SEARCH · Engineering Papers
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
The icy-conglomerate model of comet nuclei has dominated all others since its introduction. It provided a basis for understanding the non-gravitational motions of comets which had perplexed dynamicists up to that time, and provided a focus for understanding cometary composition and origin. The image of comets as dirty snowballs was quickly adopted. Comet nuclei including their trail mass loss rates and refractory to volatile mass ratios are described.
The process of e(+/-) pair production by relativistic nuclei on ambient photons is considered. The process is important for cosmic-ray nuclei in interstellar and intergalactic space as well as in galactic and extragalactic compact objects. The rate of this process is given by an integral of the cross section over the photon angular and energy distribution. In the case of isotropic photons, the angular integration is performed to provide an expression for the rate at given photon energy in the nucleus rest frame. The total rate then becomes a single integral of that rate over the photon energy distribution. Formulas are also given for the fractional energy loss of a relativistic nucleus colliding with a photon of a given energy in the rest frame. The nucleus energy-loss rate is integrated over the photon angular distribution in the case of isotropic photons, and simple fits are provided.
A model is presented which assumes the existence of cold dense clouds near the central engine of active galactic nuclei (AGNs). The effects of such clouds on the observed spectrum are explored. It is shown that this model is consistent with the complicated observed spectra and variability behavior of most extensively studied Seyfert nuclei. The results are compared with other proposed models. The existing observational evidence appears to support the 'cloud-model'.
The broad emission lines in the spectra of active galactic nuclei respond to variations in the luminosity of the central continuum source with a delay due to light-travel time effects within the emission-line region. It is therefore possible through the process of 'reverberation mapping' to determine the geometry and kinematics of the emission-line region by careful monitoring of the continuum variations and the resulting emission-line response. In this review, I will discuss progress in application of the reverberation mapping technique. I will describe the underlying assumptions and limitations of the method, discuss how the results obtained to date are changing our understanding of active nuclei, and outline several new questions that might be addressed through further reverberation mapping programs.
Results are presented from detailed balance calculations, and a compilation of atomic data and other model calculations designed to support upcoming ISO and current observing programs involving IR coronal emission lines, together with a table with a complete line list of infrared transitions within the ground configurations 2s2 2p(k), 3s2 3p(k), and the first excited configurations 2s 2p and 3s 3p of highly ionized astrophysically abundant elements. The temperature and density parameter space for dominant cooling via IR coronal lines is presented, and the relationship of IR and optical coronal lines is discussed. It is found that, under physical conditions found in Seyfert nuclei, 14 of 70 transitions examined have significant population inversions in levels that give rise to IR coronal lines. Several IR coronal line transitions were found to have laser gain lengths that correspond to column densities of 10 exp 24-25/sq cm which are modeled to exist in Seyfert nuclei. Observations that can reveal inverted level populations and laser gain in IR coronal lines are suggested.
As the most promising model for the x-ray emission from a class of active galactic nuclei, represented by radio quiet quasars and Seyfert nuclei, a nonthermal pair cascade model, where soft photons are Comptonized by non thermal electron positron pair plasmas produced by gamma rays, is introduced. The simplest model of this kind, the 'homogeneous spherical cascade model', is presented. Many characteristics of this model are qualitatively similar to the homogeneous cascade model. An important difference is that gamma ray depletion is much more efficient in the surface cascade, and consequently this model satisfies the severe observational constraint imposed by the gamma ray background radiation.
Nine planetary nebula nuclei (PNNS) are now known to be pulsating variables. All of them are O VI or PG 1159-type objects, with extreme hydrogen deficiency and high abundances of C and O. They are non-radial pulsators, with typical periods of 10 to 30 min, and their probable descendants are the shorter-period GW Vir (PG 1159-035) pulsating white dwarfs whose PNe have presumably dispersed. The pulsating PNNs are promising candidates for asteroseismological study, although there are already indications that such analyses will be complicated by highly variable power spectra. We present preliminary results from two campaigns of global photometry on the pulsating nuclei of NGC 1501 and Sanduleak 3, and from ground- and space-based searches for radial-mode pulsations in PNNs and white dwarfs.
We analyze the ultraviolet (UV) emission line and continuum properties of five low-redshift active galactic nuclei (four luminous quasars: PKS 0405-123, H1821 + 643, PG 0953 + 414, and 3C 273, and one bright Seyfert 1 galaxy: Mrk 205). The HST spectra have higher signal-to-noise ratios (typically approximately 60 per resolution element) and spectral resolution (R = 1300) than all previously published UV spectra used to study the emission characteristics of active galactic nuclei. We include in the analysis ground-based optical spectra covering H beta and the narrow (O III) lambda lambda 4959, 5007 doublet. New results are obtained and presented.
Recent nucleosynthesis and hydrodynamic calculations of the consequences of accretion onto massive ONeMg white dwarf stars show that under certain circumstances significant amounts of the beta-unstable nuclei can be produced and ejected by the resulting explosion. We use these calculations as a guide in order to obtain the conditions under which the heating of the ejected material by the nonthermal electrons and positrons produced by the decays of the beta-unstable nuclei is sufficient to overcome the cooling from adiabatic expansion and lead to the production of X-ray-emitting coronal gas. These conditions are as follows: (1) a mass fraction for Na-22 of the order of 10(exp -3) or greater, (2) an expansion velocity in the range approximately 10(exp 2) - 10(exp 3) km/s, (3) a photospheric radius of approximately 10(exp 14) cm, (4) if the density distribution in the atmosphere satisfies a power law, then the exponent must be less than 3 for heating to overcome adiabatic cooling. Both the simulations of the outburst and the model atmosphere fits to the observed energy distributions, however, imply that the exponent is greater than or = 3 during the early phases of the outburst. Nevertheless, for a value of the exponent of 2, we predict the time when hot coronal gas can form during the expansion phases of the envelope.
The broad-line region (BLR) in Seyfert galaxy nuclei exhibits correlated variations in continuum and emission-line luminosity that are qualitatively consistent with photoionization of ionization-bounded (optically thick) clouds. However, evidence is growing that a nonnegligible fraction of the BLR cloud population is optically thin to the Lyman continuum and fully ionized in hydrogen. We consider the implications of this nebular component for observed line emission and find that inclusion of thin clouds in photoionization calculations can resolve several outstanding puzzles of Seyfert variability, notably the behavior of the C IV lambda 1549/Ly-alpha ratio as a function of continuum luminosity. A similar population of thin clouds located along our line of sight can account for observed ultraviolet absorption features and 'warm absorber' behavior at X-ray energies. The Baldwin effect for active galaxies, a negative correlation between ultraviolet emission-line equivalent width and continuum luminosity, can also be explained in detail by a decrease in the covering factor of an optically thin component with increasing source luminosity. The luminosity dependence of covering factor may result from outflows of thin clouds that proceed more efficiently in intrinsically brighter sources. The presence of absorption features in active galactic nuclei (AGNs) that are blueshifted and attain the highest velocities in broad absorption line features associated with luminous QSOs would be consistent with this interpretation.
High Charge and Energy Semiempirical Nuclear Fragmentation Model (HZEFRG1) computer program developed to be computationally efficient, user-friendly, physics-based program for generating data bases on fragmentation of atomic nuclei. Data bases generated used in calculations pertaining to such radiation-transport applications as shielding against radiation in outer space, radiation dosimetry in outer space, cancer therapy in laboratories with beams of heavy ions, and simulation studies for designing detectors for experiments in nuclear physics. Provides cross sections for production of individual elements and isotopes in breakups of high-energy heavy ions by combined nuclear and Coulomb fields of interacting nuclei. Written in ANSI FORTRAN 77.
As a promising model for the X-ray emission from radio-quiet quasars and Seyfert 1 nuclei, we present a nonthermal disk-corona model, where soft photons from a disk are Comptonized by the nonthermal electron-positron pairs in a coronal region above the disk. Various characteristics of our model are qualitatively similar to the homogeneous, spherical, nonthermal pair models previously studied, but the important difference is that in our disk-corona model gamma-ray depletion is far more efficient, and, moreover, the gamma-ray annihilation line is much less prominent. Consequently, this model naturally satisfies the observed constraints on active galactic nuclei.
A model is presented which assumes the existence of cold dense clouds near the central engine of Active Galactic Nuclei (AGNs). The effects of such clouds on the observed spectrum are explored. It is shown that this model is consistent with the complicated observed spectra and variability behavior of most extensively studied Seyfert nuclei. The results are compared with other proposed models. The existing observational evidence appears to support the "cloud-model."
This five-year Long-Term Space Astrophysics grant provided the support for several major steps in advancing our knowledge of the internal structure of active galactic nuclei. The single largest portion of this program had to do with the development and application of techniques for "reverberation mapping", the use of spectral monitoring of several different bands related by radiation reprocessing to infer the internal geometry of sources. Major steps were taken in this regard, particularly in establishing the distribution in radius of emission line material, and in relating the apparent reprocessing of continuum bands to the underlying structure of the accretion disk. Another major effort built directly upon these results. Once the case for continuum reprocessing was made by the monitoring, it next behooved us to understand the spectral output of AGN as a result of this reprocessing. As a result, our view of continuum production in AGN is now much better focussed on the key problems. A third focus of effort had to do with the nature of X-ray variability in AGN, and what it can tell us about the dynamics of extremely hot material in the immediate outskirts of the supermassive black holes that form the central engines of active galactic nuclei. In addition to these primary efforts, this grant also supported many other, smaller projects. Several of these were demonstrations of how the material spewed out of AGN in relativistic.ets generate the radiation by which we observe them. J Finally, the portion of this study that had to do with continuum production by accretion disks in AGN led naturally to several papers in which new developments were presented having to do with "advection-dominated accretion disks", those disks in which accretion appears to proceed at a substantial rate, but in which radiation processes are weak.
As the most promising model for the X-ray emission from a class of Active Galactic Nuclei (AGNs) represented by radio-quiet quasars and Seyfert nuclei, here we introduce the non-thermal pair cascade model, where soft photons are Comptonized by non-thermal electron-positron pair plasmas produced by (gamma)-rays. After summarizing the simplest model of this kind, the "homogeneous spherical cascade model", our most recent work on the "surface cascade model" is presented, where a geometrical effect is introduced. Many characteristics of this model are qualitatively similar to the homogeneous cascade model. However, an important difference is that (gamma)-ray depletion is much more efficient in the surface cascade, and consequently this model naturally satisfies the severe observational constraint imposed by the (gamma)-ray background radiation.
Cometary nuclei preserve a cosmo-chemical record of conditions and processes in the primordial solar nebula, and possibly even the interstellar medium. However, that record is not perfectly preserved over the age of the solar system due to a variety of physical processes which act to modify cometary surfaces and interiors. Possible structural and/or internal processes include: collisional accretion, disruption, and reassembly during formation; internal heating by long and short-lived radionuclides; amorphous to crystalline phase transitions, and thermal stresses. Identified surface modification processes include: irradiation by galactic cosmic rays, solar protons, UV photons, and the Sun's T Tauri stage mass outflow; heating by passing stars and nearby supernovae; gardening by debris impacts; the accretion of interstellar dust and gas and accompanying erosion by hypervelocity dust impacts and sputtering; and solar heating with accompanying crust formation. These modification processes must be taken into account in both the planning and the interpretation of the results of a Comet Nucleus Sample Return Mission. Sampling of nuclei should be done at as great a depth below the surface crust as technically feasible, and at vents or fissures leading to exposed volatiles at depth. Samples of the expected cometary crust and near-surface layers also need to be returned for analysis to achieve a better understanding of the effects of these physical processes. We stress that comets are still likely less modified dm any other solar system bodies, but the degree of modification can vary greatly from one comet to the next.
The research developed the fundamental techniques of the emulsion chamber methods that permit measurements of the composition and energy spectra of cosmic rays at energies ranging from 1 GeV/n to over 1,000 TeV/n. The research program consisted of exploring new principles and techniques in measuring very high energy cosmic nuclei with large-area emulsion chambers for high statistics experiments. These tasks have been accomplished and their use was essential in successful analysis of the balloon-borne emulsion chamber experiments up to 10(exp 14) eV. It also provided the fundamental technologies for designing large-area detectors that are aimed at measuring the composition at above 1015 eV region. The latter is now partially succeeded by a NASA Mission Concept, Advanced Cosmic Composition Experiments on the Space Station (ACCESS). The cosmic ray group at the University of Alabama in Huntsville has performed technological R & D as well as contributing to the Japanese-American-Emulsion-Chamber-Experiments (JACEE) Collaboration with the regular data analysis. While primary research support for other institutions' efforts in the JACEE experiments came from NSF and DOE, primary support for the University of Alabama in Huntsville was this contract. Supplemental tasks to standardize the data base and hardware upgrades (automatized microscope) had this institutions cooperation. Investigation of new techniques in this program consisted of development of a fast calorimetry, magnetic/scattering selection of high momentum tracks for a pairmeter, and high statistics momentum measurements for low energy nuclei (E < 1 TeV/n). The highest energy calorimetry and a pairmeter have been considered as strawman instruments by the GOAL (Galactic Origin and Acceleration Limit) proposal of the NASA Cosmic Ray Working Group for long- duration balloon flights. We accomplished the objectives of the GOAL program with three circumpolar, Antarctic JACEE balloon flights during 1992 - 1994.
he purpose of this grant was to support theoretical research on the nature of accretion in active galactic nuclei. In the brief time of the award (one year), four papers that appeared in refereed journals were written, as well as two invited reviews in conference proceedings These papers significantly advanced our understanding of the structure of the most important parts of bright accretion disks around accreting black holes, such as active galactic nuclei.