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NASA NTRS2002

Plasmaspheric Density Troughs: Global IMAGE EUV Observations and Analysis via Global Core Plasma Modeling

To date, the IMAGE EUV camera has observed several plasmaspheric density trough features inside the plasmapause under a wide range of geomagnetic activity. From the perspective of EUV, a density trough feature appears as a channel of diminished pixel counts which spans a width of L-shell (DELTA L) and magnetic local time (MLT) inside the plasmapause. Plasmaspheric density troughs are found to be morphologically complex possessing considerable spatial and temporal variability. We present an analysis of the evolution of trough DELTA L and MLT extent as functions of associated D (sub ST) and K (sub p) history. Trough features range in size from 0.16 less than or equal to DELTA L less than or equal to 1.2 with azimuthal extent from 1500 less than or equal to MLT less than or equal to 1200. All cases of plasmaspheric density troughs studied to date appear to have evolved as a result of the inner edge of the afternoon/evening plasma drainage plume being wrapped around through the nightside plasmasphere. The structure of plasmaspheric density trough features is further probed by analyzing simulated EUV images produced by forward modeling artificially introduced regions of depleted density into both static and dynamic global core plasmaspheric models. Forward modeling suggests that (1) L-shell refilling of density troughs during storm recovery can be modeled as filling from the ionosphere toward the equator (i.e., bottom-up refilling), and (2) that an erosion process is operating within flux tubes beyond the outer L-shell wall of the observed density troughs.

Adrian, M. L.↗

NASA NTRS1974

Nuclei of planetary nebulae

The nuclei of planetary nebulae are examined both observationally and theoretically. It is seen that the region occupied by these stars in the logT-logL diagram is quite wide but consistent with a general progression of stars from high to low luminosity, with a noticeable but not large increase in luminosity during the early phase. The 'evolutionary path' is intrinsically quite wide and may indicate the evolution of stars under different conditions or non-monotonic passage along the mean path. Among the several theoretical approaches to this subject, only the double shell burning models seem to offer enough luminosity and short enough timescales to match the observations.

Odell, C. R.↗

NASA NTRS1989

Influence of heating mode on three-dimensional mantle convection

Numerical models of three-dimensional thermal convection in highly viscous spherical shells with different combinations of internal and basal heating consistently have upwelling concentrations in the form of cylindrical plumes and downwelling in planar sheets. As the proportion of internal heating increases, the number of upwelling plumes increases, and downwelling sheets become more vigorous and time-dependent. With any amount of basal heating, the entire convective pattern, during its evolution, is anchored to the upwelling plumes. As the proportion of internal heating increases, the heat flow carried by the upwelling plumes remains a large fraction of the basal heat flow. Downwelling sheets carry only a minor fraction (approximately 30 percent) of the basal heat flow (even when the shell is entirely heated from below), but they advect almost all of the internally generated heat. The relatively large number of plumes in the earth's mantle (inferred from hotspots), the possibility that downwelling slabs are vigorous enough to penetrate the lower mantle, and the small fraction of terrestrial surface heat flow carried by plumes all suggest that the mantle is predominantly heated from within.

Bercovici, D.↗

NASA NTRS1976

The evolution of rotating stars. 1: Method and exploratory calculations for a 7 solar mass star

A method was developed which allows us to study the evolution of rotating stars beyond the main sequence stage. Four different cases of redistribution of angular momentum in an evolving star are considered. Evolutionary sequences for a 7 solar mass star, rotating according to these different cases, were computed from the ZAMS to the double shell source stage. Each sequence was begun with a (typical) equatorial velocity of 210 km/sec. On the main sequence, the effects of rotation are of minor importance. As the core contracts during later stages, important effects arise in all physically plausible cases. The outer regions of the cores approach critical velocities and develop unstable angular velocity distributions. The effects of these instabilities should significantly alter the subsequent evolution.

Endal, A. S.↗

NASA NTRS1983

The evolution of the infrared emission from the Type II supernova 1980k in NGC 6946 - The dust formation model

The paper presents 1-4 micron photometry of supernova 1980 k in NGC 6946 obtained over a period of 1 year following the outburst. During the period between 1980 November 1 and December 19, the infrared emission probably originated from the extended atmosphere of the expanding star. The JHKL colors and a 1.3-2.6-micron spectrum observed during this period correspond to those of a blackbody with an average temperature of about 5000 K. Observations around 1981 May 31 showed that the supernova developed an infrared excess after 1980 December. This infrared excess persisted through 1981 October and is consistent with the appearance of thermal emission from about 700 to 900 K dust in addition to a hotter photosphere. The similarity of this behavior to that of the infrared evolution of some novae suggests that dust formation may be occurring in the supernova ejecta. The hypothesis, that the emission arises from preexisting grains in a circumstellar shell which are heated by the supernova outburst, is also consistent with the data.

Dwek, E.↗

NASA NTRS1988

Stellar nucleosynthesis and chemical evolution of the solar neighborhood

Current theoretical models of nucleosynthesis (N) in stars are reviewed, with an emphasis on their implications for Galactic chemical evolution. Topics addressed include the Galactic population II red giants and early N; N in the big bang; star formation, stellar evolution, and the ejection of thermonuclearly evolved debris; the chemical evolution of an idealized disk galaxy; analytical solutions for a closed-box model with continuous infall; and nuclear burning processes and yields. Consideration is given to shell N in massive stars, N related to degenerate cores, and the types of observational data used to constrain N models. Extensive diagrams, graphs, and tables of numerical data are provided.

Clayton, Donald D.↗

NASA NTRS1993

Hydrodynamics of Relativistic Fireballs

Many models of gamma-ray bursts involve a fireball, which is an optically thick concentration of radiation energy with a high ratio of energy density to rest mass. We examine analytically and numerically the evolution of a relativistic fireball. We show that, after an early rearrangement phase, most of the matter and energy in the fireball is concentrated within a narrow shell. The shell propagates at nearly the speed of light, with a frozen radial profile, and according to a simple set of scaling laws. The spectrum of the escaping radiation is harder at early times and softer later on. Depending on the initial energy-to-mass ratio, the final outcome of a fireball is either photons with roughly the initial temperature or ultrarelativistic baryons. In the latter case, the energy could be converted back to gamma-rays via interaction with surrounding material.

Piran, Tsvi↗

NASA NTRS1977

Planetary magnetism and the interiors of the moon and Mercury

Various theories regarding lunar and Mercurian magnetic properties are discussed in terms of the thermal evolutions of these bodies. In particular, the extinct dynamo, the primordial field of external origin, local mechanisms, and the active dynamo hypotheses are reviewed. The theory involving magnetization by an internal dynamo is applied to Mercury, noting that it implies the existence of a molten metallic core, or shell. Possible sources of the energy required for core differentiation are discussed, including accretional heating, long-lived radioactive isotopes, and other radioactive heat sources. Thermal processes which might keep the core molten are suggested along with processes permitting the flow of heat through the mantle. Conclusions suggested by the dynamo hypothesis are reviewed in terms of current models of the thermal evolution of Mercury and the moon.

Cassen, P.↗

NASA NTRS1981

Convective overshooting in the evolution of very massive stars

Possible convective overshooting in stars of 30-120 solar masses are considered, including a merger between the convective core and the intermediate zone, and penetration by the outer convection zone into the hydrogen-shell region when the star is a supergiant. Convective mixing between the core and inner envelopes is found to lead to a brief renewal of hydrogen burning in the core, and a moderate widening of the main sequence bond in the H-R diagram. Deep penetration by the outer convection zone is found to force the star out of the red supergiant configuration and into a configuration near the main sequence. This would account for the apparent spread of the uppermost part of the main sequence and the concentration of luminous supergiants towards earlier spectral types. In addition, heavy mass loss need not be assumed to achieve the points of agreement, and are tentatively considered unimportant from an evolutionary point of view.

Stothers, R.↗

NASA NTRS1977

Star dust

Recent infrared techniques have revealed that the dust which is a major constituent of the universe, is composed of refractory grains produced by certain classes of stars, condensed in their atmospheres and blown into interstellar space by the radiation pressure of these stars. In some cases stars are surrounded by dust shells which consist of carbon refractories in the case of a carbon-rich environment, and metallic silicates of the kind that produced terrestrial planets in the case of oxygen-rich environments. A few of these infrared stars (called cygnids) exhibit a unique morphology that suggests the formation of a planetary stage in the evolution of a planetary nebula. Comets which are bright in the infrared and believed to be the remnants of the most primitive material in the solar nebula, are found to inject the astrophysical dust into our solar system together with asteroidal debris. Certain novae are also found to condense grains which are blown out in their shells after the explosion.

Ney, E. P.↗

NASA NTRS2004

The Origin and Evolution of Deep Plasmaspheric Notches

Deep plasmaspheric notches can extend over more than 2 RE and 3 hours MLT in the magnetic equatorial plane. They can last for days and exhibit varying structure. In this presentation, the low L-shell portion of the recovery-time plasmaspheric convection plume will be explored as the source for these deep evacuations in density. Interactions with the ring current will be explored as the source of features observed in these density depletions.

Gallagher, D.↗

NASA NTRS1977

Evolution of the moon: The 1974 model

The interpretive evolution of the moon can be divided now into seven major stages beginning sometime near the end of the formation of the solar system. These stages and their approximate durations in time are as follows: (1) The Beginning: 4.6 billion years ago, (2) The Melted Shell: 4.6 to 4.4 billion years ago, (3) The Cratered Highlands: 4.4 to 4.1 billion years ago, (4) The Large Basins: 4.1 to 3.9 billion years ago, (5) The Light-colored Plains: 3.9 to 3.8 billion years ago, (6) The Basaltic Maria: 3.8 to 3.0(?) billion years ago, and (7) The Quiet Crust: 3.0(?) billion years ago to the present. The contributions of the Apollo and Luna exploration toward the study of those stages of evolution are reviewed.

Schmitt, H. H.↗

NASA NTRS1985

Molecular cloud evolution and star formation

The present state of knowledge of the relationship between molecular clouds and young stars is reviewed. The determination of physical parameters from molecular line observations is summarized, and evidence for fragmentation of molecular clouds is discussed. Hierarchical fragmentation is reviewed, minimum fragment scales are derived, and the stability against fragmentation of both spherically and anisotropically collapsing clouds is discussed. Observational evidence for high-velocity flows in clouds is summarized, and the effects of winds from pre-main sequence stars on molecular gas are discussed. The triggering of cloud collapse by enhanced pressure is addressed, as is the formation of dense shells by spherical outflows and their subsequent breakup. A model for low-mass star formation is presented, and constraints on star formation from the initial mass function are examined. The properties of giant molecular clouds and massive star formation are described. The implications of magnetic fields for cloud evolution and star formation are addressed.

Silk, J.↗

NASA NTRS1994

Search for important weak interaction nuclei in presupernova evolution

A search is made for the most important electron captures and beta-decays after core silicon burning in massive stars. A nuclear statistical equilibrium code is used to compute isotopic abundances. Electron capture and beta-decay rates are estimated for the 150 most abundant isotopes in a simplified fashion which generally includes the strongest transitions. These estimates are made for nuclei in the fp-shell and use techniques similar to Fuller, Fowler, & Newman (1982a), and are compared to them. The general behavior of the rate of change of Y(sub e) is examined. These methods are then used to follow a typical stellar trajectory, seeking the most important weak interactions during the formation of the iron core. Ranked lists of nuclei are given, to prioritize more detailed studies on individual nuclei. Beta-decays are found to ben an important modification to the evolution below the rate of change of Y(sub e) approx. = 0.46 as the core approaches a state of dynamic equilibrium between electron captures and beta-decays.

Aufderheide, Maurice B.↗

NASA NTRS1994

Self-gravity and dissipation in polar rings

Studies of inclined rings inside galaxy potentials have mostly considered the influence of self-gravity and viscous dissipation separately. In this study, we construct models of highly inclined ('polar') rings in an external potential including both self-gravity and dissipation due to a drag force. We do not include pressure forces and thus ignore shock heating that dominates the evolution of gaseous rings inside strongly nonspherical potentials. We adopt an oblate spheroidal scale-free logarithmic potential with axis ratio q = 0.85 and an initial inclination of 80 deg for the self-gravitating rings. We find that stellar (dissipationless) rings suffer from mass loss during their evolution. Mass loss also drives a secular change of the mean inclination toward the poles of the potential. As much as half of the ring mass escapes in the process and forms an inner and an outer shell of precessing orbits. If the remaining mass is more than approximately 0.02 of the enclosed galaxy mass, rings remain bound and do not fall apart from differential precession. The rings precess at a constant rate for more than a precession period tau(sub p) finding the configuration predicted by Sparke in 1986 which warps at larger radii toward the poles of the potential. We model shear viscosity with a velocity-dependent drag force and find that nuclear inflow dominates over self-gravity if the characteristic viscous inflow time scale tau(sub vi) is shorter than approximately 25(tau(sub p)). Rings with (tau(sub vi))/(tau(sub p)) less than or approximately equal to 25 collapse toward the nucleus of the potential within one precession period independent of the amount of self-gravity. Our results imply that stars and gas in real polar rings exhibit markedly different dynamical evolutions.

Dubinski, John↗

NASA NTRS1992

SiC particles from asymptotic giant branch stars - Mg burning and the s-process

The question of whether isotopically anomalous SiC particles found in meteorites originate in AGB stars is addressed. It is shown that if the peak helium shell flash temperatures of massive (6-9 solar masses) stars are about 10 percent larger than they are normally assumed to be, alpha particle reactions with the magnesium will become significant. Then the (Mg-29)(alpha, n)Si-29 reaction produces a large excess of Si-29. With a light element nuclear reaction network, the evolution of the silicon isotopic composition during AGB evolution is calculated. It is found that the experimentally determined correlation between excess Si-29 and excess Si-30 in SiC particles from carbonaceous chondrites can indeed be naturally produced in this way. It is suggested that if the large isotopically anomalous SiC particles carrying nearly pure-process krypton and xenon do indeed originate in AGB stars, those stars were massive and had peak shell flash temperatures near 450 KM.

Brown, Lawrence E.↗

NASA NTRS1985

The Crab Nebula and related supernova remnants; Proceedings of the Workshop, George Mason University, Fairfax, VA, October 11, 12, 1984

Papers are presented on the Crab Nebula's composition, helium distribution, outer structure and jet, and evolution. Attention is given to line emission from supernova remnants and charge transfer reactions, a magnetohydrodynamic model of the Crab Nebula and its radiation, inferences made using data on the pulsed flux from the crab pulsar, a new interpretation of the crab pulsar X-ray interpulse radiation, and evolutionary models of the Crab Nebula's progenitor. Other topics include the evolution of the centimeter flux of 3C58 and the Crab Nebula, a search for a shock wave around the Crab Nebula, high resolution radio studies of the Crab Nebula, supernova shell structure, and the nature of the remnant 0540-693 and its implications for the study of crablike remnants. Papers are also presented on X-ray observations of: Crab-like remnants, the Crab Nebula, the Vela X region, W28, and 3C400.2. Other papers include the 50 millisecond pulsar in the Large Magellanic Cloud and the X-ray pulse emission mechanism, optical emission from the plerionic core of CTB 80, and one-arcminute resolution observations of W50.

Kafatos, M. C.↗

NASA NTRS1996

Analysis of IUE Observations of Supernovae

This program supported the analysis of IUE observations of supernovae. One aspect was a Target-of-Opportunity program to observe bright supernovae which was applied to SN 1993J in M81, and another was continuing analysis of the IUE data from SN 1987A. Because of its quick response time, the IUE satellite has continued to provide useful data on the ultraviolet spectra of supernovae. Even after the launch of the Hubble Space Telescope, which has much more powerful ultraviolet spectrometers, the IUE has enabled us to obtain early and frequent measurements of ultraviolet radiation: this information has been folded in with our HST data to create unique observations of supernova which can be interpreted to give powerful constraints on the physical properties of the exploding stars. Our chief result in the present grant period was the completion of a detailed reanalysis of the data on the circumstellar shell of SN 1987A. The presence of narrow high-temperature mission lines from nitrogen-rich gas close to SN 1987A has been the principal observational constraint on the evolution of the supernova's progenitor. Our new analysis shows that the onset of these lines, their rise to maximum, and their subsequent fading can be understood in the context of a model for the photoionization of circumstellar matter.

Kirshner, Robert P.↗