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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 271 records · Page 15
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NASA NTRS1986

The temperature structure in accretion flows onto massive protostars

Radiation transfer problems involved in the infall of dust and gas during star formation are studied. Dust properties are discussed, and modifications of spherical radiative transfer equations are presented that permit forward scattering by dust to be treated for the small size of the star relative to the inner radius of the shell. A procedure for deriving the stellar radiation field incident on the inner edge of the shell is developed. The temperature correction procedure of Cassinelli and Hartmann (1975) for extended stellar atmospheres is modified so that the multitemperature nature of the grains in the cloud may be derived. Temperature distributions for three schematic models in which the density is prespecified are discussed. Radiative acceleration of grains is addressed, showing that the proper mean opacity differs by a large factor from the Rosseland mean opacity that is commonly used. Emergent fluxes for the models are given.

Wolfire, Mark G.↗
NASA NTRS1987

Opacity project - Astrophysical and fusion applications

An overview is presented of a project to calculate large quantities of accurate atomic data for radiative processes of importance in the precise determination of opacities in stellar atmospheres, and for astrophysical and laboratory applications in general. Work is in progress on the oscillator strengths, photoionization cross sections, damping constants, etc., for all atoms and ions in hydrogen through neon isoelectronic sequences going up to iron.

Pradhan, A. K.↗
NASA NTRS1987

Thermal diffusion in partially ionized gases - The case of unequal temperatures

Previous calculations of thermal diffusion coefficients in partially ionized gases are extended to the case of unequal neutral and ion temperatures and/or temperature gradients. Formulas are derived for the general case of a major gas as well as for minor atoms and ions. Strong enhancements of minor-ion thermal diffusion coefficients over their values in the fully ionized gas are found when the degree of ionization in the main gas is relatively low. However, compared to the case of equal temperatures, the enhancements are less strong when the neutrals are cooler than the ions. The specific case of the H-H(+) mixture, which is important in the study of solar and stellar atmospheres, is discussed as an application.

Geiss, J.↗
NASA NTRS1987

Remote sensing of atomic oxygen - Some observational difficulties in the use of the forbidden O I 1173-A and O I 1641-A transitions

Recent sounding rocket and satellite studies suggest that simultaneous measurements of the O I 989-A and 1304-A resonance lines and of the forbidden 1172.6-A and 1641.3-A transitions would form the basis of a useful remote sensing technique for measuring the O I density and optical opacity of a planetary or stellar atmosphere. Because the 1172.6-A and 1641.3-A emissions are weak lines and are emitted in a wavelength region rich in spectral features, it is important to determine whether typical flight instruments can make measurements with sufficient spectral purity so that the remote sensing observations will yield accurate results. A detailed, high-resolution study of the far UV emission features in the regions surrounding the atomic oxygen transitions at 1172.6 and 1641.3 A was made. These spectra, which were excited by electron impact on O2 and N2, are presented in an attempt to display some potential sources of interference in aeronomical measurements of these O I lines. Both atomic and molecular emissions are found, and the spectral resolution necessary to make unambiguous measurements is discussed.

Erdman, P. W.↗
NASA NTRS1986

Consistent transport coefficients in astrophysics

A consistent theory for dealing with transport phenomena in stellar atmospheres starting with the kinetic equations and introducing three cases (LTE, partial LTE, and non-LTE) was developed. The consistent hydrodynamical equations were presented for partial-LTE, the transport coefficients defined, and a method shown to calculate them. The method is based on the numerical solution of kinetic equations considering Landau, Boltzmann, and Focker-Planck collision terms. Finally a set of results for the transport coefficients derived for a partially ionized hydrogen gas with radiation was shown, considering ionization and recombination as well as elastic collisions. The results obtained imply major changes is some types of theoretical model calculations and can resolve some important current problems concerning energy and mass balance in the solar atmosphere. It is shown that energy balance in the lower solar transition region can be fully explained by means of radiation losses and conductive flux.

Fontenla, Juan M.↗
NASA NTRS1987

Uniform period spacings in white dwarf models

Theoretical models of g-mode oscillations in white dwarfs are investigated analytically. Numerical results of period-spacing computations for DOV, DAV, and DBV models are presented in tables, and the relationships between period spacings and the composition and structure of the stellar atmospheres are discussed.

Kawaler, Steven D.↗
NASA NTRS1988

Failure of continuum methods for determining the effective temperature of hot stars

It is demonstrated here that, for hot stars, methods based on the integrated continuum flux are completely unreliable discriminators of the effective temperature. Absorption line profiles provide much more accurate values of these parameters. It is not necessary to invoke nonradiative energy and momentum effects to explain the spectroscopic appearance of O-type stars of very different spectral type; rather, the observed spectra can be well modeled and fully interpreted by normal interaction of gas and radiation in stellar atmospheres of differing effective temperature and gravity.

Hummer, D. G.↗
NASA NTRS1988

Cosmological quantum chromodynamics, neutron diffusion, and the production of primordial heavy elements

A simple one-dimensional model is used to describe the evolution of neutron density before and during nucleosynthesis in a high-entropy bubble left over from the cosmic quark-hadron phase transition. It is shown why cosmic nucleosynthesis in such a neutron-rich environment produces a surfeit of elements heavier than lithium. Analytical and numerical techniques are used to estimate the abundances of carbon, nitrogen, and heavier elements up to Ne-22. A high-density neutron-rich region produces enough primordial N-14 to be observed in stellar atmospheres. It shown that very heavy elements may be created in a cosmological r-process; the neutron exposure in the neutron-rich regions is large enough for the Ne-22 to trigger a catastrophic r-process runaway in which the quantity of heavy elements doubles in much less than an expansion time due to fission cycling. A primordial abundance of r-process elements is predicted to appear as an excess of rare earth elements in extremely metal-poor stars.

Applegate, J. H.↗
NASA NTRS1988

X-ray induced stellar mass loss near active galactic nuclei

The effects of UV and X-ray radiation on stars in active galactic nuclei (AGN) are critically evaluated. Mass loss rates in X-ray-induced winds are evaluated for realistic red giant models, and the effects of the ablation of stellar envelopes by radiation pressure are considered. The importance of X-ray-induced mass loss in the standard quasar model is evaluated and whether it can provide a source of accretion fuel or emission-line clouds is discussed. It is concluded that thermal winds driven by X-ray heating are a minor total supply of mass to AGN, but that thermal plus line-driven winds and stellar ablation may increase the mass loss and improve the chances for supplying a fraction of the necessary mass supply to the central object. It is speculated that when steady winds are inefficient, complex time-dependent processes due to X-ray energy injection deep into a stellar atmosphere could still release significant mass from stars.

Voit, G. Mark↗
NASA NTRS1988

The analysis of spectra of novae taken near maximum

A project to analyze ultraviolet spectra of novae obtained at or near maximum optical light is presented. These spectra are characterized by a relatively cool continuum with superimposed permitted emission lines from ions such as Fe II, Mg II, and Si II. Spectra obtained late in the outburst show only emission lines from highly ionized species and in many cases these are forbidden lines. The ultraviolet data will be used with calculations of spherical, expanding, stellar atmospheres for novae to determine elemental abundances by spectral line synthesis. This method is extremely sensitive to the abundances and completely independent of the nebular analyses usually used to obtain novae abundances.

Stryker, L. L.↗
NASA NTRS1988

Doppler imaging of AR Lacertae at three epochs

Observations from IUE were used to study the structure of the lower chromosphere of AR Lacertae in the light of Mg II k. Sequences of LWR/P-HI images distributed around the binary period at three epochs were obtained. Discrete plage-like regions of enhanced Mg II surface flux in this system are identified. There are temporal variations in the Mg II flux on timescales of hours as well as substantial changes in chromospheric morphology on timescales of years. Even with the limited S/N attainable with the IUE, one can map the gross structures of active stellar atmospheres. With such information, one can begin to study the true 3-D structure of the atmospheres of late-type stars.

Walter, Frederick M.↗
NASA NTRS1988

On the surface magnetic fields of rapidly rotating stars with winds

The consistency of estimates of angular velocities and magnetic field strength obtained from magnetic rotator models is examined. Upper and lower bounds for the surface magnetic field are derived with constraints of interior theory using magnetohydrodynamic considerations. The formulas for the bounds are applied to rapidly rotating Be stars and Wolf-Rayet type stars using Eddington-Vogt speed relevant for the radiative diffusion zone. It is suggested that stars which have equatorially enhanced winds driven by magnetic rotator effects must have fields such that the Aflven speed is close to the speed of the Eddington-Vogt currents at the base of the stellar atmosphere.

Maheswaran, Murugesapillai↗
NASA NTRS1989

Rototranslational absorption spectra of H2-H2 pairs in the far infrared

On the basis of ab initio induced dipole components and a well-tested isotropic potential, the collision-induced absorption rototranslational profiles of hydrogen have been determined using an exact wave-mechanical theory. The main induction mechanisms, particularly the quadrupole-induced 0223,2023 components, are found to be consistent with observations. It is demonstrated that the present theory can be used to predict spectra at temperatures where no measurements exist, as is needed for the modeling of planetary and stellar atmospheres.

Meyer, Wilfried↗
NASA NTRS1989

The effects of Compton scattering in X-ray spectra

The ways in which Compton scattering on free electrons influences the continuous spectrum and temperature structure of a hot stellar atmosphere, where most of its radiation is emitted in the 1-10 keV energy range, are discussed. The numerical results pertain to the continuum spectra of X-ray bursters. The formulation and development of the numerical code, which reproduces angle-averaged relativistic Compton scattering of thermal (unpolarized) X-rays in a plane-parallel atmosphere in hydrostatic and radiative equilibrium, are described.

Madej, Jerzy↗
NASA NTRS1989

Properties, detectability and origin of interstellar diamonds in meteorites

Laboratory studies suggest that microdiamonds will be quite elusive in interstellar space, being detectable only if more than 10 per cent of the carbon is present as diamond. The diamonds apparently formed by condensation in cool stellar atmospheres, and were then impregnated with isotopically anomalous noble gases, such as Xe, from a supernova.

Lewis, Roy S.↗
NASA NTRS1989

Solar and stellar flares - Questions and problems

Topics which can be discussed quantitatively for both solar and stellar flares are reviewed. Physical conditions in stellar flares are briefly discussed and radio flares are considered, emphasizing the role of maser process in such flares. The penetration of particle beams and photons is examined, addressing the question whether an electron beam can penetrate to the chromosphere of a flare star. The existence of momentum balance in stellar flares is investigated, and solar-stellar differences regarding subsurface source of flare power are described. The significance for the interstellar medium of the flare star mass loss rate is addressed, and differences in mass loss mechanism between the sun and flare stars are discussed. Reconnection modeling of flare energy release is studied, and the geometrical structure of the flare site in a stellar atmosphere is described. The connection between coronal heating and flares is considered, and the origins of flare magnetic fields are discussed.

Mullan, D. J.↗
NASA NTRS1990

Theoretical study of the spectroscopy of ZrO

The absorption bands of ZrO have been observed in stars, particularly S stars. Here, theoretical transition probabilities are presented for the dipole-allowed transitions between the six lowest singlet and triplet states of ZrO. The results should be sufficiently reliable to provide opacity data for use in modeling stellar atmospheres. The theoretical radiative lifetime for the e 3Pi state is less than the experimental value as measured by the decay of resonant fluorescence. However, the theoretical electronic transition moments for the gamma system and the B 1Pi - X 1Sigma(+) system are much smaller than those deduced from emission studies. The calculated lifetime for the C 1Sigma(+) state is in excellent agreement with the laser-induced fluorescence studies. The as yet unobserved E 1Phi - A 1Delta band system is found to be relatively strong.

Langhoff, Stephen R.↗
NASA NTRS1989

Time-dependent corona models - Scaling laws

Scaling laws are derived for the one-dimensional time-dependent Euler equations that describe the evolution of a spherically symmetric stellar atmosphere. With these scaling laws the results of the time-dependent calculations by Korevaar (1989) obtained for one star are applicable over the whole Hertzsprung-Russell diagram and even to elliptic galaxies. The scaling is exact for stars with the same M/R-ratio and a good approximation for stars with a different M/R-ratio. The global relaxation oscillation found by Korevaar (1989) is scaled to main sequence stars, a solar coronal hole, cool giants and elliptic galaxies.

Korevaar, P.↗