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At least 1,405 records · Page 78
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NASA NTRS1986

Theoretical and Experimental Beam Plasma Physics (TEBPP)

The theoretical and experimental beam plasma physics (TEBPP) consists of a package of five instruments to measure electric and magnetic fields, plasma density and temperature, neutral density, photometric emissions, and energetic particle spectra during firings of the particle injector (SEPAC) electron beam. The package is developed on a maneuverable boom (or RMS) and is used to measure beam characteristics and induced perturbations field ( 10 m) and mid field ( 10 m to 100 m) along the electron beam. The TEBPP package will be designed to investigate induced oscillations and induced electromagnetic mode waves, neutral and ion density and temperature effects, and beam characteristics as a function of axial distance.

Roberts, B.↗
NASA NTRS1986

CID-720 aircraft Langley Research Center preflight hardware tests: Development, flight acceptance and qualification

The testing conducted on LaRC-developed hardware for the controlled impact demonstration transport aircraft is discussed. To properly develop flight qualified crash systems, two environments were considered: the aircraft flight environment with the focus on vibration and temperature effects, and the crash environment with the long pulse shock effects. Also with the large quantity of fuel in the wing tanks the possibility of fire was considered to be a threat to data retrieval and thus fire tests were included in the development test process. The aircraft test successfully demonstrated the performance of the LaRC developed heat shields. Good telemetered data (S-band) was received during the impact and slide-out phase, and even after the aircraft came to rest. The two onboard DAS tape recorders were protected from the intense fire and high quality tape data was recovered. The complete photographic system performed as planned throughout the 40.0 sec of film supply. The four photo power distribution pallets remained in good condition and all ten onboard 16 mm high speed (400 frames/sec) cameras produced good film data.

Pride, J. D.↗
NASA NTRS1985

Studies of hot B subdwarfs. II - Energy distributions of three bright sdB/sdOB stars in the 950-5500 A 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 Stromgren 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, hygrogen 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 NTRS1985

Delamination and debonding of materials

The general topics consist of stress analysis, mechanical behavior, and fractography/NDI of composite laminates. Papers are presented on a dynamic hybrid finite-element analysis for interfacial cracks in composites, energy release rate during delamination crack growth in composite laminates, matrix deformation and fracture in graphite-reinforced epoxies, and the role of delamination and damage development on the strength of thick notched laminates. In addition, consideration is given to a new ply model for interlaminar stress analysis, a fracture mechanics approach for designing adhesively bonded joints, the analysis of local delaminations and their influence on composite laminate behavior, and moisture and temperature effects on the mixed-mode delamination fracture of unidirectional graphite/epoxy.

Johnson, W. S.↗
NASA NTRS1986

Comments on using absolute spectrophotometry of Wolf-Rayet stars

Garmany et al. (1984) have conducted a study involving spectrophotometric scans of 13 Wolf-Rayet stars. They have found that the application of a 'standard' reddening law to the observed data gives spurious results in many cases. They concluded also that previous attempts to determine the intrinsic continua and the effective temperatures of Wolf-Rayet stars are inadequate. In the present study the conclusions of Garmany et al. are evaluated. According to this evaluation, it has not been demonstrated by Garmany et al., beyond a reasonble doubt, that the interstellar extinction law varies greatly from Wolf-Rayet star to Wolf-Rayet star. The procedure followed by Garmany et al. to find the apparent shape of the ultraviolet continuum of a Wolf-Rayet star is unsatisfactory for a number of reasons.

Underhill, A. B.↗
NASA NTRS1986

HD 47755, a new eclipsing binary

The IUE spectra of the close binary star HD 47755 have been examined in order to determine its geometry, chemical composition, and light curve. UBV fluxes in the spectra, when dereddened for E(B-V) = 0.09 yield an effective temperature of 16,500 K. The ratio of the mean radii of the stars is found to agree well with an old blueband spectrophotometric value. Eclipses in the binary have been observed and a complex green light curve is derived. It is suggested that the wind from at least one of the components of HD 47755 is the source of the complexity in the light curve. The geometry of the HD 47755 is compared to that of V 641 Mon, A definite cluster member of NGC 2264. The interstellar line spectrum is found to be similar to that of V 641 Mon and the column densities for a few interstellar ions are given in a table. Evaluation of the nonastrometric evidence indicates that HD 47755 is also a member of NGC 2264.

Koch, R. H.↗
NASA NTRS1986

Spectral flux from low-density photospheres - Numerical results

Radiative transfer through sharp, quasi-static atmospheres whose opacity is dominated by hydrogen is considered at densities low enough that scattering usually dominates absorption and radiative excitations usually dominate collisional excitations. Numerical results for the continuum spectral flux are obtained for effective temperatures T(e) = 6000-16,000 K and scale heights Delta-R = 10 to the 10th - 10 to the 14th cm. Spectra are significantly different than if LTE level populations were assumed. Comparison with observations of the Type II supernova 1980k tends to increase the value of the Hubble constant previously obtained by the Baade (1926) method.

Hershkowitz, S.↗
NASA NTRS1986

New waves at multiples of the plasma frequency upstream of the earth's bow shock

The first observations of waves at harmonics higher than the second of the electron plasma frequency are reported. The observations were made by the ISEE 1 spacecraft upstream of the earth's bow shock. The waves are interpreted as electromagnetic radiation at the fundamental and up to the fifth harmonic of the plasma frequency, with effective temperatures decreasing from 5 x 10 to the 17th K to 10 billion K over this range. Two models are proposed for the emission of the waves.

Cairns, I. H.↗
NASA NTRS1986

The dependence of He I 10830 A absorption strength upon X-ray emission in late-type stars

Measurements of He I 10830 A absorption equivalent widths and fractional X-ray luminosities L(x)/L(bo) are presented for a large sample of late-type stars. A correlation of increasing 10830 A strength with increasing L(x)/L(bo) is evident in dwarfs, giants, and supergiants later than and including spectral type F7. No equivalent correlation is apparent in main-sequence stars earlier than dF7 (B-V of about 0.5). The data suggest that coronal X-ray irradiation constitutes an important source of He I excitation in late-type stars. The fact that X-ray luminosity depends exponentially on 10830 A equivalent width is attributed to the production of X-rays in active regions covering only a fraction of the stellar surface. It is conjectured that the absence of 10830 A X-ray correlation in early dF stars is due to the increased importance of electron collisional excitation of He I. This effect is discussed in light of evidence that the nature of magnetic activity changes significantly across the color range spanned by dF stars; a change related directly to the increasing depth of stellar convection with decreasing effective temperature.

Zarro, D. M.↗
NASA NTRS1986

Transition to unstable ion flow in parallel electric fields

The stability of ionospheric O(+)-H(+) outflows accelerated by a nonambipolar parallel electric field is considered under conditions where the ion motion initially develops adiabatically and the ambient plasma is vertically stratified with an effective temperature that increases with altitude. Such conditions are expected near the bottom of the auroral acceleration region where ion and electron streaming instabilities first develop. It is shown for a particular equilibrium profile that the differentially accelerated ion flows become unstable within about 100 km from their entry point in the acceleration region. At O(+)/H(+) density ratios less than about 9, the instability is dominated by a violent H(+)-O(+) two-stream interaction which couples the O(+) and H(+) acoustic modes, and which mediates a transition to nonadiabatic acceleration. At higher altitudes and/or larger O(+)/H(+) density ratios, a much weaker resonant instability exists, which is driven by the relative drift between electrons and O(+) or H(+) ions. The results suggest that the H(+)-O(+) two-stream instability may be a viable mechanism for heating upflowing auroral ions.

Bergmann, R.↗
NASA NTRS1986

The chemical composition of the cool white dwarf LP 701-29

New observations and a new synthetic spectrum of the cool white dwarf LP 701-29 are presented. The analysis suggests that the atmosphere of this star is helium dominated, with enhanced metal abundances compared to the usual abundances of white dwarfs. The best fit is achieved with an effective temperature of 4500 K and an abundance of heavy elements (especially Ca) of the order 0.01 of the solar ratio of metals to helium.

Kapranidis, S.↗
NASA NTRS1986

Optical observations of very low ionization HII regions in the large Magellanic cloud

Several very low ionization isolated HII regions were detected on a prism-objective plate of the Large Magellanic Cloud. Most of the objects show a very weak (OIII) lambda 5007 emission line and, on the other hand the (OII) lambda 6584 doublets are very intense. This kind of objects seem to be ideal in order to determine accurate N and O abundance, avoiding the use of large ionization correction factors in the N abundance determination. Spectrophotometric observations of these regions were carried out with the 4 m telescope and the 2-D Frutti spectrograph at Cerro Tololo, and with the 1.52 m and the Image Dissector Scanner (IDS) at La Silla, ESO. The wavelength range lambda lambda 3700 to 7000 A was covered. Calibrated fluxes of the emission lines detected were measured, and from these data preliminary results of physical conditions of the gas as well as some ionic abundances were derived. Comparisons of the observations with ionization structure models show that the effective temperatures of the ionizing stars are less than 35,000 K. Possible abundances gradients across the large megallanic cloud are discussed.

Pena, M.↗
NASA NTRS1986

Synthetic spectra of cool stars for the wavelength range 2550-3200 A

In order to interpret the ultraviolet spectra of cool stars obtained with IUE and to determine the physical conditions in their outer atmospheres, synthetic IUE spectra of stars cooler than the Sun, mainly the Sun and the metal-deficient, carbon-rich CH stars were calculated. Results are encouraging. Synthetic spectra for the Sun over the range 2550 to 3300 A give roughly the observed level of overall emission and relative strengths or various absorption features (Fe II, Mg II, Mg I, and Fe I) although they still require extra sources of continuous opacity longward of 2650 A. Spectra of the CH stars have roughly the correct shape in the ultraviolet continuum for the expected effective temperature, although these models are problematic in that the electron density, hence H(-) opacity, is directly proportional to the Mg-Si-Fe density and the ratio of line to continuum opacity is only slightly affected by changing metallicity.

Eaton, Joel A.↗
NASA NTRS1987

Ultraviolet Spectra of Two Magnetic White Dwarfs and Ultraviolet Spectra of Subluminous Objects Found in the Kiso Schmidt Survey

Low resolution International Ultraviolet Explorer (IUE) spectroscopic observations of two magnetic white dwarfs BPM25114 and K813-14 were obtained using both the SWP and LWP cameras. The first object has an observed magnetic field of 4 x 10(7) Gauss and the second has one of 3 x 10(7) Gauss. Both objects have overall spectral energy distributions appropriate for cool DA white dwarfs with T(eff) near 10,000 K and accordingly show strong lambda lambda 1400 and 1600 absorption in their spectra. Compared to non-magnetic DA white dwarfs of comparable effective temperature, there are some differences in the profiles, presumably produced by the magnetic fields in these objects. In addition, the ultraviolet spectra of a number of hot subluminous stars in the Kiso Schmidt survey were observed.

Wegner, Gary A.↗
NASA NTRS1985

Ion reflection, gyration, and dissipation at supercritical shocks

Topics on collisionless plasmas are discussed and include: (1) Ion reflection is the dominant ion dissipation mechanism at nearly perpendicular, supercritical shocks. (2) An increasing fraction of the ions incident on a supercritical shock is reflected as the Mach number increases. The actual fraction reflected can be predicted using the Rankine Hugoniot conservation relations. (3) The effective temperature associated with the dispersion in velocity space associated with ion reflection accounts for a large fraction of the temperature rise observed across supercritical, quasi perpendicular shocks.

Gosling, J. T.↗
NASA NTRS1986

Ultraviolet observations of Be stars. II - Superionization in Be stars of different spectral types

The results of an investigation of the dependence of the strength/presence of lines from superionized species (N V, C IV, Si IV) on spectral type and v sin i in the ultraviolet spectra of Be stars are presented. No evidence is found for N V in stars later than B2; this apparent absence may, however, simply reflect problems with the IUE reduction procedure. The C IV and Si IV lines pervade the ultraviolet spectra of all B subtypes, with a trend toward increasing strength with increasing effective temperature. A similar trend in the 'edge velocity' of the feature with spectral type is found. The data also suggest a possible trend toward the presence of stronger C IV and a higher edge velocity in stars with larger values of v sin i. The observed temporal variability in N V, C IV, and Si IV in some Be stars is discussed.

Marlborough, J. M.↗
NASA NTRS1986

The evolution of very low mass stars

The results of numerical evolutionary calculations for stars with masses in the range of 0.01-0.10 solar mass are presented. The stellar models by which these stars are followed from the early stages of contraction through deuterium burning to the very late stages of degenerate cooling at ages comparable to that of the Galaxy are described, and the methodology used to investigate the major uncertainties in the input physics are discussed. It is found that, for brown dwarfs with masses substantially below the end of the hydrogen-burning main sequence, the evolution of the effective temperature and bolometric luminosity are fairly well determined, despite the residual uncertainties in the input physics. In particular, the evolution is remarkably insensitive to the choice of the atmospheric opacity law at low temperatures and to the amount of mismatch in specific entropy across the stellar envelope. The minimum mass for a star to attain main-sequence status is moderately sensitive to the assumed input physics, particularly the surface boundary conditions.

Nelson, L. A.↗
NASA NTRS1987

The moderately interacting Algol binary RS Cephei

In agreement with previous observers, it is found that the eclipsing binary RS Cep is an Algol-type semidetached system. From IUE spectra and Lick Observatory optical scans, the spectral types of the components are determined as B9.7e V + G8 III-IV. The effective temperature of the primary star is found to be 9670 K (with an uncertainty of about 150 K). From the photometric elements, the spectral types, and the semidetached nature of the system, a satisfactory model of the system is developed, giving masses 2.4 and 0.4 solar mass, radii 2.3 and 7.6 solar radii, and separation of the components 32 solar radii. This model leads to a distance of 910 pc; the color excess is E(B - V) = 0.03 mag. Balmer emission lines are observed in the optical scans, so that the system is a Be star. An IUE spectrum obtained during the total primary eclipse reveals a number of UV emission lines, typical for interacting Algols and W Ser stars. Power emitted in these lines is about twice as high as that in TT Hya or U Cep, therefore, RS Cep is a relatively active Algol system. A broad 'bulge' of excess relative flux is observed roughly between wavelengths 210 and 270 nm in the totality spectrum, and interpreted as broad blends of emission lines of Fe II; the same Fe II lines, formed in a circumstellar shell or disk, cause additional absorption in the out-of-eclipse spectra.

Plavec, Mirek J.↗