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NASA NTRS1984

Geological rhythms and cometary impacts

Time-series analysis reveals two dominant, stable long-term periodicities approximately equal to 33 + or - 3 and 260 + or - 25 million years in the known series of geological and biological upheavals during the Phanerozoic Eon. Because the cycles of these episodes agree in period and phase with the cycles of impact cratering on earth, these results suggest that periodic comet impacts strongly influence global tectonism and biological evolution. These two periodicities could arise from interactions of the solar system with interstellar clouds as the solar system moves cyclically through the Galaxy.

Rampino, M. R.↗

NASA NTRS1984

Cosmogony as an extrapolation of magnetospheric research

A theory of the origin and evolution of the Solar System which considered electromagnetic forces and plasma effects is revised in light of information supplied by space research. In situ measurements in the magnetospheres and solar wind can be extrapolated outwards in space, to interstellar clouds, and backwards in time, to the formation of the solar system. The first extrapolation leads to a revision of cloud properties essential for the early phases in the formation of stars and solar nebulae. The latter extrapolation facilitates analysis of the cosmogonic processes by extrapolation of magnetospheric phenomena. Pioneer-Voyager observations of the Saturnian rings indicate that essential parts of their structure are fossils from cosmogonic times. By using detailed information from these space missions, it is possible to reconstruct events 4 to 5 billion years ago with an accuracy of a few percent.

Alfven, H.↗

NASA NTRS1985

Theoretical investigation of the interstellar CH3NC/CH3CN ratio

Calculations have been performed to determine the abundance ratio of the metastable isomer CH3NC to the stable isomer CH3CN in dense interstellar clouds. According to gas phase, ion-molecule treatments, these molecules are both synthesized via protonated ion precursors. A calculation has been made of the ratio of the formation rates of the protonated precursor ions, CH3NCH(+) and CH3CNH(+), synthesized via the radiative association reaction between CH3(+) and HCN, which is thought to be the dominant formation process of the two isomeric ions. The calculations, which involve both ab initio quantum chemistry and equilibrium determinations, lead to a predicted CH3NCH(+)/CH3CNH(+) formation rate ratio between 0.1 and 0.4. If this ratio is maintained in the neutral species formed from the precursor ions, theory predicts a sizable abundance for methyl isocyanide (CH3NC) and lends credence to its tentative observation.

Defrees, D. J.↗

NASA NTRS1985

Spacelab 2 - A preview

The Spacelab 2 mission, which is scheduled for Space Shuttle Challenger launch in July of 1985, will carry four telescopes for solar study, a dual X-ray telescope for observation of galaxy clusters, and a helium-cooled IR telescope for studies of interstellar clouds and other extended sources. The largest cosmic ray detector carried to space thus far will also be part of the payload. Life science experiment packages will examine the vitamin D chemistry of human blood under zero-G conditions, and the manner in which pine tree seedlings sense gravity and respond to it. Spacelab 2 will carry a crew of seven, including three mission specialists and two payload specialists.

Henize, K. G.↗

NASA NTRS1985

Terrestrial record of the solar system's oscillation about the galactic plane

A new study is presented of the observational evidence pertaining to the theory which attributes the episodic component of the earth's impact cratering record over the past 600 Myr to gravitational encounters between the solar system and interstellar clouds that cause comets to fall into the solar system and impact the earth. Contrary to a claim by Thaddeus and Chanan (1985), the vertical scale height of the clouds seems to be sufficently small and the sun's vertical trajectory sufficiently large for the modulating effect of the sun's galactovertical motion to be detectable in the terrestrial record of impact cratering with at least a 50 percent a priori probability.

Stothers, R. B.↗

NASA NTRS1985

Protostars and planets - Overview from a planetary perspective

Today the simultaneous formation of a star and planetary system is understood to be natural and coupled consequences of the collapse of an interstellar cloud of molecular gas and dust. Ideas put forward to account for the existence of the solar system seem generally applicable to star formation throughout the universe. If this is indeed the case, then planetary systems should be present in large numbers throughout the universe. Even the gross structural features of the planetary system in the solar system seem largely to be the result of nearly deterministic processes, rather than the result of chance alone; many planetary systems throughout the universe may resemble this one. This chapter summarizes and examines some of the implications of ideas about the formation of stars and planetary systems.

Levy, E. H.↗

NASA NTRS1986

Formation of the sun and its planets

Aspects of star formation are considered, taking into account those observations and models which have a relation to the formation of the solar system. Attention is given to the galactic structure, the stellar properties, planetary indications, interstellar clouds, observations of forming stars, the conditions for cloud collapse, models of star formation, the modeling of secondary features, and aspects of dynamical evolution. Aspects of cosmochemistry are also explored. Questions of chronology are discussed along with nuclide variations, chemical variations among chondritic meteorites, and chemical variations among differentiated objects. A description of planet formation is provided, taking into consideration, gas-dust interaction, sun and accretion disk dynamics, planetesimal swarms, planet-disk interactions, planetary systems, terrestrial planet formation, and gaseous protoplanet contraction.

Kaula, W. M.↗

NASA NTRS1986

A comparison of high-resolution radio and optical data for the northeast region of the Cygnus Loop

Radio observations of the northeastern part of the Cygnus Loop obtained with the VLA at 18 cm with a resolution of 4.7 arcsec are presented and discussed. In general, the brightest radio filaments match the brightest optical filaments very well, particularly those which are bright in H-alpha. The agreement in position, morphology, and relative brightness is particularly good where all four optical species are bright. However, a few bright radio features have no bright optical counterparts. The general radio and optical structure of the region can be explained by a model in which shocked interstellar clouds form filaments due to unstable cooling and compression. The features appear to be ropelike, although it is not possible to rule out that they are irregular sheets seen edge-on. A large gradation in cooling on a scale of parsecs is evidenced by the sequence of the optical species in emission, with a corresponding increase in density generally evidenced by the continuum radio emission.

Straka, W. C.↗

NASA NTRS1986

Cool interstellar matter in early-type galaxies

IRAS fluxes of early-type galaxies have been examined. From studying a magnitude-limited sample, it seems that although the statistics are poor, perhaps a third of these objects and possibly even more have an appreciable amount of dust. In general, the infrared emission is strongest at 100 microns, and quite often the far-infrared luminosity of an early-type galaxy can be well in excess of 10 to the 8th solar luminosities. The data are most easily understood if the infrared results from dust reprocessing of starlight. Within 3 kpc of the center of an early-type galaxy, there may be more cold matter than hot gas; recent models that the interstellar media of these objects are composed primarily of hot gas appear to be oversimplifications. The X-ray data do indicate that the thermal pressure in the early-type galaxies often is sufficiently high that in view of the expected low temperatures of their interstellar clouds, it can be speculated that the cold material that is present may be forming low-mass stars.

Jura, M.↗

NASA NTRS1986

The abundance of interstellar sulphur and zinc in high density sight-lines

On the basis of early absorption line studies of individual lines of sight with the Copernicus satellite, chlorine, sulphur and zinc were classed together as elements which showed little or no depletion, relative to hydrogen, in the interstellar medium. The abundances of other less volatile elements, such as Fe and Mg were found to vary widely from one sight-line to another with gas-phase abundances in some cases being orders of magnitude below their solar counterparts. Detailed studies are reported of the depletion/density behavior of two other volatile elements which were previously considered to be virtually undepleted, S and Zn, using equivalent width data from both Copernicus and IUE observations. The results provide further evidence that the established dependence of depletion on n bar (H) extends to volatile elements and show that their use as tracers of metallicity, or for estimating hydrogen column densities, may lead to large errors in sight-lines through dense regions. It now appears that such elements may take part in the surface chemistry of grains and be important constituents of grain mantle material, although they probably do not contribute significantly to the bulk mass of grains. Due to the very similar atomic masses and ionization potentials of sulphur and phosphorous, the thermal velocity distributions of the singly ionized species of these elements in interstellar clouds should be very similar. However, a comparison of Doppler widths (b-values) derived for SIT and PIT in the same sight-lines from the Bohlin et al Copernicus equivalent width measurements has revealed an unexpected systematic discrepancy of a factor of approx. 1.7. This Discrepancy indicates that the normally adopted oscillators strengths of the PII lambda lambda 1153 and 1302 A lines may require revision.

Harris, A. W.↗

NASA NTRS1987

Infrared properties of dust grains derived from IRAS observations

The analysis of several diffuse interstellar clouds observed by the Infrared Astronomy Satellite (IRAS) is presented. The 60/100 micron flux ratios appear to be nearly constant in clouds with up to 1 sup m visual extinction at the center. Observations of a highly regular cloud in Chamaeleon show that the 12/100 micron ratio peaks at an intermediate radial distance and declines towards the center of the cloud. These observations indicate that nonequilibrium emission accounts only for the 12 and 25 micron bands; strong emission observed at the 60 micron band is probably due to equilibrium thermal radiation. The correlation of the 12 micron emission with a red excess observed for a high latitude cloud, L1780, is shown to be consistent with the assumption that both features are due to fluorescence by the same molecular species.

Chlewicki, G.↗

NASA NTRS1986

Comment

Frank et al.'s (1986) suggestion that the earth is impacted by small (100-Mg) comets at the rate of 20/min is reconsidered in the light of recent comet models. The possibility of such a swarm of comets is found to be problematical because of the large number of comets required and because thermal stability arguments, while not excluding such a possibility, imply that it is implausible. In a reply, Frank et al. reaffirm their findings that such small comets, following direct elliptical orbits about the sun, are distributed in a disk centered on the sun and lying near the ecliptic plane at heliocentric radial distances 1-10,000 AU. The presence of these comets in the inner solar system is attributed to the gravitational action of the outer planets, passing stars, or interstellar clouds on comets is more distant orbits.

Mckay, C. P.↗

NASA NTRS1986

Comets

The nature, history, and evolution of comets are considered. Cometary ions, formed by photoionization and other processes, are forced into a highly structured ion tail by the interaction with the solar wind. The importance of comets to solar-system studies lies in the possibilities that they are well-preserved samples of either the interstellar cloud which collapsed to form the solar system or the planetesimals from which the outer planets accumulated, and that they provided either the prebiotic complex molecules from which life evolved or some volatiles necessary for the evolution of these molecules.

Neugebauer, M.↗

NASA NTRS1987

Unusual stable isotope ratios in amino acid and carboxylic acid extracts from the Murchison meteorite

The isotopic composition of hydrogen, nitrogen, and carbon in amino acid and monocarboxylic acid extracts from the Murchison meteorite has been determined. The unusually high D/H and N-15/N-14 ratios in the amino acid fraction are uniquely characteristic of known interstellar organic materials. The delta D value of the monocarboxylic acid fraction is lower but still consistent with an interstellar origin. These results confirm the extraterrestrial origin of both classes of compound and provide the first evidence suggesting a direct relationship between the massive organosynthesis occurring in interstellar clouds and the presence of prebiotic compounds in primitive planetary bodies.

Epstein, S.↗

NASA NTRS1987

The structure of the time-dependent interstellar shocks and grain destruction in the interstellar medium

A new theoretical analysis of the structure of interstellar shocks and the grain dynamics of these shocks is presented. The basic hydrodynamic equations for J-shocks in interstellar gas are given in which shock-driving pressure is allowed to be weakly time-dependent and the grains are treated as a separate two-dimensional fluid. Specific equations for the grain dynamics in the cooling postshock gas are derived. An analytic theory for the propagation of a shock driven into an interstellar cloud by the blast wave of a supernova remnant which sweeps over the cloud is developed. An improved calculation of the grain charge in postshock gas is described, giving a simple analytic approximation for the results. The consequences of including these processes in the numerical code of Seab and Shull (1983) are addressed, including the effect of realistic magnetic fields in low-density gas.

Mckee, Christopher F.↗

NASA NTRS1987

Oscillator strengths for visible and ultraviolet observations of interstellar molecules

Optical observations of molecules in diffuse interstellar clouds are discussed. The role of oscillator strengths in determining cloud properties such as densities, temperatures, and particle and photon fluxes is examined. The use of an emission technique, molecular and atomic lifetime measurements, and absorption measurements to estimate oscillator strengths is described. It is noted that the lifetime and absorption measurements are more advantageous than the emission technique to estimate oscillator strengths.

Smith, Peter L.↗

NASA NTRS1987

Evolutionary models of interstellar chemistry

The goal of evolutionary models of interstellar chemistry is to understand how interstellar clouds came to be the way they are, how they will change with time, and to place them in an evolutionary sequence with other celestial objects such as stars. An improved Mark II version of an earlier model of chemistry in dynamically evolving clouds is presented. The Mark II model suggests that the conventional elemental C/O ratio less than one can explain the observed abundances of CI and the nondetection of O2 in dense clouds. Coupled chemical-dynamical models seem to have the potential to generate many observable discriminators of the evolutionary tracks. This is exciting, because, in general, purely dynamical models do not yield enough verifiable discriminators of the predicted tracks.

Prasad, Sheo S.↗

NASA NTRS1987

Energetic N(+) ions in the interstellar medium

Energetic N(+) ions are produced in interstellar clouds by reactions of He(+) ions and N2 with sufficient energy to overcome the exoergicity of the reaction of N(+) with H2 and thereby from NH(+) ions. The reacting N(+) ions also slow down by elastic collisions with the ambient H2 molecules. The equilibrium energy distributions of the N(+) ions in a gas with temperature between 10 and 70 K are derived and the effective reaction rate coefficients are calculated. At 30 K, the rate coefficient is 1.6 x 10 to the -10th cu cm/sec.

Yee, J. H.↗