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Cameron, A. G. W.

Publications and source records attributed to Cameron, A. G. W..

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Accumulation processes in the primitive solar nebula.

Particle accumulation processes are discussed for a variety of physical environments, ranging from the collapse phase of an interstellar cloud to the different parts of the models of the primitive solar nebula constructed by Cameron and Pine (1973). The interstellar collapse phase, the settling of particles to the control plane, processes in the convective zone, the accumulation of planetary bodies, and gas accretion are discussed. Attention is given to the formation of satellites, angular momentum aspects of star formation, the formation of comets, the formation of Mercury, the T Tauri solar wind, the evolution of the terrestrial planets, the radial distances of the planets, and the dissipation of excess solids.

Cameron, A. G. W.↗
NASA NTRS1973

The early evolution of the solar system

The problems of relating collapse conditions in an interstellar cloud to a model of the primitive solar nebula are discussed. In such a nebula there is a radial force balance between gravity, the pressure gradient, and centrifugal forces due to the rotation. Approximate values are given for the combinations of temperature and density throughout the nebula, from a maximum of about 2000 K near the center to less than 200 K in the outer portion. These conditions are based upon the compression adiabats in the terminal stages of the collapse of an interstellar cloud. One general conclusion, of great importance for accumulation of bodies within the solar system, is that interstellar grains should not be completely evaporated at distances in the nebula beyond about one or more astronomical units.

Cameron, A. G. W.↗
NASA NTRS1973

Interstellar grains in museums

It is argued that carbonaceous chondrites, particularly of type I, are probably collections of interstellar grains which have been mildly transformed through exposure to higher than normal temperatures, resulting in a loss of volatile materials.

Cameron, A. G. W.↗
NASA NTRS1972

Effects of sudden mixing in the solar core on solar neutrinos and ice ages.

Some numerical experiments with a solar model have been conducted in connection with the hypothesis regarding the effects of mixing in the solar core. Questions concerning a plausible mechanism by which such a mixing could be produced are explored. The variation of solar luminosity throughout the numerical experiments is shown. In connection with a great change in luminosity after a second mixing, it is suggested that the earth is presently undergoing an ice age.

Ezer, D.↗
NASA NTRS1972

Orbital eccentricity of Mercury and the origin of the moon.

The results of some recent work conducted by Anderson (1972) are considered. If Anderson is correct in his conclusion that the bulk composition of the moon resembles that of the Allende inclusions, then the natural place for the formation of the moon in the solar system is inside the orbit of Mercury, through planetary accumulation from the condensed material to be found there. This explains the anomalous large eccentricity of the orbit of Mercury.

Cameron, A. G. W.↗
NASA NTRS1972

The p-process in explosive nucleosynthesis.

The limiting conditions consistent with p-process synthesis in supernova envelopes are inferred from calculations of the appropriate rates of proton capture and neutron photodisintegration. Temperatures in excess of 2 x 10 to the 9th power K are required, for proton mass densities of the order of 100 g cm to the minus 3rd power, if significant production of p-process nuclei is to take place on a hydrodynamic time scale. It is concluded that these processes must be appropriate to very different stellar or supernova environments.

Truran, J. W.↗
NASA NTRS1972

Origin and early development of the planets.

Extensive numerical calculations have been carried out to determine the structure of the primitive solar nebula. The resulting structure shows a dominance of the radial pressure gradient near the center, and a dominance of the centrifugal forces throughout most of the outer region. Thermal convection and Eddington-Sweet circulation currents can be expected to cause a rapid dissipation of the disk, requiring only a few centuries. It is therefore postulated that the planets were formed in an exceedingly hot initial state due to their rapid rates of accumulation. Beyond one or two astronomical units from the center, the original interstellar grains should not be completely destroyed during the formation of the disk, and these will form the nuclei for subsequent chemical condensations and chemical accumulation. Inside this distance, nucleation of condensed materials must occur within the nebular gases.

Cameron, A. G. W.↗
NASA NTRS1972

A numerical hydrodynamic study of coalescence in head-on collisions of identical stars.

The specific results reported refer to head-on collisions between identical polytropes of index 3 having solar mass and radius. If the polytropes were initially at rest at infinity, then about 5% of the combined mass is lost by ejection following collision. The volatilized mass fraction rises to about 18% for an initial relative collision velocity of 1000 km/sec at infinite separation, and to about 60% for the 2000 km/sec case. Since the initial kinetic and gravitational energies balance for a relative velocity of 1512 km/sec at infinity, it may be seen that net coalescence persists to velocities somewhat in excess of this figure. Mass ejection takes place in two ways simultaneously: (1) by a rapid sideward expulsion of fluid in a massive lateral sheet normal to the collision axis, and (2) as a result of two recoil shocks which lead momentum flows backward along this axis. The lateral effect has similarities to the expansion of gas into a vacuum i.e., shocks are not involved. However, the ejection of material from the rear colliding hemisphere due to the recoil shocks predominates at low collision velocities. As the velocity increases, both effects strengthen, but the lateral expulsion intensifies more rapidly than the recoil shocks.

Seidl, F. G. P.↗
NASA NTRS1972

Surface composition of magnetic neutron stars.

The relative abundances of seven constituent nuclei, He4, C12, O16, Ne20, Mg24, Si28, and Fe56, are calculated as a function of time for neutron star atmospheres within which exist magnetic fields of the order of 10 to the 13th G. The opacity, equation of state of the electrons, and cooling rate of the magnetic star are discussed, and it is shown to be a reasonable approximation to assume an atmosphere to be isothermal. The effects of particle diffusion are included in the nuclear reaction network. Computations are performed both for a constant mass atmosphere and for an atmosphere in which mass is being ejected. It is found that the final abundances are model-independent, as long as the initial model contains predominantly He4. The relative abundances are compared to the cosmic ray spectrum. For both the constant-mass and mass-loss atmospheres, nucleosynthesis proceeds virtually completely to Fe56. However the outermost layers of the envelope, in which no mass is being ejected, are composed almost entirely of He4 with trace amounts of Fe56. After the loss of about 10 to the 21st g, only Fe56 is ejected from atmospheres expelling mass.

Rosen, L. C.↗
NASA NTRS1972

Neutron stars.

Particular attention is given to the physical concepts underlying neutron stars. The characteristics of an equation of state which is suitable for a neutron star configuration are discussed. Cold neutron star models are examined, including the properties of a particular model in the intermediate range of stable models. Questions regarding the thermal energy of a neutron star are discussed together with aspects of vibrational energy, rotational energy, magnetic energy, and problems in connection with the Vela pulsar.

Cameron, A. G. W.↗
NASA NTRS1971

The evolution of hydrogen-helium stars.

Investigation of the premain sequence evolution and the main sequence evolution of stars of 5, 10, 20, 30, 100, and 200 solar masses. Normal stars in this entire mass range normally convert hydrogen into helium by the CN-cycle on the main sequence. The present hydrogen-helium stars of 5 and 10 solar masses must reach higher central temperatures in order to convert hydrogen to helium by the proton-proton chains. Consequently, the mean densities in the stars are greater, and the surface temperatures are higher than in normal stars. In the stars of 20 solar masses and larger, the proton-proton chains do not succeed in supplying the necessary luminosity of the stars by the time the contraction has produced a central temperature near 10 to the 8th K. At that point triple-alpha reactions generate small amounts of C12, which then acts as a catalyst in the CN-cycle, the rate of which is then limited by the beta-decays occurring within the cycle. During the evolution of these more massive stars, the central temperature remains in the vicinity of 10 to the 8th K, and the surface temperature on the main sequence approaches 100,000.

Ezer, D.↗
NASA NTRS1971

Evolutionary models of nucleosynthesis in the galaxy.

A model of the galaxy is constructed and evolved in which the integrated influence of stellar and supernova nucleosynthesis on the composition of the interstellar gas is traced numerically. Our detailed assumptions concerning the character of the matter released from evolving stars and supernovae are guided by the results of recent stellar evolutionary calculations and hydrodynamic studies of supernova events. It is difficult to visualize an epoch of massive star formation in the collapsing gas cloud which formed our galaxy which would enrich the gas rapidly enough to account for the level of heavy element abundances in halo population stars; we have therefore proposed a stage of star formation which is entirely pregalactic in character. We suggest that the Jeans' length-sized initial condensations in the expanding universe discussed by Peebles and Dicke may provide the appropriate setting for this first generation of stars. Guided by these considerations, and by the need for a substantial quantity of 'unseen' mass to bind our local group of galaxies, we have constructed a model of the galaxy in which this violent early phase of massive star formation produces both (1) approximately 25% of the level of heavy elements observed in the solar system and (2) an enormous unseen mass in the form of black holes.

Truran, J. W.↗
NASA NTRS1971

Nuclear processes associated with peculiar A-type stars.

A discussion is given of the various roles that nuclear reactions may play in production of anomalous abundances of elements in peculiar A stars. The effects which may be expected to occur both in the surface nuclear reactions and in some possible internal reactions that can occur in advanced stages of stellar evolution are considered. It is suggested that various features of peculiar A stars may require simultaneous operation of two or more of the processes of surface diffusion, surface nuclear reactions, and internal nuclear reactions.

Cameron, A. G. W.↗
NASA NTRS1971

Cosmic rays from supernovae and comments on the Vela X pre-supernova

A possible history of the production of elements in the galaxy is presented, based on assumptions about the end points of stellar evolution and of the general evolution of the galaxy. A wide range of quantities involving the relative abundances of nucleosynthesis products observed in the solar system, and various galactic quantities such as the current rate of supernova production and the present gas content of the galaxy, were considered. These assumptions were utilized in a computer program in which the gas content of the galaxy is gradually turned into stars. The stars are continually enriched in the products of nucleosynthesis as they approach the ends of their evolutionary lifetimes. It is suggested that supernova explosions are associated with the mass range of about 4-8 solar masses. Possible theories on the type of stellar explosive event represented by the Vela supernova are discussed.

Cameron, A. G. W.↗
NASA NTRS1971

The chemical evolution of the galaxy

Galaxy chemical evolution, discussing stellar mass loss, supernova explosions, white dwarf formations, nucleosynthesis, cosmochronology, etc

Cameron, A. G. W.↗