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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Thermonuclear reactions in cool accreting neutron stars and burst phenomena

The ignition of accreting materials on neutron stars is explored using strongly coupled plasma analytical techniques. The calculations cover the ignition temperature and density at the bottom of the accreted envelope of a neutron star. Emphasis is placed on low-temperature ignitions which take place at high densities. The investigation is extended to the accretion of material from a white dwarf in the form of pure He, C + O, or O + Ne + Mg. It is shown that electrons are strongly degenerate in low-temperature flashes, where the ignition is more dependent on density than on temperature. Precursor flashes 0.4-0.7 the intensity of the main burst will appear before the main bursts. The intensity relationship indicates that the appropriate model for an X-ray burst from a neutron star accreting from a white dwarf is a He shell flash in the presence of a hydrogen-rich atmosphere. The flash will have a maximum energy of 2 x 10 to the 43 ergs and could last as long as 40,000 sec.

Miyaji, S.↗
NASA NTRS1986

Some topics in the magnetohydrodynamics of accreting magnetic compact objects

Magnetic compact objects (neutron stars or white dwarfs) are currently thought to be present in many accreting systems that are releasing large amounts of energy. The magnetic field of the compact star may interact strongly with the accretion flow and play an essential role in the physics of these systems. Some magnetohydrodynamic (MHD) problems that are likely to be relevant in building up self-consistent models of the interaction between the accreting plasma and the star's magnetosphere are addressed in this series of lectures. The basic principles of MHD are first introduced and some important MHD mechanisms (Rayleigh-Taylor and Kelvin-Helmholtz instabilities; reconnection) are discussed, with particular reference to their role in allowing the infalling matter to penetrate the magnetosphere and mix with the field. The structure of a force-free magnetosphere and the possibility of quasistatic momentum and energy transfer between regions linked by field-aligned currents are then studied in some detail. Finally, the structure of axisymmetric accretion flows onto magnetic compact objects is considered.

Aly, J. J.↗
NASA NTRS1988

Late-Archaean crustal growth in the Lewisian Complex of Northwest Scotland: Diachroneity in magmatic accretion and implications for models of crustal growth

The Lewisian Complex of northwestern Scotland is an Archaean basement terrain that was reworked during the Proterozoic. Previous geochronological studies have established that the complex represents an entirely new crustal addition during late-Archaean times, and the difference between a Sm-Nd accretion date of 2920 + or - 50 Ma and a Pb/Pb metamorphic date of 2680 + or - 70 Ma was interpreted as indicating a crustal accretion-differentiation superevent (CADS) lasting 240 + or - 110 Ma. A combined Rb-Sr, Pb/Pb, and Sm-Nd isotopic study reported was applied to the Lewisian CADS to attempt to address the problem of whether this comprised a short regional scale magmatic accretion event followed approximately 200 Ma later by regional high-grade metamorphism, or a long period of episodic or semi-continuous magmatic accretion prior to high-grade metamorphism.

Whitehouse, M. J.↗
NASA NTRS1988

Accretion-disk modeling of the UV spectrum of quasars

The thin accretion disk spectrum, including the effect of electron scattering, is calculated for a grid of the accretion parameters. It is shown that the luminosity and spectral slope in the UV and optical bands are uniquely determined by these parameters and depend only weakly on the viscosity parameter. When the model calculations are compared with observations of a sample of QSOs and Seyfert 1 nuclei, the black hole mass is in the range 10 to the 8th to 10 to the 9.5 solar for the QSOs and 10 to the 7.5 to 10 to the 8.5 solar for the Seyferts and for low-luminosity objects. The correlation between the spectral slope and luminosity of QSOs in the UV can be explained by evolution along curves of constant black hole mass and decreasing accretion rate. The relation between the spectrum and the accretion parameters can be used to constrain the cosmological evolution of the objects and imposes severe constraints on the pure luminosity evolution interpretation function for QSOs and Seyferts.

Wandel, Amri↗
NASA NTRS1988

Thick accretion disks ten years later

The theory of radiation pressure supported thick accretion disks is discussed and observational evidence for their existence is reviewed. Theoretical aspects examined include luminosity, spectra, jets, and variability. Observational data of the binary system SS 433, compact X-ray sources (showing quasi-periodic oscillations), and active galactic nuclei are presented. The observed central masses, total luminosities, accretion rates, efficiencies, funnels, jets, and variabilities of objects thought to be thick accretion disks are given in a table and the history of the discovery of thick accretion disks is summarized.

Abramowicz, Marek A.↗
NASA NTRS1989

Pressure gradient torque in highly supersonic nonaxisymmetric accretion

The contribution of a pressure gradient torque to the angular momentum transfer rate in highly supersonic nonaxisymmetric accretion flows is considered. This study takes into account the contribution due to the pressure variation in the postaccretion-shock region which is significant for high Mach number accretion. For the case of accretion flow with Mach (infinity) of not less than 5, the overall accretion torque is shown to approach a constant value.

Ho, Cheng↗
NASA NTRS1989

Active galactic nuclei. III - Accretion flow in an externally supplied cluster of black holes

This third paper in the series modeling QSOs and AGNs as clusters of accreting black holes studies the accretion flow within an externally supplied cluster. Significant radiation will be emitted by the cluster core, but the black holes in the outer halo, where the flow is considered spherically symmetric, will not contribute much to the overall luminosity of the source because of their large velocities relative to the infalling gas and therefore their small accretion radii. As a result, the scenario discussed in Paper I will refer to the cluster cores, rather than to entire clusters. This will steepen the high-frequency region of the spectrum unless inverse Compton scattering is effective. In many cases accretion flow in the central part of the cluster will be optically thick to electron scattering, resulting in a spectrum featuring optically thick radiative component in addition to power-law regimes. The fitting of these spectra to QSO and AGN observations is discussed, and application to 3C 273 is worked out as an example.

Pacholczyk, A. G.↗
NASA NTRS1990

Modeling of surface roughness effects on glaze ice accretion

A series of experimental investigations focused on studying the cause and effect of roughness on accreting glaze ice surfaces were conducted. Detailed microvideo observations were made of glaze ice accretions on 1 to 4 inch diameter cylinders in three icing wind tunnels (the Data Products of New England six inch test facility, the NASA Lewis Icing Research Tunnel, and the B. F. Goodrich Ice Protection Research Facility). Infrared thermal video recordings were made of accreting ice surfaces in the Goodrich facility. Distinct zones of surface water behavior were observed; a smooth wet zone in the stagnation region with a uniform water film; a rough zone where surface tension effects caused coalescence of surface water into stationary beads; a horn zone where roughness elements grow into horn shapes; a runback zone where surface water ran back as rivulets; and a dry zone where rime feathers formed. The location of the transition from the smooth to the rough zone was found to migrate with time towards the stagnation point. The behavior of the transition appeared to be controlled by boundary layer transition and bead formation mechanisms at the interface between the smooth and rough zones. Regions of wet ice growth and enhanced heat transfer were clearly visible in the infrared video recordings of glaze ice surfaces. A simple multi-zone modification to the current glaze ice accretion model was proposed to include spatial variability in surface roughness.

Hansman, R. John, Jr.↗
NASA NTRS1990

Optical veiling, disk accretion, and the evolution of T Tauri stars

High-resolution spectra of 31 K7-M1 T Tauri stars (TTs) in the Taurus-Auriga molecular cloud demonstrate that most of these objects exhibit substantial excess emission at 5200 A. Extrapolations of these data consistent with low-resolution spectrophotometry indicate that the extra emission is comparable to the stellar luminosity in many cases. If this continuum emission arises in the boundary layers of accreting disks, more than about 30 percent of all TTs may be accreting material at a rate which is sufficiently rapid to alter their evolution from standard Hayashi tracks. It is estimated that roughly 10 percent of the final stellar mass is accreted in the TT phase. This amount of material is comparable to the minimum gravitationally unstable disk mass estimated by Larson and it is speculated that the TT phase represents the final stages of disk accretion driven by gravitational instabilities.

Hartmann, Lee W.↗
NASA NTRS1990

Neutron viscosity in accretion disks

Neutron viscosity is investigated as a possible mechanism for the dissipation of kinetic energy into luminosity in the innermost parts of accretion disks around compact objects. Simplified models are presented of the self-consistent, steady-state accretion flows in which viscosity is provided by neutron collisions with accreting ions. Ion temperatures are determined by balancing the heating of ions by viscous dissipation to their cooling by Coulomb collisions with the electrons, providing a self-consistent solution between neutron production and their impact on energy dissipation. The results indicate that neutrons can indeed provide the necessary dissipation to sustain the steady-state accretion of matter at rates of about 10 to the -8th solar mass/yr or less, and electron temperatures of about 100 keV-1 MeV. Neutrons thus present a promising way of modeling bright Galactic X-ray sources.

Guessoum, Nidhal↗
NASA NTRS1990

Structure and evolution of irradiated accretion disks. I - Static thermal equilibrium structure. II - Dynamical evolution of a thermally unstable torus

The thermal equilibrum structure and dynamical behavior of externally irradiated accretion disks are investigated. For radiative disks only the surface layer is heated, while for convective disks the heat penetrates deeply into the disk. For sufficiently strong radiation and given irradiation flux F(irr), the disk is completely stabilized against thermal instabilities of the sort invoked to explain dwarf novae. For moderately strong irradiation there is still an unstable branch in the thermal equilibrium curve. In typical soft X-ray transients, the disk is unstable against the dwarf-nova type instability. Fixed F(irr) on accretion disk annuli reduces the amplitude and the quiescent times and increases the outburst duration of the resultant light curves. Varying F(irr) in proportion to the mass accretion rate at the disks's inner edge results in light curves with a plateau in the decay from outbursts. In the case when irradiation is suddenly switched on, a temperature inversion results which leads to the formation of an accretion corona.

Tuchman, Y.↗
NASA NTRS1991

Nonlinear evolution of accretion disks induced by radiative feedback processes

The existence of accretion disks around young stellar objects has recently become widely accepted. The luminosity of some young stellar objects is highly variable and is generally attributed to the release of gravitational energy from matter funneled onto them by accretion disks. The inward transport of matter through these disks is coupled to the outward transfer of angular momentum. This transfer is most likely to be regulated by the mixing of adjacent annuli through the process of large-scale turbulence. Most of the accretion energy generated by this process emerges near the inner edge of the disk. This radiation may be intercepted by the disk and may modify the vertical and viscous evolution of the disk itself. If surface heating can stabilize the disk against the dominant viscous process, then in systems with large accretion rates, angular momentum transport and mass flow through the disk will be quenched. Using this result, it is shown that such a mechanism can induce feedback through the disk which produces oscillations in the luminosity of the central object. This oscillation can become chaotic in certain regimes and might thus explain the highly variable nature of many T Tauri systems and their outbursting counterparts: the FU Orionis objects.

Bell, K. R.↗
NASA NTRS1991

Nonlinear instability of the accretion line

The instability of the flow along the accretion line in the two-dimensional Bondi-Hoyle-Lyttleton type accretion flow is studied. By means of numerical simulations, the nonlinear regime of the instability is explored, thus extending a previous work that dealt with the linear regime using the WKB approximation. In contrast to the behavior in the linear regime, in the nonlinear regime the radial and tangential modes are coupled. The radial instability, which manifests itself as large density and velocity variations on short distances, grows far beyond the linear regime. The tangential instability, on the other hand, stays in the linear regime and is dominant by long-wavelength perturbations. This results from both the influence of the radial instability and the incoming material (the material accreted onto the accretion line).

Soker, Noam↗
NASA NTRS1991

Generation of ultrahigh-energy gamma rays in accreting x ray pulsars

Relativistic protons producing ultrahigh energy gamma rays as a result of nuclear collisions ought to be generated in close proximity to the surface of a neutron star due to accretion. The main features of the mechanism in question are a high efficiency of conversion of the gravitational energy of the accreting matter into acceleration energy and a high efficiency of the acceleration itself. It is shown that in accretion to a neutron star with a strong magnetic field, a loss cone type distribution of accreting protons is formed, which due to instability effectively generates small scale Alfven and proton cyclotron waves, as well as nonlinear waves (magneto-acoustic and Alfven solitons). The electric field of the moving solitons may accelerate the protons to energies of greater than 10(exp 15) eV. The region of acceleration is not locally isolated, but extends from its surface. New possible sources of ultrahigh energy gamma rays are predicted. They may be binary x ray systems containing neutron stars with magnetic fields of about 10(exp 9) gauss.

Gnedin, Yu. N.↗
NASA NTRS1992

Compton backscattered annihilation line emission: A new diagnostic of accreting compact sources

It is shown that Compton scattering of 511 keV electron-positron annihilation radiation produces a line like feature at approx. 170 keV from backscattered photons. Assuming a simple model of an accretion disk around a compact source, the spectrum is explored of the spectrum of Compton scattered annihilation line emission for a range of conditions. It is further shown that such Compton baskscattering of annihilation line emission from the inner edge of an accretion disk could account for the previously unidentified 170 keV line emission and high energy continuum observed from a variable, compact source, or sources, of annihilation radiation near the Galactic Center. Identification of the observed 170 keV line as an annihilation line reflection feature provides strong new evidence that the source of the emission is an accreting compact object. Further study of these features in existing spectra and in forthcoming GRO observation of these and other sources can provide unique new diagnostics of the innermost regions of accretion disks around compact objects.

Lingenfelter, Richard E.↗
NASA NTRS1992

Cooling of the magma ocean due to accretional disruption of the surface insulating layer

Planetary accretion has been considered as a process to heat planets. Some fraction of the kinetic energy of incoming planetesimals is trapped to heat the planetary interior (Kaula, 1979; Davies, 1984). Moreover, blanketing effect of a primary atmosphere (Hayashi et al., 1979; Sasaki, 1990) or a degassed atmosphere (Abe and Matsui, 1986; Zahnle et al., 1988) would raise the surface temperature of the Earth-size planets to be higher than the melting temperature. The primordial magma ocean was likely to be formed during accretion of terrestrial planets. In the magma ocean, if crystallized fractions were heavier than melt, they would sink. But if solidified materials were lighter than the melt (like anorthosite of the lunar early crust) they would float to form a solid shell surrounding the planet. (In an icy satellite, solidified water ice should easily float on liquid water because of its small density.) The surface solid lid would prevent efficient convective heat transfer and slow the interior cooling. Consider that the accretion of planetesimals still continues in this cooling stage. Shock disruption at planetesimal impact events may destroy the solid insulating layer. Even if the layer survives impacts, the surface layer is finally overturned by Rayleigh-Taylor instability, since accreting materials containing metals are heavier than the surface solidified lid of silicates.

Sasaki, Sho↗
NASA NTRS1992

The fate of accreted CNO elements in neutron star atmospheres - X-ray bursts and gamma-ray lines

The fate of incident C-12, N-14, and O-16 in accreting neutron star atmospheres is described. When the accreting material is stopped by Coulomb collisions with atmospheric electrons, all incoming elements heavier than helium thermalize at higher altitudes in the atmosphere than the accreting protons. The incoming protons and helium then destroy the elements via nuclear spallation reactions. A small fraction of the nuclear reactions cause nuclear excitation and subsequent gamma-ray emission. The probability for a nucleus to survive this bombardment depends on how long it spends in the hazardous region of the atmosphere. The fractions of incident C-12, N-14, and O-16 that survive proton bombardment are calculated as a function of the accretion rate, and the mass and radius of the neutron star. The subsequent paucity of CNO nuclei decreases hydrogen-burning rates in the deep regions of the atmosphere, thereby reducing the amount of helium available for the unstable nuclear flashes that cause type I X-ray bursts. The gamma-ray line emission from this collisional deceleration scenario is determined.

Bildstein, Lars↗
NASA NTRS1991

On the 'flip-flop' instability of Bondi-Hoyle accretion flows

A simple physical interpretation is advanced by means of an analysis of the shock cone in the accretion flows past a compact object and with an examination of the accretion-line stability analyses. The stability of the conical shock is examined against small angular deflections with attention given to several simplifying assumptions. A line instability is identified in the Bondi-Hoyle accretion flows that leads to the formation of a large opening-angle shock. When the opening angle becomes large the instability becomes irregular oscillation. The analytical methodology is compared to previous numerical configurations that demonstrate different shock morphologies. The Bondi-Hoyle accretion onto a compact object is concluded to generate a range of nonlinear instabilities in both homogeneous and inhomogeneous cases with a quasiperiodic oscillation in the linear regime.

Livio, Mario↗