local and regional scales. Ring structure is generally dominated by collisional and gravitational dynamics. On the basis of such features as orbital resonances with various satellites, which drive spiral density and bending waves, the ring mass density and local vertical thickness can be determined.

Cuzzi, J. N.↗

NASA NTRS1984

Atlas of Saturn's rings

A preliminary summary is made from a compilation of Voyager Saturn ring data. The data base includes optical thickness radial profiles at 3.6 and 13.6 cm radio wavelength measurements, 1300 and 2640 A occultations of delta Sco, albedos in the 3000-6000 A range, time and azimuthal resolved albedo profiles, particle impacts in the G ring, and number densities of magnetospheric charged particles. Sample data tracks are provided for the A, C, and D rings and the Cassini division. Finally, the trajectory of the Voyager 2 pass through the ring system is plotted.

Collins, S. A.↗

NASA NTRS1983

Saturn's upper atmosphere from the Voyager 2 EUV solar and stellar occultations

The temperature and composition of the upper atmosphere of Saturn have been inferred from Voyager 2 ultraviolet-spectrometer-occultation measurements made by observing the sun and the star Delta Scorpii while they were being occulted by Saturn. The observations analyzed here provide atmospheric parameters from 2900 km down to 960 km above the 1-bar level referred to the equator. The temperature in the model simulation of the data is 420 + or - 30 K down to about 1600 km. Below 1600 km the temperature decreases with a variable lapse rate down to 120 + or - 30 K near the methane homopause located at 1010 + or - 40 km. A constant temperature at 120 K is applied in the model for the region of the methane homopause down to 960 km where the present analysis terminates. Column amounts of H2 and H were measured from 2900 km down to about 1100 km, giving respective densities of about 1.0 x 10 to the 8th/cu cm and about 5.5 x 10 to the 6th/cu cm near the exobase at 2500 km. Near the methane homopause the H2 density is = 1.2 x 10 to the 12th/cu cm with a CH4 number-density mixing ratio of 0.00006. The eddy-diffusion coefficient in the vicinity of the methane homopause is estimated to be 5.0 x 10 to the 6th sq cm/sec. The atomic hydrogen density profile suggests a downward H flux of 1.8 x 10 to the 9th sq cm/sec from near the exobase down to a terminal boundary of about 1200 km.

Smith, G. R.↗

NASA NTRS1983

Eccentric ringlet in the Maxwell gap at 1.45 Saturn radii Multi-instrument Voyager observations

The results of Voyager experiments to characterize the eccentric ringlet at 1.45 Saturn radius are discussed. The Voyager I carried out microwave occultation trials with a dual-frequency transmitter, while Voyager 2 observed the occultation of delta Scorpii using an UV spectrometer and a photopolarimeter. The ringlet was associated with a basically empty region next to an edge of the C ring which decreased to a few hundred meters thickness. The width of the ringlet in the gap was estimated to vary from 38-88 km, and it maintains an elliptical shape. It precesses within the 265 km gap it occupies, and has a double-peaked core 15 km wide. The bulk of the ringlet is particles of less than 1 cm radius.

Esposito, L. W.↗

NASA NTRS1983

Saturn's pole - geometric correction based on voyager UVS and radio occultations

Measurements of of the opacity of Saturn's rings acquired during occultation experiments at radio (3.6 cm) and ultraviolet wavelengths were initially reduced to radial position rho using a standard pole vector. Common features in the two data sets from this reduction were offset by distances Delta rho(i). These offsets have been attributed to an error in the pole direction. Because the viewing geometries were quite different for the two experiments, the set of differences of Delta rho(i) provides a sensitive measure of corrections needed to refine the Saturn pole direction. The new standard pole vector in 1950.0 coordinates has right ascension alpha = 38.409 + or - 0.016 deg and declination delta = 83.324 + or - 0.002 deg.

Simpson, R. A.↗

NASA NTRS1982

Identification of 1980S27 and 1980S26 resonances in Saturn's A ring

The Voyager 2 ultraviolet spectrometer observation of a stellar occultation by Saturn's rings contains evidence for features associated with at least 31 resonances due to the F ring shepherd satellites 1980S27 and 1980S26. A limited analysis of density wave patterns associated with eight of these resonances yields surface mass densities for the A ring consistent with previous estimates.

Holberg, J. B.↗

NASA NTRS1982

Voyager absolute far-ultraviolet spectrophotometry of hot stars

Voyager observations in the 912-1200 A spectral region are used to indirectly intercompare absolute stellar spectrophotometry from previous experiments. Measurements of hot stars obtained by the Voyager 1 and 2 ultraviolet spectrometers show considerably higher 912-1200 A continuum fluxes than the recent observations of Brune et al. (1979) and Carruthers et al. (1981). The intercomparisons show all observations in basic agreement near 1200 A. The Carruthers et al. flux measurements are preferred down to 1050 A at which point the Voyager and Brune et al. values are respectively 60% higher and 60% lower. Below 1050 A the diasgreement among the observations becomes very large and the fluxes predicted by model atmospheres have been adopted. The pure hydrogen line-blanketed model atmosphere calculations of Wesemael et al. 1980) in comparison with Voyager observations of HZ 43 are used to adjust the Voyager calibration below 1050 A. This adjusted Voyager calibration, which is in good agreement with current model atmosphere fluxes for both early-type stars and DA white dwarfs, will be used for Voyager astronomical observations.

Holberg, J. B.↗

NASA NTRS1982

Identification of resonance features within the rings of Saturn

The Voyager 2 UV spectrometer observed a stellar occultation by the rings of Saturn, which located ring features with an accuracy of 12 km. A high-resolution (3 km) optical depth atlas of the rings shows at least nine features, including four density wave patterns, identified with satellite resonances. Analysis of these density wave patterns yields the first surface mass densities for the A ring and, together with the optical depth atlas, a total ring mass of 6.4 x 10 to the -8th Saturn masses.

Holberg, J. B.↗

NASA NTRS1982

Titan's upper atmosphere - Composition and temperature from the EUV solar occultation results

It is inferred from observation of an occultation of the sun by Titan, using the Voyager 1 UV spectrometer, that temperatures are 176 + or - 20 K near the evening terminator and 196 + or - 20 K near the morning terminator, and that the major atmospheric constituent is N2, with a density of 2.7 + or - 0.2 x 10 to the 8th/cu cm at 3840 km. A layer of absorbing molecules, possibly polymers, is found near both morning and evening terminators. A photochemical model suggests that the homopause is located at 3500 + or - 70 km, with an eddy diffusion coefficient of 1(+2, -0.7) x 10 to the 8th/sq cm per sec, which decreases to about 1000 sq cm/sec in the lower stratosphere as N2 to the -2/3 power.

Smith, G. R.↗