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At least 1,171 records · Page 65
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NASA NTRS1990

Theoretical models of gas dynamics and star formation in interacting ring galaxies

A series of one and two dimensional hydrodynamic simulations of a ring wave in interstellar gas disks was completed. These calculations included nonlinear source terms to model the effects of interstellar interactions and star formation, as well as the spatial-temporal gas flow. Toomre's kinematical model was merged with the Arnold, Shandarin, and Zeldovich 'pancake' theory of caustics in galaxy formation. The resulting theory can describe almost all the structure in restricted three-body simulations of single-pass collisions, even with multi-component potentials. Off-center galactic collisions were studied to understand the dynamics involved. Multi-color optical and near-infrared observations of faint tidal features were performed in about two dozen interacting galaxies selected from the Arp atlas. This sample provided evidence for ongoing star formation in tidal structures, and even enhancements of star formation in some cases. The task of assembling the data for gas-rich, late-type galaxies, was undertaken to see if a more coherent picture of the gas distribution would emerge from the more complete data. Analytic solutions of the equations with subsonic flows to balance gas consumption for expulsion form a galactic fountain were also derived.

Struck-Marcell, Curtis↗
NASA NTRS1990

Ca II and Na I absorption in the QSO S4 0248 + 430 due to an intervening galaxy

Observations of the QSO S4 0248 + 430 and a nearby anonymous galaxy are presented. Two absorption components are found in both Ca II H and K and Na I D1 and D2 at z(a) = 0.0515, 0.0523. Column densities of log N(Ca II) = 13.29, 13.50, and log N(Na I) = 13.79, 14.18 are found for z(a) = 0.0515, 0.0523 absorption systems, respectively. The column density ratios imply considerable calcium depletion and disk-type absorbing gas. At least one and possibly both absorption components are produced by high-velocity gas. A broadband image of the field shows an asymmetrical armlike feature or possible tidal tail covering and extending past the position of the QSO. The presence of this extended feature and the apparent difference between the absorption velocities and galaxy rotation velocity suggest that the absorbing gas is not ordinary disk gas, but rather is a result of tidal disruption.

Womble, Donna S.↗
NASA NTRS1990

Changes in the orbital periods of close binary stars

A number of close binary stars show erratic changes in their orbital periods on time scales of order 5-10 yr. Recently it has been proposed that the period changes are the result of changes in the quadrupole moment of one star, caused in turn by an alteration of the internal structure of that star. Magnetic pressure, which either distorts the shape of the star or changes its tidally induced quadrupole moment, is suggested as the driving force behind the alteration. Here, the amount of energy required to distort one component of a binary and match the observed period changes is estimated. The rate at which energy is produced or lost is governed by the thermal time scale of the star, and the estimates indicate that the observed period changes would take at least 1000 yr for the tidal quadrupole mechanism, and of order 60 yr to match a period change in V471 Tau which took only 4 yr.

Marsh, T. R.↗
NASA NTRS1991

Evolution of luminous IRAS sources - CCD imaging

The paper presents optical imaging of a sample of 64 luminous IRAS galaxies which cover a wide range of IRAS spectra and luminosity and also include a range of optical types. The objects are almost all in current or recent tidal interaction. The connections between the strength and age of the interaction, the IR spectrum and luminosity, and the optical colors, and other properties of the galaxies are discussed. The principal result is that the powerful IR sources with steep IR spectra are stronger and dynamically younger interacting systems, while the flat IR spectrum objects are older. Star formation, dust obscuration, and the timescales for nuclear activity compared with IR and tidal events are discussed, and a self-consistent evolution scenario connecting the luminous IR sources is described.

Hutchings, J. B.↗
NASA NTRS1991

Dynamical simulations of the interacting galaxies in the NGC 520/UGC 957 system

Numerical simulations of the interacting galaxies in the NGC 520/UGC 957 system are presented. Two sets of models were produced to investigate the postulated three-galaxy system of two colliding disk galaxies within NGC 520 and the dwarf galaxy UGC 957. The first set of models simulated a dwarf perturbing one-disk galaxy, which tested the possibility that NGC 520 contains only one galaxy disturbed by the passage of UGC 957. The resulting morphology of the perturbed single disk in the simulation fails to reproduce the observed tidal tails and northwest mass condensation of NGC 520. A second set of models simulated two colliding disks, which tested the hypothesis that NGC 520 itself contains two galaxies in a strong collision and UGC 957 is unimportant to the interaction. These disk-disk models produced a good match to the morphology of the present NGC 520. It is concluded that (1) NGC 520 contains two colliding disk galaxies which have produced the brighter southern half of the long tidal tail and (2) UGC 957, which may originally have been a satellite of one of the disk galaxies, formed the diffuse northern tail as it orbited NGC 520.

Stanford, S. A.↗
NASA NTRS1991

Forced nutations of the earth: Influence of inner core dynamics. I - Theory. II - Numerical results and comparisons. III - Very long interferometry data analysis

A treatment is presented of the nutation problem for an oceanless, elastic, spheroidally stratified earth, with the dynamical role of the inner core explicitly included in the formulation. Solving the enlarged system of equations shows that a new almost diurnal eigenfrequency emerges. A rough estimate places it not far from the prograde annual tidal excitation frequency, so that possible resonance effects on nutation amplitudes need careful consideration. Tables are provided that exhibit the sensitivities of various relevant quantities, the eigenfrequencies and the coefficients which appear in the resonance expansion, as well as the nutation amplitudes at important tidal frequencies, to possible errors in the earth parameters which enter the theory set forth. Finally, the analysis of 798 VLBI experiments performed between July 1980 and February 1989 and the determination from this analysis of corrections to selected coefficients in the International Astronomical Union 1980 theory of the nutations of the earth are discussed.

Mathews, P. M.↗
NASA NTRS1990

Microwave and optical lunar transponders

The scientific areas which used data from the Lunar Laser Ranging Experiment, collected from measurements to the Apollo 11, 14, and 15 and Lunakhod 2, include lunar science (i.e., studies of variations in the lunar angular orientation from that for uniform rotation, lunar tidal displacements, and the lunar mass distribution), geodynamics, astrometry, and gravitational physics. This paper argues that the placement of microwave and optical transponders on the moon would improve the accuracy of laser range measurements by nearly two orders of magnitude and would simplify the measurements. The K-band microwave transponders would be operated at the lunar base and at two remote sites on the moon surface, yielding much improved lunar libration and tidal displacement measurements. A two-wavelength laser transponder also would be operated at the lunar base, allowing accurate tropospheric propagation corrections to be made. This would introduce major improvements in measurements of the lunar orbit and of the earth's rotation, and in tests of general relativity.

Bender, P. L.↗
NASA NTRS1991

The environmental distribution of late proterozoic organisms

Along present day coast lines, the environmental distributions of prokaryotic and protistan populations are often sharply delimited. Realized habitat ranges are generally narrower than those circumscribed by physiological tolerances, suggesting the importance of organism-organism interactions in the determination of population distributions. Microfossil populations preserved in silicified carbonates, dolomites, and shales of the 700-800 Ma old Akademikerbreen Group, Svalbard, and elsewhere indicate that the environmental distributions were defined equally clearly during the Proterozoic Eon. The Draken Conglomerate Formation is a tidal flat/lagoonal complex in which we have distinguished five principle biofacies containing a total of 42 taxa. Supratidal to subtidal gradients include the increasing abundance and diversity of both mat dweller microbenthos and allochthonous (principally planktonic) organisms, as well as a taphonomically important pattern of decreasing sheath thickness among mat builder microorganisms. The seaward barriers of Akademikerbreen lagoons were oolitic shoals, and these contain about a dozen endolithic and epilithic species not observed elsewhere in the group. Subtidal environments below fair weather wave base are represented by mudstones of the Svanbergfjellet Formation. These contain abundant and diverse cyanobacteria-like fossils generally similar to but specifically different from those found in tidal flat sediments, as well as diverse unicellular protists (some of impressive morphological complexity) and at least half a dozen cellularly preserved metaphyte populations. In all, more than 80 species are distributed among Akademikerbreen lithologies. Fossil assemblages from Svalbard and elsewhere illustrate the potential for a much finer paleoecological, biostratigraphic, and, hence, evolutionary resolution of the early fossil record.

Knoll, A. H.↗
NASA NTRS1991

Libration in the earth's rotation

External luni-solar torque exerted on the difference (B-A) of the earth's two equatorial principal moments of inertia gives rise to two types of librational motions in the earth's rotation: the semidiurnal libration in spin and the prograde diurnal libration in polar motion. Formulas for the librations considering a realistic earth model and their tidal decompositions are derived and evaluated. The spin libration has a maximum peal-to-peak amplitude of 0.90 milliarcseconds, that of the polar libration is 0.06 milliarcseconds. Implications concerning their detectability and role in the tidal variation of earth rotation are discussed.

Chao, B. F.↗
NASA NTRS1992

Ventilation-perfusion relationships in the lung during head-out water immersion

Mechanisms of altered pulmonary gas exchange during water immersion were studied in 12 normal males: 6 young (aged 20-29) and 6 older (aged 40-45). It is concluded that, in young subjects with closing volume (CV) less than expiratory reserve volume (ERV), gas exchange was enhanced during immersion, because normal ventilation-perfusion relations were preserved, and by mass balance, the ventilation/O2 uptake changes elevated arterial P(O2). In older males with CV greater than ERV and 52 percent of tidal volume below CV, immersion-induced airways closure during tidal breathing was associated with minimally increased shunt that did not significantly impair gas exchange. It is suggested that airways closure of this degree is of little importance to gas exchange.

Derion, Toniann↗
NASA NTRS1992

Sub-daily resolution of earth rotation variations with Global Positioning System measurements

Data from a worldwide Global Positioning System (GPS) tracking experiment have been used to determine variations in earth rotation (UT1-UTC) over a time period of three weeks. Kalman filtering and smoothing enabled changes in UT1-UTC over intervals of 2 to 24 hrs to be detected with the GPS data. Internal consistency checks and comparisons with other solutions from very long baseline interferometry (VLBI) and satellite laser ranging (SLR) indicate that the GPS UT1-UTC estimates are accurate to about 2 cm. Comparison of GPS-estimated variations in UT1-UTC with 2-hr time resolution over 4 days with predicted variations computed from diurnal and semidiurnal oceanic tidal contributions strongly suggests that the observed periodic sub-daily variations of about 0.1 msec are largely of tidal origin.

Lichten, Stephen M.↗
NASA NTRS1991

North-Australian tropical seas circulation study

This investigation is intended to fully address the stated objective of the TOPEX/POSEIDON mission (National Aeronautics and Space Administration, 1986). Hence, we intend to use TOPEX/POSEIDON altimetry data to study the large-scale circulation of the Coral Sea Basin and the Arafura Sea and the mass exchange between these and adjoining basins. We will obtain data from two such cruises in 1993 and 1994 and combine them with TOPEX/POSEIDON radar altimetry data to identify interannual and seasonal changes in: (1) the location of the major ocean currents and the South Equatorial Current bifurcation in the Coral Sea; (2) the source region of the South Tropical Counter Current (STCC); and (3) the water exchange between the Coral Sea and the adjoining seas. We will also estimate seasonal and interannual variations in the horizontal transport of mass and heat associated with near-surface geostrophic and wind-driven currents. In addition, the tidal components of the Coral Sea will be studied to provide a correction for altimetry subtidal sea level changes and to develop a regional numerical model for tidal forcing in the Great Barrier Reef (GBR) and Papua New Guinea Reef regions.

Burrage, Derek↗
NASA NTRS1992

Studies of regional and global tectonics and the rotation of the earth using very-long-baseline interferometry

Work is continuing on the study of atmospheric gradients. We include a preprint entitled 'The effect of turbulence on atmospheric gradient parameters determined from ground-based radiometric and space geodetic measurements'. Work has begun on a study of solid Earth tidal deformations using the VLBI data set. We have examined deformations at the semi-diurnal tidal period using the IRIS data set.

Shapiro, Irwin I.↗
NASA NTRS1993

Environmental influences on galaxy evolution

We investigate the role of mergers and interactions in the evolution of galaxies by studying galaxies in compact groups. Compact groups of galaxies have high spatial densities and low velocity dispersions making these regions ideal laboratories in which to study the effect of interactions and mergers. Based on a detailed spectroscopic and multi-color imaging study, we find that both the isophotal shapes and the stellar kinematics indicate that many of the elliptical galaxies in compact groups have been affected by tidal interactions. At the same time, however, we find that only a few elliptical galaxies in compact groups have evidence for the young stellar populations that would be expected if they are the result of recent merger of two spiral galaxies. Therefore, we conclude that tidal interactions affect galaxy properties at the current epoch, but the bulk of basic galaxy formation and transformation must have occurred at much higher redshift.

Zepf, Stephen E.↗
NASA NTRS1992

A massive early atmosphere on Triton

The idea of an early greenhouse atmosphere for Triton is presented and the conditions under which it may have been sustained are quantified. The volatile content of primordial Triton is modeled, and tidal heating rates are assessed to set bounds on the available energy. The atmospheric model formalism is presented, and it is shown how a massive atmosphere could have been raised by modest tidal heating fluxes. The implications of the model atmospheres for the atmospheric escape rates, the chemical evolution, and the cratering record are addressed.

Lunine, Jonathan I.↗
NASA NTRS1993

Energetic ion anisotropies in the geomagnetic tail. I - A statistical survey. II - Magnetic field and substorm characteristics

A comprehensive data set from the ISEE 2 spacecraft is used here to perform a statistical study of events in the earth's central magnetotail that are characterized by high anisotropies of energetic ions. In about 75 percent of the cases the anisotropy vector deviates no more than 45 deg from the tidal axis. High-anisotropy samples within 45 deg of the tidal axis are dominated by the earthward fraction. High ion anisotropies are observed continuously for longer than 1 min only in a few cases. The probability of observing high ion anisotropies is significantly enhanced beyond about 16 R(E) downtail distance within a few earth radii of the neutral sheet and on the duskside of the magnetotail. The analysis is extended with respect to the local magnetic field and to the relationship between energetic ion anisotropies and substorm phases. It is found that the events can be well organized in terms of substorm expansion phase and substorm recovery. These relations and the magnetic field characteristics during the events support the notion that the near-earth source of tailward streaming ions is identical with a substorm neutral line.

Kettmann, Georg↗
NASA NTRS1993

High-latitude oceanic variability associated with the 18.6-year nodal tide

Water column temperature variations which indicate that the upper ocean is responding to 18.6-yr tidal forcing are presented. An enhanced high-latitude response to this forcing is predicted by equilibrium tide theory, and it should be most evident at latitudes poleward of about 50 deg. Critical information on regional climate variations might be found in the subsurface waters of the North Pacific and possibly other high-latitude oceans. The 18.6-yr nodal tidal cycle must be considered in circulation models of the ocean and atmosphere that examine time scales of decades or longer. Because the temperature range associated with the cycle is 0.5-1 C, short-term climate and biological populations could be affected.

Royer, Thomas C.↗
NASA NTRS1993

Discovery of orbital decay in SMC X-1

Three observations of the binary X-ray pulsar SMC X-1 with the Ginga satellite, conducted on 3 days in 1987 May, 8.4 days in 1988 August-September, and 4 days in 1989 July-August, are reported. Based on the three observation epochs, the rate of change in the orbital period is estimated at (-3.36 +/- 0.02) x 10 exp -6/yr. An interpretation of the orbital decay is made in the context of tidal evolution with allowance for the effect of the increasing moment of inertia of the companion star due to its nuclear evolution. While the orbital decay is thought to be driven by tidal interactions, it is suggested that the asynchronism between the orbit and the rotation of the companion star is most likely maintained by the evolutionary expansion of the companion star (Sk 160) rather than via the Darwin instability.

Levine, A.↗