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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1979

The excitation of density waves at the Lindblad and corotation resonances by an external potential

The linear response of a differentially rotating two-dimensional gas disk, both with and without self-gravity, to a rigidly rotating external potential is calculated on the assumptions that the speed of sound is much smaller that the orbital velocity and that the external potential varies on the scale of the disk radius. The results show that: (1) the external potential exerts torques on the disk only at the Lindblad and corotation resonances; (2) the torque is positive at the outer Lindblad resonance and negative at the inner Lindblad resonance; (3) the torque at corotation has the sign of the radial vorticity gradient; and (4) the torques are of the same order of magnitude at both types of resonance and independent of the speed of sound in the disk. It is found that the external potential also excites density waves in the vicinity of the Lindblad and corotation resonances, that the long trailing wave is excited at a Lindblad resonance, and that short trailing waves are excited at the corotation resonance. The behavior of particle disks is briefly discussed, and the external torques on particle disks are proven to be identical to those on gas disks

Goldreich, P.↗
NASA NTRS1986

Density waves in the solar nebula - Differential Lindblad torque

The differential torque exerted by Lindblad resonances on a perturbing object embedded in a two-dimensional nonself-gravitating disk with density, pressure and sound speed gradients is quantified. First-order corrections are made to account for Keplerian rotation and the presence of the gradients. The total torque is calculated by summing over all resonances in the absence of local wave damping. When applied to the primordial solar nebula disk, the calculations show that disks that cool with increasing heliocentric distance will cause decay of the orbit of the perturbing object. Conditions in which the perturber will escape orbit delay are also described. The characteristic drift time will be no greater than the stochastic accretion time scales. Implications of the calculations for planetary formation are discussed.

Ward, W. R.↗
NASA NTRS1987

On the confinement of planetary arcs

The necessary conditions for planetary arc confinement are investigated, concentrating on the limiting case where the arc is marginally confined. It is shown that periodic perturbations due to a discrete Lindblad resonance can induce a secular change of Jacobian energy as well as orbital eccentricity. If the particles are subject to the perturbation of only one inner (outer) Lindblad resonance, the Lindblad resonance's location must be exterior (interior) to that of the corotation resonance if the perturbations are to lead to confinement. The trajectories of particles that are subject to inelastic collisions and pertubations due to corotation and Lindblad resonances are computed, showing that the confinement mechanism can indeed operate provided the perturbation due to the Lindblad resonance is sufficiently strong to balance viscous effects. The possibility that internal nonradial oscillations of Neptune may provide the necessary perturbations for arc confinement is considered.

Lin, D. N. C.↗
NASA NTRS2011

The Updated IAU MDC Catalogue of Photographic Meteor Orbits

The database of photographic meteor orbits of the IAU Meteor Data Center at the Astronomical Institute SAS has gradually been updated. To the 2003 version of 4581 photographic orbits compiled from 17 different stations and obtained in the period 1936-1996, additional new 211 orbits compiled from 7 sources have been added. Thus, the updated version of the catalogue contains 4792 photographic orbits (equinox J2000.0) available either in two separate orbital and geophysical data files or a file with the merged data. All the updated files with relevant documentation are available at the web of the IAU Meteor Data Center. Keywords astronomical databases photographic meteor orbits 1 Introduction Meteoroid orbits are a basic tool for investigation of distribution and spatial structure of the meteoroid population in the close surroundings of the Earth s orbit. However, information about them is usually widely scattered in literature and often in publications with limited circulation. Therefore, the IAU Comm. 22 during the 1976 IAU General Assembly proposed to establish a meteor data center for collection of meteor orbits recorded by photographic and radio techniques. The decision was confirmed by the next IAU GA in 1982 and the data center was established (Lindblad, 1987). The purpose of the data center was to acquire, format, check and disseminate information on precise meteoroid orbits obtained by multi-station techniques and the database gradually extended as documented in previous reports on the activity of the Meteor Data Center by Lindblad (1987, 1995, 1999 and 2001) or Lindblad and Steel (1993). Up to present, the database consists of 4581 photographic meteor orbits (Lindblad et al., 2005), 63.330 radar determined orbit: Harvard Meteor Project (1961-1965, 1968-1969), Adelaide (1960-1961, 1968-1969), Kharkov (1975), Obninsk (1967-1968), Mogadish (1969-1970) and 1425 video-recordings (Lindblad, 1999) to which additional 817 video meteors orbits published by Koten el al. (2003) were

Porubcan, V.↗
NASA NTRS1981

The origin of the eccentricities of the rings of Uranus

The effect of gravitational perturbations from a nearby satellite on the eccentricity e of a narrow particulate ring is considered. The perturbations near a resonance in an eccentric ring may be divided into corotation and Lindblad terms. For small e, the corotation terms damp e, whereas the Lindblad terms excite e. In the absence of saturation the corotation terms win by a small margin, and e damps. However, if the perturbations open gaps at the strongest resonances, then the Lindblad terms win, and e grows. This result offers an explanation for the existence of both circular and eccentric rings around Uranus. It is also shown that eccentricity changes induced by circular rings on eccentric satellite orbits are similar to those induced by satellites with circular orbits on eccentric rings.

Goldreich, P.↗
NASA NTRS1991

Numerical exploration of planetary arc dynamics

After analytically characterizing the response of a test particle to the combined effects of corotation and Lindblad resonances, due to the satellites of Neptune, as two coupled dynamical systems whose coupling depends on the collective effects in the arc, a formula is obtained for the energy imparted by the Lindblad resonance. This energy confines the arc material around the corotation resonance radius. The fact that the gradient of the torque density around the arc must be negative confirms the constraints defined by Lin et al. (1987) on the relative positions of the corotation and the Lindblad resonances if the arc is to be stable. These results are tested with a direct numerical simulation for two different arc configurations.

Sicardy, Bruno↗
NASA NTRS1993

The IR properties of ringed galaxies and the IRAS database

Our study of the Infrared Astronomy Satellite (IRAS) properties of ringed galaxies has been largely successful. We have identified what we think is the probable cause of the differences in the IRAS properties among non-interacting barred galaxies as the pattern speed of the bar. The key to identifying this parameter has been our focusing the study on outer-ringed galaxies where we know precisely what is present in the central regions (from available BVI CCD images in our library of images). The theory is that outer rings, through their morphology and other characteristics, can be identified with the outer Lindblad resonance, one of the major resonances in galaxy structure. Using a library of n-body simulations for comparison, we can reliably infer both low and high pattern speed galaxies from the appearance of outer rings and the existence of other ring features. It is clear that in some barred galaxies, the bar pattern speed is high enough to avoid an inner Lindblad resonance, hence such objects do not contain nuclear or circumnuclear star formation. The IRAS observations are most sensitive to nuclear star formation in early-type barred galaxies, and will thus select those barred galaxies where the pattern speed is low enough to allow an inner Lindblad resonance to exist. High pattern speed barred galaxies therefore weaken the correlation between bars and infrared excess. This finding helps to reconcile the inconsistent results found between different studies on the correlation between bars and far-IR emission.

Buta, Ronald J.↗
NASA NTRS1977

Solar influence on meteor rates and atmospheric density variations at meteor heights

Lindblad (1967) has concluded that there was an inverse relation between meteor rates and the solar cycle brought about by an increase in atmospheric density gradient at the height of meteor ionization. The present paper investigates Lindblad's conclusion more fully by using three long series of continuous radar meteor data from New Zealand and Canada. The results confirm a clear variation of total rate from year to year, inversely correlated with the annual sunspot number. Although meteor rates call for a density gradient variation inversely related to the solar cycle, direct evidence for such a variation remains nonexistant. Possibly the effect is being obscured by other density changes occurring at these heights. Analysis of meteor rates within the same one-year period in the two hemispheres has established that seasonal rate changes brought about by the variation of the angle between the latitude of the observing station and the apex of the earth's way override change of density gradient in at least one of the hemispheres and possibly both in controlling meteor rates within the year.

Ellyett, C.↗
NASA NTRS1988

On disk-planet interactions and orbital eccentricities

While Lindblad resonances both within and without a perturber's orbit excite its eccentricity, the present study of the eccentricity evolution due to the density wave interaction between a planetesimal and a Keplerian disk notes that coronation resonances in these regions lose their eccentricity damping effectiveness if the object is embedded in a continuous disk without a gap. Attention is given to another class of Lindblad resonances which, under these conditions, operates on disk material coorbiting with the perturber; these resonances thereby become the most important source of eccentricity damping. A model problem indicates that eccentricity ultimately undergoes decay.

Ward, William R.↗
NASA NTRS1989

Resonance excitation of spiral density waves in a gaseous disk. I - A linear theory

The wave propagation in a gaseous disk which is self-gravitating and viscous is studied. Waves are generated by an external disturbance and are maintained in motion among forces due to rotation, self-gravity, and pressure. A linear theory is developed for the general case in which all factors are taken into account. The general solution can be applied to the photostellar disk of the solar nebula and the circumstellar disk of a binary in which the effects of self-gravity and pressure are equally important. The theory confirms in great detail that a long wave is excited at one of the Lindblad resonances gravitationally by the external periodic potential, propagates toward the corotation, is reflected before reaching there at the Q-barrier, and finally propagates in the reverse direction toward and past the Lindblad resonance as a short wave.

Yuan, Chi↗
NASA NTRS1992

Near-resonant excitation and propagation of eccentric density waves by external forcing

An overview is presented of the astronomical evidence that relatively massive, distended, gaseous disks form as a natural by-product of the process of star formation, and also the numerical evidence that SLING-amplified eccentric modes in the outer parts of such disks can drive one-armed spiral density waves in the inner parts by near-resonant excitation and propagation. An ordinary differential equation (ODE) of the second order that approximately governs the nonlocalized forcing of waves in a disk satisfying Lindblad resonance almost everywhere is derived. When transformed and appended with an extra model term, this ODE implies, for free waves, the usual asymptotic results of the WKBJ dispersion relationship and the propagation Goldreich-Tremaine (1978) formula for the resonant torque exerted on a localized Lindblad resonance. An analytical solution is given for the rate of energy and angular momentum transfer by nonlocalized near-resonant forcing in the case when the disk has power-law dependences on the radius of the surface density and temperature.

Ostriker, Eve C.↗
NASA NTRS1994

Kinematic signature of a rotating bar near a resonance

Recent work based on H I, star count and emission data suggests that the Milky Way has rotating bar-like features. In this paper, I show that such features cause distinctive stellar kinematic signatures near Outer Lindblad Resonance (OLR) and Inner Lindblad Resonance (ILR). The effect of these resonances may be observable far from the peak density of the pattern and relatively nearby the solar position. The details of the kinematic signatures depend on the evolutionary history of the 'bar' and therefore velocity data, both systematic and velocity dispersion, may be used to probe the evolutionary history as well as the present state of Galaxy. Kinematic models for a variety of sample scenarios are presented. Models with evolving pattern speeds show significantly stronger dispersion signatures than those with static pattern speeds, suggesting that useful observational constraints are possible. The models are applied to the proposed rotating spheroid and bar models; we find (1) none of these models chosen to represent the proposed large-scale rotating spheroid are consistent with the stellar kinematics and (2) a Galactic bar with semimajor axis of 3 kpc will cause a large increase in velocity dispersion in the vicinity of OLR (approximately 5 kpc) with little change in the net radial motion and such a signature is suggested by K-giant velocity data. Potential future observations and analyses are discussed.

Weinberg, Martin D.↗
NASA NTRS2003

Gap-Opening, Disk Clearing, and the Survival of the Regular Satellites of Jupiter and Saturn

It is only recently that the theory of disk-companion interactions yields migration rates due to the gas tidal torque that are in agreement with numerical simulations and up to an order of magnitude slower than previous estimates. Also, for a weakly turbulent disk, the gap size is controlled primarily by the damping length of acoustic waves launched by the secondary at Lindblad resonances, which in turn depends on whether the waves are 2D or 3D. At least for small azimuthal wavenumbers this damping length is of the order of the radial location of the Lindblad resonance. This may have important consequences for disk dispersal in satellite systems. In the case of Jupiter, it means that the inner Galilean satellites may have jointly opened a gap. On the other hand, in Saturn's system the satellites inside of Titan are probably too small to have opened gaps in the gas disk at the time of their formation; but the possibility exists that, by effectively clearing the gas disk inside its own orbit, Titan may have allowed smaller satellites to survive, depending on whether Titan can clear the disk in a timescale comparable to the migration rates due to gas drag and gas tidal torque for these objects.

Estrada, P. R.↗
NASA NTRS1973

Yet Another Stream Search Among 2401 Photographic Meteors

Two streams previously listed (one of them with a classification on Ceplecha's system in terms of beginning height) by Cook are shown probably not to exist, a possibility already pointed out by Cook. One stream that he questioned was revised as to membership and then classified. Four streams are added to the list and one of these is classified. Previous reports exist for three of these streams, while one is new. The two Piscid streams of Lindblad and his alpha Triangulid stream are regrouped into two streams, one already called the Andromedids by Cook and the other still called the Piscids; the alpha Triangulids are absorbed into the Andromedids. The Piscids are classified along with the iota Aquarids. The classifications of the Taurids and the Andromedids remain unchanged.

Cook, A. F., II↗
NASA NTRS1971

Theory of spiral structure.

The question whether the galactic spiral arms are material objects or wave patterns is discussed. A semiempirical approach is adopted in presenting the concept of density waves. The theory of density waves is considered, giving attention to a survey of theoretical developments by analytical methods, the implication of a spiral pattern of density waves, spirals with moderately small pitch angle, and the origin and permanence of galactic spirals. The theoretical aspects discussed are tested against more detailed observations in the Milky Way system. It is pointed out that the density wave concept introduced by Lindblad, including the material concentration of both gas and stars, is the essential basis for the spiral structure of disk-shaped galaxies.

Lin, C. C.↗
NASA NTRS1980

Molecular clouds and galactic spiral structure

Two large-scale 2.6 mm CO surveys of the galactic plane, one in the first quadrant (l = 12 to 60 deg, b = -1 to +1 deg), the other in the second (l = 105 to 139 deg, b = -3 to +3 deg), have provided evidence that, contrary to previous findings, molecular clouds constitute a highly specific tracer of spiral structure. Molecular counterparts of five of the classical 21-cm spiral arms have been identified: the Perseus arm, the local arm (including Lindblad's local expanding ring), the Sagittarius arm, the Scutum arm, and the 4-kpc arm. The region between the local arm and the Perseus arm is apparently devoid of molecular clouds, and the interarm regions of the inner Galaxy appear largely so. CO spiral structure implies that the mean lifetime of molecular clouds cannot be greater than 100 million years, the time required for interstellar matter to cross a spiral arm. Conservation of mass then sets a limit on the fraction of the interstellar medium in the form of molecular clouds: it cannot exceed one-half at any distance from the galactic center in the range 4-12 kpc.

Cohen, R. S.↗
NASA NTRS1980

Disk-satellite interactions

The rate at which angular momentum and energy are transferred between a disk and a satellite which orbit a central mass is calculated. It is shown that the angular momentum and energy transfer at Lindblad resonances tends to increase the satellite's orbit to lowest order in eccentricity, whereas the transfer at corotation resonances tends to decrease it. The results are applied to the interaction between Jupiter and the protoplanetary disk. The angular momentum transfer is shown to be so rapid that substantial changes in both the structure of the disk and the orbit of Jupiter must have taken place on a time scale of a few thousand years.

Goldreich, P.↗
NASA NTRS1984

On the '3 kiloparsec arm' - Resonance excitation of linear and nonlinear waves by an oval distortion in the central region

The mechanism responsible for the massive rapidly expanding Galactic feature known as the 3-kpc arm (Rougoor and Oort, 1960) is investigated theoretically in the framework of a density-wave model of the Galaxy. The arm is attributed to outgoing waves excited at the outer Lindblad resonance (OLR) of a small rotating oval distortion near the Galactic center. Linear and nonlinear models are developed, and it is shown that a density crest at 3.5 kpc with expansion velocity 53 km/s can be generated by a distortion turning at 118 km/s kpc with a 10-percent perturbation of the main gravitational field at the OLR (located at 3 kpc). The wave character of the nonlinear solution is found to be the same as that of the linear solution and to give good agreement with the observations in terms of expansion velocity, size, shape, and mass; only the observed asymmetry of the arm is not reproduced.

Yuan, C.↗