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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Cusp region particle precipitation and ion convection for northward interplanetary magnetic field

Data from Atmosphere Explorer D for periods of strong northward interplanetary magnetic field are discussed. In the dayside magnetospheric cusp region energy time spectrograms of suprathermal positive ion fluxes exhibit a characteristic 'V' pattern as the spacecraft moves toward higher latitudes; that is, with the peak in the energy spectrum falling in energy and then rising again. Convection velocities follow this pattern closely with strong eastwest flows (with antisunward components) occurring in the equatorward half of the 'V' and significant sunward flows occurring in the poleward half of the 'V'. These patterns can be understood qualitatively in terms of a model of ionospheric electric potential produced by the known dependence of Birkeland current densities on magnetic activity.

Burch, J. L.↗
NASA NTRS1981

Vacuum UV and soft X-ray optical emissions from electron impact on metals

Spectrograms from 2200 to 20 A of optical emissions produced by a simple lamp using fast (3-10-keV) electron impact on tungsten and tantalum targets are presented. The lamp dissipated up to 5 kW/sq cm on the target surface, simultaneously cleaning the surface and generating bremsstrahlung in the soft X-ray region with 3-keV electron impact and longer wavelength transition radiation at the higher energy. In both the bremsstrahlung and transition radiation modes, the lamp emissions were of sufficient strength and reproducibility to qualify the two mechanisms as potential processes for use in a secondary standard light source. Calculated spectral distributions of transition radiation for normally incident electrons on tungsten, tantalum, and aluminum targets, with a viewing direction 45 deg from the normal, are also shown.

Hughes, R. H.↗
NASA NTRS1981

Energetic magnetosheath ions and the interplanetary magnetic field orientation

Times when energetic ions are absent and present in ISEE 1 magnetosheath plasma spectrograms are correlated with ISEE 3 IMF orientation measurements. The study indicates that when the plasma at the spacecraft is traced along a streamline to the bow shock surface, the angle between the surface normal at that point and the IMF is greater than 60 deg when the energetic ions are absent and less than 60 deg when they are present. The pattern is consistent with the ions coming from the same regions of the bow shock where intermediate and diffuse ions are found on the upstream side. The 60 deg criterion is used to draw schematic patterns of the location of energetic ions in the magnetosheath as a function of IMF orientation. Some orientations result in layers adjacent to the magnetopause and other orientations give layers adjacent to the bow shock.

Crooker, N. U.↗
NASA NTRS1981

Electron densities for 10 planetary nebulae derived from the semiforbidden C III lambda 1907/1909 ratio. II

Electron densities for IC 351, IC 2165, J900, IC 3568, NGC 6644, NGC 6891, IC 4997, NGC 7009, Hu 1-2, and IC 5217 are derived from high-dispersion semiforbidden C III spectrograms. For seven of these nebulae, the derived electron density is larger than the values derived from either surface brightness measurements or forbidden line ratios. Only one object, Hu 1-2, shows pronounced splitting of the semiforbidden C III emission lines due to a large expansion velocity.

Feibelman, W. A.↗
NASA NTRS1981

Two young stars in L 43

Optical and infrared photometry has been obtained for the two stars (RNO 90 and RNO 91) illuminating reflection nebulae in the small, filamentary dust cloud L 43. Spectrograms of RNO 90 in the blue and red have also been obtained, which show that it is a T Tauri star. Both stars have strong infrared excesses, placing them firmly in the 'dust' position of the H - K, K - L diagram. The absorption reversals superimposed on the Balmer lines from H-alpha to H-delta in RNO 90 display an interesting effect. They appear to move progressively redward from a position near the blue edge of the emission line at H-alpha to a nearly central location at H-delta. L 43 is located near the star-forming end of the Sco OB2 association, but is relatively isolated from the major center of activity, the Rho Oph cloud. There are a large number of similar-looking small clouds in its vicinity, only one of which is known to have an associated T Tauri star (AS 209). The complexity of this, the nearest star-forming region in an OB association, is illustrated.

Herbst, W.↗
NASA NTRS1981

Photographic observations of comets at Lowell Observatory

Observations of Halley's comet at Lowell Observatory cover a period from November 10, 1909 to May 17, 1911: 334 direct photographs were taken, 118 objective prism spectra, and 32 slit spectrograms of the nucleus, many of them including 5 arc minutes of the surrounding coma. Just one morning's observations are illustrated as an example of utilizing every possible resource available at the Observatory at the time. Observational improvements developed since then are described and some suggestions for the coming return of Halley's comet are made.

Giclas, H. L.↗
NASA NTRS1982

Ducted auroral kilometric radiation

Certain discrete, intense wave signals attributed to auroral kilometric radiation (AKR) were observed with ISEE-l while it was within the plasmaspheric shadow zone for direct propagation. It is believed that wave ducting by thin depletions of the plasma density aligned with the magnetic field accounts for such signals, and that their discrete nature is caused by the satellite intercepting individual ducts. These ducts, which were also observed as coincident decreases of the upper hybrid resonance frequency, appeared to be twenty-percent depletions roughly one hundred kilometers across. The AKR, which is emitted approximately perpendicular to the magnetic field, apparently entered these ducts equatorward of the source after the waves had been refracted parallel to the duct axis. A diffuse background was also observed which is consistent with the leakage from similar ducts at lower L-values. These observations establish the existence of ducted AKR, its signature on the satellite wave spectrograms, and new evidence for depletion ducts within the plasmasphere.

Calvert, W.↗
NASA NTRS1982

The remarkable spectrum of some material ejected by Eta Carinae

Some of the outer condensations of the very massive star Eta Carinae have emission-line spectra which convey clues to the nature of the star from which they were ejected. Ground-based spectrograms as well as ultraviolet IUE data on the brightest such condensation is described. Excess nitrogen presumably resulting from the C/N/O cycle implies that the star is at least moderately evolved. The facts seem consistent with recent suggestions that turbulent mixing, combined with mass loss, causes a very massive star like Eta Carinae to evolve quasi-homogeneously.

Davidson, K.↗
NASA NTRS1982

Identification of Fe I lines in the ultraviolet solar spectrum

High-resolution ultraviolet echelle spectrograms of the sun obtained from a rocket-borne spectrograph have been used for the identification of Fe I absorption lines in the wavelength range 1770-2020 A. Wavelength measurements, with a precision of about 15 mA, are tabulated for 425 Fe I features, along with available laboratory wavelenghs or predicted wavelengths calculated from atomic energy level data. The classification system of Moore has been extended to include possible additional multiplets with 126 newly classified lines; 234 unclassified lines were observed and identified with laboratory Fe I spectra

Mccabe, M. K.↗
NASA NTRS1982

Halley's Comet

Since 240 B.C., Chinese observers have documented a nearly unbroken record of scientifically useful observations of Periodic Comet Halley (P/Halley). Investigations of the comet's motion by Western astronomers are discussed, taking into account the first successful prediction of a cometary return by Halley (1705), computations conducted by Rosenberger (1830), and studies performed by Cowell and Crommelin (1910). Comet Halley's motion and nongravitational forces are considered along with meteor showers associated with P/Halley. The physical properties of P/Halley are examined, giving attention to the visual observations, the light curve of P/Halley, the coma, the tails, direct photographs, spectrograms, and the emission spectrum of P/Halley. Other subjects explored are related to the cometary nucleus, the mass of P/Halley, the rotation period and axial inclination, the composition, a nominal model of P/Halley's coma, and plans for investigations in connection with the coming apparition of Comet Halley.

Newburn, R. L., Jr.↗
NASA NTRS1982

The optical spectrum of R136a - The central object of the 30 Doradus nebula

R136 (HD 38268) is the brightest member of the compact cluster of stars at the center of the 30 Doradus Nebula, a massive H II region in the Large Magellanic Cloud (LMC). The far-ultraviolet luminosity is equivalent to at least 1000 ordinary O type stars. Optically, R136 is an unresolved patch corresponding to a linear extent of about 0.7 pc. Several components are known to be present, of which R136a is the brightest. A quantitative description of the optical spectrum of R136a has not been available in the literature. The present investigation is concerned with the analysis of spectrograms in the visible region. It is found that the optical spectrum of R136a shows broad but rather weak emission lines of ions which are frequently seen in early WN-type stars. The centroid of H-alpha is displaced by approximately 100 km/s redward of the nebular H-alpha emission. The Balmer series is visible, in absorption, up to H11.

Ebbets, D. C.↗
NASA NTRS1982

Ultraviolet shell formation at V1016 Cygni

A recent 330 minute high-resolution International Ultraviolet Explorer (IUE) spectrogram of V1016 Cygni shows splitting of the 1550 A component of the C IV resonance doublet, suggesting that a nebular shell is in the process of formation. An expansion velocity of 15 km/s is indicated by the IUE data and corresponds to the low range encountered with normal planetary nebulae, but IUE data of V1016 Cyg, taken approximately one year earlier, do not show split C IV lines. The recent IUE observations are consistent with VLA contour maps at 1.3 cm made by Newell and Hjellming (1981) and by Hjellming and Bignell (1982) that morphologically show an extended shell-type object. Thus, V1016 Cyg may have undergone an event which forms a planetary nebula.

Feibelman, W. A.↗
NASA NTRS1983

Velocity field and physical conditions in the active lenticular galaxy NGC 3998

A rotating and expanding flattened distribution of gas is suggested by measurements of the emission line velocity field for the line elliptical/lenticular galaxy NGC 3998, using seven long slit spectrograms in five position angles. Expanding material kinetic energy values of 10 to the 53rd to 10 to the 54th ergs, together with the flat spectrum radio source and nucleus X-ray emission, indicate pronounced nuclear activity. Spectrophotometry of the galactic nucleus shows emission line strengths typical of shocks rather than of photoionization, and line ratios indicate a postshock temperature of 60,000 K and a preshock density of 25 particles/cu cm. Both the stars and the ionized gas of the galaxy have central velocity dispersions of 260 km/s. In view of the high rotational velocity of the stars, NGC 3998 is a lenticular rather than elliptical galaxy.

Blackman, C. P.↗
NASA NTRS1983

Profiles and intensity ratios of the C IV lambda 1548, 1550 emission lines in planetary nebulae

The C IV resonance doublet at lambda 1548, 1550 is an important diagnostic tool in the study of planetary nebulae. The predicted theoretical intensity ratio of 2:1 is, however, rarely observed in the high dispersion IUE spectrograms. Values of 0.8-2.0 are observed for a sampling of 11 planetary nebulae of various excitation classes and for three proto-planetary nebulae. A diversity of line profiles is observed. In addition to the C IV doublet, weak subsidiary lines are seen. The spacing between these pseudo-lines happens to correspond to the separation of the C IV lines but are believed to be due to hot pixels and do not conform to the radial velocity displacement of the C IV lines.

Feibelman, W. A.↗
NASA NTRS1983

IUE observations of the perplexing bipolar planetary nebula NGC 2346

A preliminary analysis of International Ultraviolet Explorer (IUE) low-dispersion spectrograms of NGC 2346, obtained at various phases of the eclipsing nucleus, shows features typical of a high-excitation planetary nebula. Variations in line profiles, intensities, and stellar continuum are observed as a function of time. An orbital period of 16.0 days was determined from 1.8m(v) variations measured by the IUE Fine Error Sensor (FES) and agrees with the period derived from radial velocity measurements by Mendez.

Feibelman, W. A.↗
NASA NTRS1983

The remarkable spectrum of the RV Tauri star R Scuti at deep minimum

In September 1981 R Sct H-alpha declined in brightness to V about 8.5, an exceptionally faint minimum. A 15 A/mm spectrogram of the H-alpha region of R Sct two days past minimum brightness, when the visual magnitude was about 8.3, was obtained. This is apparently the first spectral observation of R Sct close to such a deep minimum. Contrary to spectroscopic observations during 'ordinary' minima of V about 6-7, at this deep minimum the expected G-K Ia absorption spectrum is not seen. Instead, a fantastic array of dozens of strong, narrow emission features and strong TiO absorption bands are present. The emission lines are due to neutral metals, and closely resemble a reversed photospheric spectrum. The deep minimum spectrum is apparently purely chromospheric, and is not produced in a stellar wind or extended atmosphere. The spectroscopic behavior of R Sct at deep minimum, as well as a number of other kinematic and photometric properties, is surprisingly similar to that of the R Coronae Borealis variables.

Howell, S. B.↗
NASA NTRS1983

Ultraviolet observations of M1-2 (= VV 8)

First IUE spectrograms of the peculiar emission-line object M1-2 (= VV 8) are presented. In the wavelength region 1150-3200 A this object is characterized by emission lines superposed on a moderately strong continuum. Some ultraviolet and optical properties of M1-2 are discussed in the context of symbiotic stars and protoplanetary and planetary nebulae. An E(B-V) extinction of 0.37 mag is obtained from the ratio of the He II flux at 1640 and 4686 A and is compared with values derived from other parameters. The ultraviolet observations suggest that M1-2 is a young, high-density planetary nebula of excitation class 5 or 6 with characteristics similar to IC 4997.

Feibelman, W. A.↗
NASA NTRS1983

Initial hot plasma composition results from the Dynamics Explorer

Initial observations from the energetic ion composition spectrometer on the DE-1 spacecraft have found mass or charge dependent processes to be operating in the vicinity of an inverted-V event in the auroral acceleration region, above the polar cap ionosphere, and in the mid-altitude dayside cusp. Separate color spectrograms for the different ion species provide a synoptic picture of these processes that can contribute significantly to our understanding of both the solar wind and the ionosphere as sources of energetic magnetospheric ions.

Shelley, E. G.↗