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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Theory of stellar coronae - An interpretation of X-ray emission from non-degenerate stellar sources

It is shown that the acoustic wave heating theory of stellar coronae explains neither Einstein Observatory coronae data nor previous UV and X-ray observations of the sun and other stars, on the evidence of data implying that magnetic fields, stellar rotation rates and convection zone parameters figure in the determination of coronal heating. Einstein Observatory results suggest that O-type star coronae are heated by the interaction of turbulent stellar winds with slowly-decaying primordial magnetic fields or by radiative instabilities in the flow. The apparent absence of coronae in Ap stars is due to the stability of atmospheres in which even weak convection is suppressed by the strong field. Dynamo action is implicated in some normal A-type stars and in F- and later-type dwarfs. Coronal characteristics of dMe and dM stars, close binaries, and K- and M-type giants are also considered.

Linsky, J. L.↗
NASA NTRS1985

Ultra high energy gamma rays, cosmic rays and neutrinos from accreting degenerate stars

Super-Eddington accretion for a recently proposed unipolar induction model of cosmic ray acceleration in accreting binary star systems containing magnetic white dwarfs or neutron stars is considered. For sufficiently high accretion rates and low magnetic fields, the model can account for: (1) acceleration of cosmic ray nuclei up to energies of 10 to the 19th power eV; (2) production of more or less normal solar cosmic ray composition; (3) the bulk of cosmic rays observed with energies above 1 TeV, and probably even down to somewhat lower energies as well; and (4) possibly the observed antiproton cosmic ray flux. It can also account for the high ultra high energy (UHE) gamma ray flux observed from several accreting binary systems (including Cygnus X-3), while allowing the possibility of an even higher neutrino flux from these sources, with L sub nu/L sub gamma is approximately 100.

Brecher, K.↗
NASA NTRS1984

Analysis of laminated composite shells using a degenerated 3-D element

A special three-dimensional element based on the total Lagrangian description of the motion of a layered anisotropic composite medium is developed, validated and employed to analyze laminated anisotropic composite shells. The element contains the following features: geometric nonlinearity, dynamic (transient) behavior and arbitrary lamination scheme and lamina properties. Numerical results of nonlinear bending, natural vibration, and transient response are presented to illustrate the capabilities of the element.

Chao, W. C.↗
NASA NTRS1984

Feige 24 - Wind/flare accretion by a hot DAZ1 degenerate

Model stellar envelopes have been constructed in order to represent the outer layers of the hot DAZ white dwarf component of the spectroscopic binary Feige 24. The results of the calculations show that the metals, discovered by Dupree and Raymond (1983) should diffuse rapidly into the interior unless a continuous source of these elements is present in the system. It is proposed that this source is a stellar wind from the red dwarf component of this detached close binary, and diffusion and accretion calculations are provided to support the hypothesis. Recent EXOSAT observations of Feige 24 provide additional evidence to test the accretion hypothesis. The accreting material may provide significant X-ray opacity for this object. This discussion is extended to other similar M dwarf-white dwarf close binary systems.

Sion, E. M.↗
NASA NTRS1986

Carbon combustion supernovae - Numerical studies of the final evolution of degenerate carbon-oxygen cores

The evolution of polytropic 1.46-solar-mass 3-Gg/cu cm (4 x 10 to the 8th)-K pure C cores is investigated theoretically by means of model computations starting from the runaway stage. The temperature in central zones is raised to the runaway temperature, and the evolution is followed in a spherical coordinate system. In models of burning without detonation, it is found that the initially spherical burning front is Rayleigh-Taylor unstable, without bipolar jets or equatorial rings; that some C is not burned to Ni; and that the Ni in the 0.8-1.0 solar mass of burned fuel produced can account for observed properties of type I supernovae.

Mueller, E.↗
NASA NTRS1986

Degenerate four-wave mixing in a mercury-argon discharge

Phase conjugation has been obtained with pump powers as low as 0.5 mW on the 546.1-nm line of atomic mercury. Collisional processes that oppose the effects of optical pumping sustain the signal. Line splittings observed in the Doppler regime are compared with theory. Reflectivities agree with those obtained from a model that includes pump absorption.

Richardson, W.↗
NASA NTRS1987

Further observations of the peculiar hot helium-rich degenerate KPD 0005 + 5106

KPD 0005 + 5106 observations including high- and low-resolution IUE and Voyager UV spectra, high-resolution spectra at optical wavelengths, and a low-resolution spectrum at red wavelengths are presented. The only detected transitions from a heavier element assignable to the stellar photosphere is the N V 1240 A doublet; hydrogen and some probable CNO ions are also detected in the emission. It is suggested that the detected nitrogen is most likely due to selective radiative acceleration processes.

Downes, Ronald A.↗
NASA NTRS1989

The detection of ionized helium and carbon in the pulsating DB degenerate GD358

Echelle observations of GD358, prototype of the class of six known pulsating DB stars and one of the hottest DB stars, are presented. Evidence for the existence of He II and C II is demonstrated. The Ly-alpha + He II region reveals no clear evidence of absorption wings flanking the geocoronal emission. The significance of the He II detection is discussed.

Sion, Edward M.↗
NASA NTRS1990

Ultraviolet Fe VII absorption lines in planetary nebula nuclei, hot subdwarfs, and hot degenerate objects

Results are reported from a search for Fe VII absorption lines in data from high-dispersion IUE-SWP observations of PN nuclei (PNN), hot subdwarfs (HSDs), and hot white dwarfs (HWDs). The data-reduction techniques employed are outlined, and the results are presented in extensive tables and sample spectra and characterized in detail. Absorption in at least one of the four Fe VII lines above 120 nm wavelength is found in 22 of 51 PNN, and possibly in 10 HSDs, in the pulsating HWD PG 1159 - 035, and in the PNN K1 - 16. It is concluded that Fe VII is more common in WD progenitors such as PNN than previously predicted and is especially typical of the more luminous low-gravity stars.

Feibelman, Walter A.↗
NASA NTRS1991

Modal interaction in linear dynamic systems near degenerate modes

In various problems in structural dynamics, the eigenvalues of a linear system depend on a characteristic parameter of the system. Under certain conditions, two eigenvalues of the system approach each other as the characteristic parameter is varied, leading to modal interaction. In a system with conservative coupling, the two eigenvalues eventually repel each other, leading to the curve veering effect. In a system with nonconservative coupling, the eigenvalues continue to attract each other, eventually colliding, leading to eigenvalue degeneracy. Modal interaction is studied in linear systems with conservative and nonconservative coupling using singularity theory, sometimes known as catastrophe theory. The main result is this: eigenvalue degeneracy is a cause of instability; in systems with conservative coupling, it induces only geometric instability, whereas in systems with nonconservative coupling, eigenvalue degeneracy induces both geometric and elastic instability. Illustrative examples of mechanical systems are given.

Afolabi, D.↗
NASA NTRS1991

A geometric nonlinear degenerated shell element using a mixed formulation with independently assumed strain fields

A mixed formulation is chosen to overcome deficiencies of the standard displacement-based shell model. Element development is traced from the incremental variational principle on through to the final set of equilibrium equations. Particular attention is paid to developing specific guidelines for selecting the optimal set of strain parameters. A discussion of constraint index concepts and their predictive capability related to locking is included. Performance characteristics of the elements are assessed in a wide variety of linear and nonlinear plate/shell problems. Despite limiting the study to geometric nonlinear analysis, a substantial amount of additional insight concerning the finite element modeling of thin plate/shell structures is provided. For example, in nonlinear analysis, given the same mesh and load step size, mixed elements converge in fewer iterations than equivalent displacement-based models. It is also demonstrated that, in mixed formulations, lower order elements are preferred. Additionally, meshes used to obtain accurate linear solutions do not necessarily converge to the correct nonlinear solution. Finally, a new form of locking was identified associated with employing elements designed for biaxial bending in uniaxial bending applications.

Graf, Wiley E.↗
NASA NTRS1996

Generation of Squeezed Light Using Photorefractive Degenerate Two-Wave Mixing

We present a quantum nonlinear model of two-wave mixing in a lossless photorefractive medium. A set of equations describing the quantum nonlinear coupling for the field operators is obtained. It is found that, to the second power term, the commutation relationship is maintained. The expectation values for the photon number concur with those of the classical electromagnetic theory when the initial intensities of the two beams are strong. We also calculate the quantum fluctuations of the two beams initially in the coherent state. With an appropriate choice of phase, quadrature squeezing or number state squeezing can be produced.

Lu, Yajun↗
NASA NTRS2000

Accretion Onto Magnetic Degenerate Stars

While the original objectives of this research program included the study of radiative processes in cataclysmic variables and the evolution of neutron star magnetic fields, the scope of the reported research expanded to other related topics as this project developed. This final report therefore describes the results of our research in the following areas: 1) Irradiation-driven mass transfer cycles in cataclysmic variables and low-mass X-ray binaries; 2) Propeller effect and magnetic field decay in isolated old neutron stars; 3) Decay of surface magnetic fields in accreting neutron stars and pulsars; 4) Finite-Difference Hydrodynamic simulations of mass transfer in binary stars.

Frank, Juhan↗
NASA NTRS1999

Silicon bulk micromachined, symmetric, degenerate vibratorygyroscope, accelerometer and sensor and method for using the same

When embodied in a microgyroscope, the invention is comprised of a silicon, four-leaf clover structure with a post attached to the center. The whole structure is suspended by four silicon cantilevers or springs. The device is electrostatically actuated and capacitively detects Coriolis induced motions of the leaves of the leaf clover structure. In the case where the post is not symmetric with the plane of the clover leaves, the device can is usable as an accelerometer. If the post is provided in the shape of a dumb bell or an asymmetric post, the center of gravity is moved out of the plane of clover leaf structure and a hybrid device is provided. When the clover leaf structure is used without a center mass, it performs as a high Q resonator usable as a sensor of any physical phenomena which can be coupled to the resonant performance.

Tang, Tony K.↗
NASA NTRS2021

Detection of an extremely strong magnetic field in the double-degenerate binary merger product HD 144941

HD 144941 is an extreme He (EHe) star, a rare class of subdwarf OB star formed from the merger of two white dwarf (WD) stars. Uniquely amongst EHe stars, its lightcurve has been reported to be modulated entirely by rotation, suggesting the presence of a magnetic field. Here we report the first high-resolution spectropolarimetric observations of HD 144941, in which we detect an extremely strong magnetic field both in circular polarization (with a line-of-sight magnetic field averaged over the stellar disk〈Bz〉 ∼ −8 kG) and in Zeeman splitting of spectral lines (yielding a magnetic modulus of〈B〉 ∼17 kG). We also report for the first time weak Hαemission consistent with an origin an a Centrifugal Magnetosphere (CM). HD 144941’s atmospheric parameters could be consistent with either a subdwarf or a main sequence (MS) star, and its surface abundances are neither similar to other EHe stars nor to He-strong magnetic stars. However, its Hαemission properties can only be reproduced if its mass is around 1 M⊙, indicating that it must be a post-MS object. Since there is no indication of binarity, it is unlikely to be a stripped star, and was therefore most likely produced in a WD merger. HD 144941 is therefore further evidence that mergers area viable pathway for the generation of fossil magnetic fields.

M E Shultz↗