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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 415 records · Page 23
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NASA NTRS1983

The waiting point approximation in R-process calculations

The steady flow approximation to the r-process has been used to evaluate the validity of the waiting point assumption (that, for a given charge number Z, neutron captures become balanced by neutron photodisintegrations) under a variety of conditions. The criterion for the accuracy of the approximation for any set of nuclei with the same value of Z is that the ratio of the total rate of neutron photodisintegration in those nuclei to the total rate of neutron captures should be close to unity. No set of conditions involving neutron production by helium thermonuclear reactions (at temperatures up to 10 to the 9th K) could be found for which the waiting point approximation is useful for all values of Z, and it is recommended that all such r-process calculations not use this approximation. At higher temperatures, such as those characteristic of explosive carbon burning (2 x 10 to the 9th K), the waiting point approximation is much better.

Cameron, A. G. W.↗
NASA NTRS1984

X-ray observations of the burst source MXB 1728 - 34

Where sufficient information has been obtained, attention is given to the maximum burst flux, integrated burst flux, spectral hardness, rise time, etc., of 96 X-ray bursts observed from March 1976 to March 1979. The integrated burst flux and the burst frequency appear to be correlated; the longer the burst interval, the larger the integrated burst flux, as expected on the basis of simple thermonuclear flash models. The maximum burst flux and the integrated burst flux are strongly correlated; for low flux levels their dependence is approximately linear, while for increasing values of the integrated burst flux, the flux at burst maximum saturates and reaches a plateau.

Basinska, E. M.↗
NASA NTRS1981

X-ray bursts: Observation versus theory

Results of various observations of common type I X-ray bursts are discussed with respect to the theory of thermonuclear flashes in the surface layers of accreting neutron stars. Topics covered include burst profiles; irregular burst intervals; rise and decay times and the role of hydrogen; the accuracy of source distances; accuracy in radii determination; radius increase early in the burst; the super Eddington limit; temperatures at burst maximum; and the role of the magnetic field.

Lewin, W. H. G.↗
NASA NTRS1985

The heavy ion composition in 3HE-rich solar flares

The 3He-rich flares show a tendency to be enriched in heavy ions, and that this enrichment covers the charge range through Fe. The discovery of this association was responsible, in part, for the discarding of 3He enrichment models which involved spallation or thermonuclear reactions, since such models were unable to produce heavy nuclei enhancement. Results of a survey of heavy nucleus abundances observed in 66 3He-rich flares which occurred over the period October 1978 to June 1982 are presented.

Mason, G. M.↗
NASA NTRS1984

Magnetorotational iron core collapse

During its final evolutionary stages, a massive star, as considered in current astrophysical theory, undergoes rapid collapse, thereby triggering a sequence of a catastrophic event which results in a Type II supernova explosion. A remnant neutron star or a black hole is left after the explosion. Stellar collapse occurs, when thermonuclear fusion has consumed the lighter elements present. At this stage, the core consists of iron. Difficulties arise regarding an appropriate model with respect to the core collapse. The present investigation is concerned with the evolution of a Type II supernova core including the effects of rotation and magnetic fields. A simple neutrino model is developed which reproduced the spherically symmetric results of Bowers and Wilson (1982). Several two-dimensional computational models of stellar collapse are studied, taking into account a case in which a 15 solar masses iron core was artificially given rotational and magnetic energy.

Symbalisty, E. M. D.↗
NASA NTRS1984

A 164-day period in gamma-ray bursts from GBS0526-66?

Evidence is presented for a pattern in the recurrence time of gamma ray bursts from GBS0526-66, showing an apparent period of 164 days. It is suggested that this pattern reflects periodic accretion to a neutron star from an eccentric binary companion. It is proposed that this system drives two types of bursts: (1) the relatively frequent but weak recurrent bursts produced by either magnetospheric gating, or thermonuclear runaway, of periodically accreting matter around the neutron star, and (2) the much rarer intense bursts produced by tidally triggered star quakes.

Rothschild, R. E.↗
NASA NTRS1984

Magnetic fields in the radiative interior of stars. I Thermal shadows and forced convection. II - Forced convection and the Li-7 abundance

A theoretical model is developed for thermal shadows of magnetic inhomogeneities and their effects in the radiative cores of stars such as the sun. It is shown that the shadows cause a slight decrease of gas pressure, thereby lowering the opacity and raising the effective heat transport. The shadows produced by the inhomogeneities induce thermal gradients and therefore forced convection. If the field strength is at least 400,000 gauss partial Li-7 depletion will occur due to destruction in convection-dominated thermonuclear reactions. The scale of the field would need to be 30,000 km.

Parker, E. N.↗
NASA NTRS1985

CNO abundances resulting from diffusion in accreting nova progenitors

The configurations of potential nova progenitors at the onset of a thermonuclear runaway (TNR) are obtained by evolving a series of white dwarf (WD) models through the quasi-static accretion phase preceding the TNR. Special attention is paid to the effect of diffusion on composition profiles. The initial parameters of the accretion phase are the WD mass, the WD luminosity, and the accretion rate. They lead to a wide range of combinations of accreted mass and heavy element content of the envelope Z(env). It is found that the WD mass is the most important parameter in determining the accreted mass. A lower limit exists for the mass of a WD capable of producing a nova-like outburst which decreases with decreasing WD mass. There is a strong correlation between the accretion rate and Z(env), higher Z(env) values resulting from lower accretion rates. Changes in WD luminosity have no dramatic effect on the outcome of the accretion phase.

Kovetz, A.↗
NASA NTRS1986

Observational properties of cosmic gamma-ray bursts

A brief overview of the major observational results obtained in gamma-ray burst studies is presented. Also discussed is to what extent the thermonuclear model, which appears at present to be the most plausible, can account for the observed properties of the bursts. The investigation of gamma-ray bursts should cover observations of the time histories of events, energy spectra, and their variablility, source localization, and inspection of the localization regions during the active and quiescent phases of the source in other wavelengths, as well as, evaluation of the statistical distributions of the data obtained.

Mazets, E. P.↗
NASA NTRS1985

On the observed properties and long-term structure and evolution of white dwarfs in cataclysmic variables

An attempt is made to explain the observed surface temperatures and luminosities of cataclysmic variable white dwarfs based on the cooling physics and time-averaged structure of the white dwarf in response to accretion. The evolutionary changes in core temperature and core luminosity as an accreting white dwarf evolves quasi-statically are compared with the cooling evolution of a nonaccreting, one solar mass, pure C-12 core. Then, the evolutionary behavior of the white dwarf envelope in mass and temperature, in response to long-term accretion, is used to predict temperatures and luminosities of white dwarfs during quiescent intervals between nova outbursts. It is shown that the observed luminosities and effective temperatures of the bare white dwarfs detected in some cataclysmic variables are the expected intrinsic values associated with classical nova thermonuclear outburst cycles. Two alternate interpretations of their observed surface temperatures and luminosities are assessed.

Sion, E. M.↗
NASA NTRS1986

X-ray burst sources

There are about 100 bright X-ray sources in the Galaxy that are accretion-driven systems composed of a neutron star and a low mass companion that fills its critical Roche lobe. Many of these systems generate recurring X-ray bursts that are the result of thermonuclear flashes in the neutron star's surface layers, and are accompanied by a somewhat delayed optical burst due to X-ray heating of accretion disk. The Rapid Burster discovered in 1976 exhibits an interval between bursts that is strongly correlated with the energy in the preceding burst. There is no optical identification for this object.

Lewin, W. H. G.↗
NASA NTRS1987

Theoretical investigations of X-ray bursts

Current theoretical understanding of the X-ray burst phenomenon is reviewed, providing a framework in which the burst radiation can be used as a diagnostic of the fundamental properties of the underlying neutron star. The typical Type I X-ray burst is detected as a rapid increase in emission to a level about a factor of 10 above that seen during the quiescent state and recurs on time scales which range from several hours to several days. The thermonuclear flash model has successfully reproduced the basic features of the X-ray burst phenomenon and thereby provided strong theoretical evidence that neutron stars are involved. Topics covered include: theory of the emission spectrum; oscillation modes and prospects for diagnosing the thermal state of neutron stars through experiments on board the X-Ray Timing Explorer or the Advanced X-Ray Astrophysics Facility; applications to the mass and radius of a neutron star.

Taam, Ronald E.↗
NASA NTRS1987

The wind-disk interaction in X-ray burst sources. II - Collimation by thick accretion disks

The wind-disk interaction model in type I cosmic X-ray bursts is here examined in the case where the accretion disk surrounding the neutron star is geometrically thick. The funnel structure along the rotation axis of such a system may collimate the radiatively driven wind following a thermonuclear flash in the star's surface layers, and could thereby enhance the radiative flux (diffusive plus advective) along the beam direction. It is shown that for plausible conditions, this mechanism can account for the super-Eddington fluxes observed in some bursts. The overall time scale for variability, the color temperature, and the spectral shape in this picture are consistent with the observed properties of type I X-ray bursts.

Melia, Fulvio↗
NASA NTRS1986

Searches for low-energy counterparts of gamma-ray bursters

The results of six studies concerning the identification of low-energy counterparts of gamma-ray bursters (GRBs) are reported. From these results, it is concluded that the near-infrared observations of GRBs from IRAS pose serious difficulties for several GRB models. No precedent or plausible explanation is found for the optical flash detected on February 8, 1984 by Pedersen et al. (1984) other than that the flash originated on a GRB. The star in the middle of the 1944 optical transient error box has an ultraviolet excess which is significant at the 95 percent confidence level, a result which strongly suggests that the object is indeed the quiescent GRB optical counterpart. If true, then GRBs are probably low-mass contact binaries caused by thermonuclear or accretion instabilities at distances of roughly 1 kpc.

Schaefer, B. E.↗
NASA NTRS1987

Exosat observations of 4U/MXB 1636-53 - On the relation between the amount of accreted fuel and the strength of an X-ray burst

During Exosat observations of 4U/MXB 1636-53 in August 1985, three bursts were observed in 6 h and 24 bursts in 79 h of uninterrupted observation. The burst times, burst fluences, peak burst fluxes, burst rise times, decay times, burst intervals, mean persistent fluxes, and the product of the latter two are listed, analyzed, and discussed. The implications of the results for thermonuclear flash models are examined. A model which fits the data well involves bursts which suffer from nuclear energy losses due to stable hydrogen burning between bursts. Thus, the percentage of 'lost' energy increases with increasing burst intervals. For individual bursts, however, there are significant deviations from the approximate linear relationship.

Lewin, W. H. G.↗
NASA NTRS1987

Positron annihilation gamma rays from novae

The potential for observing annihilation gamma rays from novae is investigated. These gamma rays, a unique signature of the thermonuclear runaway models of novae, would result from the annihilation of positrons emitted by beta(+)-unstable nuclei produced near the peak of the runaway and carried by rapid convection to the surface of the nova envelope. Simple models, which are extensions of detailed published models, of the expansion of the nova atmospheres are evolved. These models serve as input into investigations of the fate of nearby Galactic fast novae could yield detectable fluxes of electron-positron annihilation gamma rays produced by the decay of N-13 and F-18. Although nuclear gamma-ray lines are produced by other nuclei, it is unlikely that the fluxes at typical nova distances would be detectable to present and near-future instruments.

Leising, Mark D.↗
NASA NTRS1987

New insights from a global view of X-ray bursts

It is shown here that the behavior of nuclear burning on accreting neutron stars depends on the mass accretion rate per unit area sigma-dot, the temperature T, and the column density of accreted matter sigma, and is thus intrinsically a three-dimensional problem. The concepts of a nuclear fuel surface and an ignition surface in (sigma-dot, T, sigma) space is introduced. It is shown that certain cuts taken through the ignition surface correspond to particular physical situations. It is found that at high temperatures a succession of hydrogen flashes leads eventually to a pure helium flash. At low temperatures, compressional heating completely determines the burst behavior. Burst sources are expected to show highly erratic behavior if they lie near the boundaries of the H and He ignition surfaces, resolving a major discrepancy between the thermonuclear flash model and observations.

Fushiki, Ikko↗
NASA NTRS1988

On relative supernova rates and nucleosynthesis roles

It is shown that the Ni-56-Fe-56 observed in SN 1987A argues that core collapse supernovae may be responsible for more that 50 percent of the iron in the galaxy. Furthermore it is argued that the time averaged rate of thermonuclear driven Type I supernovae may be at least an order of magnitude lower than the average rate of core collapse supernovae. The present low rate of Type II supernovae (below their time averaged rate of approx. 1/10 yr) is either because the past rate was much higher because many core collapse supernovae are dim like SN 1987A. However, even in this latter case they are only an order of magnitude dimmer that normal Type II's due to the contribution of Ni-56 decay to the light curve.

Arnett, W. David↗