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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Anomalous composition and energy spectra of cosmic rays below 20 MeV/nucleon

The relative abundances and energy spectra of C, N, O, Ne, and Fe are investigated as a function of the interplanetary low energy proton intensity using observations obtained over a 17-month period by IMP-8. The oxygen intensity was enhanced when proton activity was at its lowest level, while all other heavy elements almost disappeared. Nuclei of less than 3 MeV/nucleon showed a strong intensity fluctuation strongly correlated with the interplanetary proton flux and displayed a velocity anisotropy along the garden-hose angle, indicating that they are of solar origin. The anomalous oxygen hump between 3 and 20 MeV is apparently not of solar origin. The observations are compatible with the model proposed by Fisk et al. (1974), in which these oxygen nuclei are considered to be interstellar neutral oxygen atoms which are ionized and accelerated to the observed energies by magnetic field irregularities in the outer solar system.

Klecker, B.↗
NASA NTRS1976

Color gradients in the nuclear region of M31

Spectrophotometric measurements of the nuclear region of M31 under conditions of excellent seeing with M31 near the zenith and using a 3.8-9.9-in. diameter entrance annulus for the bulge along with a 3.6-in. aperture for the nucleus, show that the nuclear region is only slightly redder and shows marginally stronger CN blends than the bulge. The data can be interpreted in terms of an increase in heavy-element abundance toward the center of M31 by a factor substantially less than two.

Thuan, T. X.↗
NASA NTRS1976

On the origin of the solar system and of Jupiter and its satellites

The evolution of the middle part of the primitive solar nebula is traced through seven stages: (1) a gravitational instability which clumps condensed material into bodies of appreciable size; (2) the formation of a larger body as a result of mutual collisions; (3) gravitational condensation of the gas in the nebula; (4) subsequent hydrodynamic collapse of the gas onto a heavy-element planetary core; (5) the attainment of a highly distended hydrostatic configuration and its subsequent slow gravitational contraction; (6) the formation of a gaseous disk around proto-Jupiter; and (7) the formation of the Galilean satellites (and Amalthea) within this disk. Different theories on the origin of the primitive solar nebula are reviewed, models of Jupiter are evaluated, and the origin and evolution of the planet are traced through the indicated stages. It is proposed that Jupiter's current excess luminosity is due mostly to a loss of internal energy generated during the initially more rapid contraction period and, to a lesser degree, to the planet's current release of gravitational energy. The question of when the icy constituent was added to the outer two Galilean satellites is considered.

Cameron, A. G. W.↗
NASA NTRS1976

High-velocity interstellar clouds

Interstellar absorption line studies have revealed that rapidly moving clouds within one kiloparsec of the sun have considerably less depletion of heavy elements than in the low-velocity diffuse clouds, and their density is somewhat greater and their thickness smaller. Evaporation of interstellar grains, resulting from grain-grain collisions in interstellar shocks, may provide a possible explanation for the observations. Some problems involved in this mechanism are discussed, particularly the likely dominant importance of the interstellar magnetic field in confining the grains within the shocked layer and in accelerating them through the associated electric field, thus offsetting the frictional force of the gas.

Spitzer, L.↗
NASA NTRS1977

The structure of the atmosphere of Uranus

Models of the interior of Uranus suggest that the abundances of such substances as CH4 are greatly enhanced with respect to solar abundances of heavy elements. Such enhancement leads to a new type of model atmosphere for Uranus, which agrees with observation if the internal energy flux is small (no more than 10%) compared with the absorbed solar energy. An important feature of the models is the presence of a cloud of CH4 droplets whose top is at a temperature of approximately 90 K and a pressure of about 4 atm. Above the cloud, the atmosphere is stable because of the rapid decrease in the thermal flux with depth. Being saturated, most of the observable gaseous CH4 is near the cloud; the CH4 abundance above the cloud, of the order of 5 km-am, is a very sensitive function of the cloud-top temperature.

Danielson, R. E.↗
NASA NTRS1977

On the fragmentation of cosmic gas clouds. III - The initial stellar mass function

Radiation from the first opaque protostellar fragments to form in a nonhomologously collapsing molecular cloud can provide a significant heat source in regions where the matter remains relatively diffuse over the initial free-fall time scale. This energy input provides a negative feedback that gradually inhibits fragmentation as larger protostellar fragments form that are also more effective radiators. A highly simplified model is described that yields an expression for the initial mass function in stellar clusters. Dynamical dissipation, driven by the more massive protostars, tends to decouple the gas and grain temperatures and steepen the mass function. Fragmentation of less massive stars appears to proceed more efficiently when the heavy-element abundance is decreased, indicating the possibility of an inverse correlation between gradients in Z and M/L. Other possible observational tests of fragmentation are also suggested.

Silk, J.↗
NASA NTRS1977

Structure of the iron fluorescence line in X-ray binaries

The paper presents an approximate solution to the iron K alpha line transfer problem in a stellar atmosphere illuminated by an external X-ray source as in a binary X-ray system. The emergent Comptonized line profile can independently yield information on the abundance of heavy elements in the primary atmosphere and on the solid angle subtended by the primary at the X-ray source.

Hatchett, S.↗
NASA NTRS1977

Grain disruption in interstellar hydromagnetic shocks

The observed abundance variations of Ca, Fe, Si, and Ti in intermediate-velocity interstellar gas suggest that grains have been disrupted in clouds with velocities as low as 20 to 50 km/s. A simplified hydromagnetic shock model for such clouds is described; the dynamical equations for charged grains in the postshock region are derived, including the collisional drag and 'betatron-acceleration' effect of a magnetic-field gradient; and the fraction of grains destroyed in evaporative collisions with other grains is calculated. It is found that for shocks of 20 to 50 km/s in which the fractional H-ionization remains low, 3-10% of the grain material may be destroyed - sufficient to explain in part the heavy-element depletion pattern in intermediate velocity clouds and the well-known correlation of N(Ca II)/N(Na I) with cloud velocity.

Shull, J. M.↗
NASA NTRS1977

Investigation of coronal holes in the radio and far-ultraviolet ranges

Results of coronal-hole observations carried out in the far-UV with a spectro-heliometer aboard Skylab are compared with corresponding results of ground-based radioheliograph measurements performed at frequencies of 80 and 160 MHz. It is found that the electron density derived from the far-UV observations for the transition region and lower corona is nearly three times greater than the value computed on the basis of the radio data. Unsuccessful attempts are made to eliminate this discrepancy by recalibrating the radio data and by recalculating the ionization equilibrium. A substantial local enhancement of the heavy-element abundance in certain parts of the transition region and inner chromosphere is considered as a possible cause of the discrepancy.

Sheridan, K. V.↗
NASA NTRS1977

On the origin of the absorption spectra of quasistellar and BL Lacertae objects

Following the earlier work of Wagoner (1967) we have reexamined the possibility that the absorption spectra of QSOs are due to gas in the disks, coronae, or halos of intervening galaxies. Comparison of the expected number of absorption systems with the number observed in all of the QSOs and BL Lacertae objects so far cataloged shows that there is a gross discrepancy in the sense that the multiplicity of absorption systems already found is far higher than the numbers expected if the absorption is due to gas in known disks or hypothetical small coronae. A discussion of the selection effects suggests that the number of absorption systems so far reported is an underestimate, so that an even greater discrepancy is likely to be present. High multiplicities can be explained only if very large halos or clouds containing heavy elements are commonly present, and there is no independent evidence for this. We conclude that the bulk of the absorption is likely to be intrinsic to the QSOs.

Burbidge, G.↗
NASA NTRS1978

On two new X-ray sources in the SMC and the high luminosities of the Magellanic X-ray sources

The discovery of two new X-ray sources, SMC X-2 and SMC X-3, in the Small Magellanic Cloud is reported. They have hard spectra, and their luminosities in the energy range 2-11 keV are 1.0 and 0.7 by 10 to the 38th power erg/sq cm per sec, respectively. It is shown that the luminosity distribution of the known Magellanic X-ray sources, which are now nine in number, is shifted toward higher luminosities with respect to that of similar sources in the Galaxy, and that the cause of the shift is probably an underabundance of heavy elements in the material accreted by the X-ray sources.

Clark, G.↗
NASA NTRS1978

A consistent age for the Universe

The constraints associated with traditional dating methods for determining the age of the universe are combined in the context of the standard cosmological model to see which age can satisfy them all. The allowed age of the universe is found to range from 13.5 to 15.5 times 1,000,000,000 years in accordance with the limits for the density parameter being 0.06 and 0.3 and the limits for the heavy element mass fraction of globular clusters being 0.0001 and 0.001. The proposed range for the age of the universe agrees with Friedmann's description of the universe.

Kazanas, D.↗
NASA NTRS1978

Optical emission from a fast shock wave - The remnants of Tycho's supernova and SN 1006

The faint optical filaments in Tycho's supernova remnant appear to be emission from a shock front moving at 5600 km/s. The intensity of the hydrogen lines, the absence of forbidden lines of heavy elements in the spectrum, and the width of the filaments are explained by a model in which a collisionless shock wave is moving into partially neutral gas. The presence of the neutral gas can be used to set an upper limit of approximately 5 x 10 to the 47th power ergs to the energy in ionizing radiation emitted by a Type I supernova. The patchy neutral gas is probably part of the warm neutral component of the interstellar medium. The existing information on the remnant of SN 1006 indicates that its emission is similar in nature to that from Tycho's remnant.

Chevalier, R. A.↗
NASA NTRS1979

Analysis of ultraviolet spectrophotometric data from Copernicus

Ultraviolet spectral data from the OAO 3 satellite are being used to study interstellar absorption lines and stellar and circumstellar lines in hot stars. The interstellar data are beneficial in analyzing the depletions of heavy elements from the gas phase and in elucidating how these depletions depend on physical conditions. Abundances in separate velocity components were determined from line profiles. Observations were carried out for interstellar abundances, both atomic and molecular, towards a number of stars. The better quality data are being analyzed for profile information and the lesser data are being used in curve-of-growth analyses. Molecular observations were carried out as well, N2 was sought; interstellar C2 was detected and its rotational excitation utilized to establish limits in interstellar cloud temperatures. An extensive search for H2O resulted in a tentative identification which will produce new information on chemical reaction rates. Interstellar depletions and grain properties in the rho Ophiuchi cloud, stellar wind variability, and circumstellar lines are also under study.

Snow, T. P., Jr.↗
NASA NTRS1978

Survey of some recent stellar-evolution calculations

The stellar-evolution program developed by Sweigart (1973) was used to compute main sequence turnoff, red giant sequences, and horizontal branch sequences for many values of helium abundance and heavy-element abundance. The immediate aim has been to produce a self-consistent set of evolutionary sequences for wide ranges in the composition and mass. The long-term aim involves application to cluster HR diagrams and the integrated properties of galaxies. Turnoff sequences, red giant sequences, and horizontal branch sequences are discussed.

Sweigart, A. V.↗
NASA NTRS1979

Particle acceleration in solar flares by cyclotron damping of cascading turbulence

It is pointed out that cyclotron damping of cascading turbulence can be a very efficient acceleration process because the turbulence is generally damped by the suprathermal tail of the particle distribution. Using a simplified approach to the transfer of wave energy down the spectrum, analytic time-dependent solutions are derived which describe the growth of the tail and its effect back on the turbulence. They suggest that with this mechanism, a sizable fraction of all the available energy ends up in a very small minority of the particles at the flare site, in accord with observations. Preferential acceleration of heavy elements is generally expected. The enhancement mechanism is very sensitive; and if He-3 is preaccelerated by even a modest amount, it could be enhanced by many orders of magnitude, as observed in some flares.

Eichler, D.↗
NASA NTRS1979

Gas drag in primordial circumplanetary envelopes - A mechanism for satellite capture

Known properties of the current solar system and Bodenheimer's (1977) model of early Jovian evolution are employed to develop a mechanism for satellite capture based on gas drag in primordial circumplanetary envelopes. In particular, the deceleration and fragmentation of two parent bodies passing through an extended primordial Jovian nebula may account for the clusters of prograde and retrograde satellites of Jupiter. Subsequently, the fragments probably underwent limited orbital evolution, and were dispersed by collision with a stray body. The heavy element cores of the outer planets may also be due to primordial gas drag capture. Nebular drag capture of the Martian satellites Phobos and Deimos, Neptune's Nereid and Triton, and Saturn's Phoebe and Iapetus is also conceivable.

Pollack, J. B.↗
NASA NTRS1979

Compositions of energetic particle populations in interplanetary space

Study of the compositions of energetic particle populations in interplanetary space is reviewed emphasizing observations of helium and heavier particles with energies below about 10 to 20 MeV/nucleon. Three dominant constituents of interplanetary particles are considered: solar flare particles, co-rotating particle streams, and a so-called 'anomalous cosmic ray' component. The recent discovery of a class of solar flare particle events with heavy elements indicates highly selective solar injection mechanisms. Observations of positive radial intensity gradients and anisotropies in co-rotating particle streams suggest the existence of large-scale interplanetary acceleration in a region extending from about 2 to 4 AU. Although present observations exclude the sun as a direct source of the 'anomalous cosmic ray' component, its origin is still debated.

Gloeckler, G.↗