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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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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

The varieties of symmetric stellar rings and radial caustics in galaxy disks

Numerical, restricted three-body and analytic calculations are used to study the formation and propagation of cylindrically symmetric stellar ring waves in galaxy disks. It is shown that such waves can evolve in a variety of ways, depending on the amplitude of the perturbation and the potential of the target galaxy. Rings can thicken as they propagate outward, remain at a nearly constant width, or be pinched off at large radii. Multiple, closely spaced rings can result from a low-amplitude collision, while an outer ring can appear well-separated from overlapping inner rings or an apparent lens structure in halo-dominated potentials. All the single-encounter rings consist of paired fold caustics. The simple, impulsive, kinematic oscillation equations appear to provide a remarkably accurate model of the numerical simulations. Simple analytic approximations to these equations permit very good estimates of oscillation periods and amplitudes, the evolution of ring widths, and ring birth and propagation characteristics.

Struck-Marcell, Curtis↗
NASA NTRS1978

Gamma ray astronomy and the origin of the light nuclei

Nuclear reactions induced by the collisions of the protons and alphas of the galactic cosmic ray with heavy nuclei of the interstellar gas are responsible for the continuous production of the light elements lithium, beryllium, and boron in the galaxy. To better than one order of magnitude, the observed ratios of these abundances to hydrogen abundance and the nuclidic abundance ratios between themselves are accounted for by simply considering the effect of fast protons and alphas with a flux and an energy spectrum as observed in galactic cosmic rays, for a period comparable with the life of our galaxy. The role of gamma ray astronomy in solving problems that occur when accurate agreement is sought with increasingly accurate data is discussed.

Reeves, H.↗
NASA NTRS1987

The role of orbital dynamics and cloud-cloud collisions in the formation of giant molecular clouds in global spiral structures

The role of orbit crowding and cloud-cloud collisions in the formation of GMCs and their organization in global spiral structure is investigated. Both N-body simulations of the cloud system and a detailed analysis of individual particle orbits are used to develop a conceptual understanding of how individual clouds participate in the collective density response. Detailed comparisons are made between a representative cloud-particle simulation in which the cloud particles collide inelastically with one another and give birth to and subsequently interact with young star associations and stripped down simulations in which the cloud particles are allowed to follow ballistic orbits in the absence of cloud-cloud collisions or any star formation processes. Orbit crowding is then related to the behavior of individual particle trajectories in the galactic potential field. The conceptual picture of how GMCs are formed in the clumpy ISMs of spiral galaxies is formulated, and the results are compared in detail with those published by other authors.

Roberts, William W., Jr.↗
NASA NTRS1990

Two-dimensional caustics in disturbed galaxy disks

Interacting galaxies show a variety of exotic morphologies besides the well-known rings, spirals, and bars. It is proposed that many of these can be identified with nonlinear, caustic structures that are known from singularity theory to be generic in collisionless dynamics. A simple kinematical model for stellar orbits is used to study the development of caustic waveforms in galaxy disks following impulsive, nonaxially symmetric (collisional) disturbances. It appears that all of the caustics known from singularity theory to be generic in two dimensions develop as transient forms in at least some moderately off-center collisions. The more complex, higher-order caustics appear most prominently in the more off-center and larger amplitude disturbances. It is further suggested that the highly nonlinear caustic waves may play an important role in interaction-induced star formation.

Struck-Marcell, Curtis↗
DOE OSTI2021

Probing the hadronic nature of the gamma-ray emission associated with Westerlund 2

ABSTRACT Star-forming regions have been proposed as potential Galactic cosmic ray accelerators for decades. Cosmic ray acceleration can be probed through observations of gamma-rays produced in inelastic proton–proton collisions at GeV and TeV energies. In this paper, we analyse more than 11 yr of Fermi–LAT data from the direction of Westerlund 2, one of the most massive and best-studied star-forming regions in our Galaxy. In particular, we investigate the characteristics of the bright pulsar PSR J1023–5746 that dominates the gamma-ray emission below a few GeV at the position of Westerlund 2 and the underlying extended source FGES J1023.3–5747. The analysis results in a clear identification of FGES J1023.3–5747 as the GeV counterpart of the TeV source HESS J1023-575, through its morphological and spectral properties. This identification provides new clues about the origin of the HESS J1023-575 gamma-ray emission, favouring a hadronic origin of the emission, powered by Westerlund 2, rather than a leptonic origin related to either the pulsar wind nebula associated with PSR J1023–5746 or the cluster itself. This result indirectly supports the hypothesis that star-forming regions can contribute to the cosmic ray sea observed in our Galaxy.

Mestre, Enrique↗
NASA NTRS1990

Extended emission and star formation in I Zw 18

The results of sky limited emission-line (H-alpha and forbidden O III 5007A) and line-free (6450 A and 5100 A) continuum imagery of the super-metal-poor blue compact galaxy I Zw 18 are reported. The main body of star formation in the galaxy is surrounded by a 3 kpc diameter extended envelope in H-alpha. In addition, there exist arcs of emission emanating from both central H II regions and winding around to the west, as well as a broader ridge of emission extending northward from the main body. The total mass of ionized material is estimated to be 3 x 10 to the 6th solar masses, or about 5 percent of the mass of the observed H I clouds. Several extended stellar continuum regions exist to the northwest of the main body, the brightest three forming a line along PA = 310 deg out to a 3 kpc distance from the central region of the galaxy. The correlation of the morphology and location of stellar continuum and emission regions with the distribution of H I is discussed. It is calculated that I Zw 18 has had previous, but disconnected, episodes of star formation, possibly from chance collisions of widely separated H I clouds.

Dufour, Reginald J.↗
NASA NTRS1986

Molecular hydrogen in the young starburst in NGC 253

Shocked molecular hydrogen has been observed around the nucleus of the nearby galaxy, NGC 253. This galaxy has a relatively modest luminosity (approx. 3 x 10 to the 10th power solar luminosities) and appears to have no distortions or companions that would indicate a possible interaction. The energy of the galaxy appears to be derived primarily from a starburst. Thus, our observations have caused us to examine the starburst process in some detail to identify how the molecular hydrogen is excited. It is proposed that the molecular hydrogen emission is produced by collisions of dense molecular clouds accelerated by supernovae explosions. Within the nucleus, this process occurs early in the life of the starbust. This suggest a sequence of nuclear starburst development; examples along this sequence from young to old would include NGC 253, M82, NGC 1097, and M31.

Rieke, G. H.↗
NASA NTRS1990

Deep Fabry-Perot imaging of NGC 6240: Kinematic evidence for merging galaxies

The authors have observed the superluminous, infrared galaxy NGC 6240 (z = 0.025) at H alpha with the Hawaii Imaging Fabry-Perot Interferometer (HIFI - Bland and Tully 1989). During the past decade, observational evidence from all wavebands indicates that the unusual appearance of NGC 6240 has resulted from a collision between two gas-rich systems, a view which is supported by our spectrophotometric data. However, the origin of the enormous infrared luminosity (4 times 10(exp 11) solar luminosity) detected by the Infrared Astronomy Satellite (IRAS) remains highly controversial, where opinions differ on the relative roles of large-scale shocks, massive star formation or a buried 'active' nucleus. These mechanisms are discussed in the light of the author's Fabry-Perot observations.

Hawthorn, J. Bland↗
NASA NTRS2004

Witnessing the Gradual Slowdown of Powerful Extragalactic Jets: The X-Ray-Optical-Radio Connection

A puzzling feature of the Chandra-detected quasar jets is that their X-ray emission decreases faster along the jet than their radio emission, resulting from an outward-increasing radio-to-X-ray ratio. In some sources this behavior is so extreme that the radio emission peak is located clearly downstream of that of the X-rays. This is a rather unanticipated behavior given that the inverse Compton nature of the X-rays and the synchrotron radio emission are attributed to roughly the same electrons of the jet's nonthermal electron distribution. In this letter we show that this morphological behavior can result from the gradual deceleration of a relativistic flow and that the offsets in peak emission at different wavelengths carry the imprint of this deceleration. This notion is consistent with another recent finding, namely, that the jets feeding the terminal hot spots of powerful radio galaxies and quasars are still relativistic with Lorentz factors GAMMA approximately 2-3. The picture of the kinematics of powerful jets emerging from these considerations is that they remain relativistic as they gradually decelerate from kiloparsec scales to the hot spots, where, in a final collision with the intergalactic medium, they slow down rapidly to the subrelativistic velocities of the hot spot advance speed.

Georganopoulos, Markos↗
NASA NTRS1974

Effect of inelastic electron-atom collisions on the Balmer decrement

Calculation of the Balmer decrement in radiatively ionized hydrogen gas as a function of temperature and density, taking into account the effect of electron-atom collisions. It is found that once the electron density exceeds 10 to the 10th power per cu cm significant deviations from the normal radiative recombination decrement begin to occur. Implications of these results for the physical conditions in the line-emitting region of the Seyfert galaxy NGC 4151 are discussed briefly.

Adams, W. M.↗
NASA NTRS1990

NGC 1614 - An IR-luminous merger but not (yet?) an active galaxy

New observations of the merging galaxy NGC 1614 are described. The system has a nuclear region of QSO-like luminosity, but shows no direct evidence for an active nucleus. It is heavily and unevenly reddened across its nucleus, while infrared imaging also shows a 'ridge' of dust. The inner spiral structure of the galaxy has normal rotation for an inclined disk, as indicated by the H-alpha emission. A linear 'tail' to the S and extended arms to the E have more positive velocities, and probably are the remains of an interacting companion and the tidal plume(s) caused by the collision. The only H I seen in emission appears to coincide with bright knots of H-alpha and forbidden O III emission of the base of the tail. The lack of direct evidence for an active nucleus indicates that if NGC 1614 is a precursor to a Seyfert-like system the AGN has not yet turned on.

Neff, S. G.↗
NASA NTRS1972

Disk and ring structure in the universe.

For a gas or dust cloud with a net angular momentum about a central mass, the stable shape is a rotating ring or disk, for a very simple reason. Collisions among the cloud's particles dissipate the energy of the random motions, while the rotational energy is maintained by the conservation of angular momentum. The angular momentum can be very easily imparted to matter. A ring or disk structure is, therefore, found in many celestial objects from asteroids to galaxies. The most impressive and obvious disk structures in the universe are the spiral galaxies. It is thought that the planetary system was formed from a flat disk of dust and gas. Much observational evidence shows that the thin, flat, concentric rings around the planet Saturn are composed of a host of individual particles.

Huang, S.-S.↗
NASA NTRS1978

Ne II 12.8 micron emission and galactic dynamics in M82

Ne II fine-structure emission at 12.8 microns from the galaxy M82 has been spatially and spectrally resolved. This radiation is relatively unaffected by obscuration, scattering, or maser amplification, and should hence provide a more reliable map of the velocity field within the galaxy than do optical or radio observations. A recessional velocity of 215 + or - 20 km/s is found for the galaxy, and a much steeper rotation curve is obtained than that from optical measurements. Furthermore, there is evidence for large-scale noncircular motion of the gas within the inner 150 pc of the nebula. The results are consistent with the idea that the peculiarities of M82 are due to collision with an intergalactic dust cloud as suggested by Solinger, Morrison, and Markert (1977).

Beck, S. C.↗
NASA NTRS1981

The abundance of argon at the galactic center

Measurements of forbidden line Ar II 6.99 micron and Pf alpha 7.45 micron made from the Kuiper Airborne Observatory are presented for Sgr A, the H II region at the center of the Galaxy. These line strengths, when combined with ground-based measurements, suggest a factor of enhancement of two in the Ar/H ratio in the galactic center region relative to that in the sun and in the solar neighborhood. The accuracy of the determination is presently limited by the uncertainty in the collision strength for Ar(+).

Lester, D. F.↗
NASA NTRS1983

The dynamics and fueling of active nuclei

It is generally believed that quasars and active galactic nuclei produce their prodigious luminosities in connection with the release of gravitational energy associated with accretion and infall of matter onto a compact central object. In the present analysis, it is assumed that the central object is a massive black hole. The fact that a black hole provides the deepest possible central potential well does imply that it is the most natural candidate for the central engine. It is also assumed that the quasar is associated with the nucleus of a conventional galaxy. A number of difficulties arise in connection with finding a suitable stellar fueling model. A simple scheme is discussed for resolving these difficulties. Attention is given to fueling in a nonaxisymmetric potential, the effects of a massive accretion disk, and the variability in the disk luminosity caused by star-disk collisions assuming that the energy deposited in the disk is radiated.

Norman, C.↗
NASA NTRS1987

Submillimeter and Far-Infrared Spectroscopy of M17 and S106: UV-Heated, Quiescent Molecular Gas?

Measurements of CO line emission toward the interface between molecular cloud and HII region in M17, and the center of the bipolar nebula S106 are discussed. The warm quiescent molecular gas is in the interface between the exciting OB stars and the surrounging molecular cloud. Comparison of the submillimeter, far-IR, and millimeter CO line intensities and profiles suggests a model in which clumps of relatively cool gas (T = 50 K) have warm surfaces or are embedded in a warm surrounding medium. Heating of the warm, quiescent molecular gas by collisions with dust grains can be excluded. Slow shocks or heating by photoelectrons are possible. The most promising mechanism is photoelectric heating. The existence of a substantial amount of warm, quiescent molecular gas in UV illuminated regions may be of importance locally in most OB star formation regions and globally in external galaxies with large rates of star formation.

Harris, A. I.↗
NASA NTRS1982

The nature of the central parsec of the Galaxy

Observations of infrared fine-structure line emission from compact clouds of ionized gas in the galactic center have been reported by Lacy et al (1979, 1980). These observations suggest the existence of a central black hole of nearly 3,000,000 solar masses and require mechanisms to generate, ionize, and dispose of the gas clouds. It is found that the best model to fulfill these requirements involves cloud generation through disruption of red giants by stellar collisions, ionization by a population of stars which is affected either by enhanced metal abundances or the death of the most massive stars, and gas disposal by star formation. Although the existence of a massive black hole cannot be ruled out, it would play no necessary role in this model and may cause the tidal disruption of stars at a rate such that their accretion into the black hole would produce more radiation than is observed.

Lacy, J. H.↗
NASA NTRS1993

A new observational and numerical study of tidal interactions in M81-M82-NGC3077 system

A nearby system of interacting galaxies M81-M82-NGC3077 triplet (D = 3.3 Mpc; Freeman & Madore 1988) has been studied using multi-wavelength observations and numerical simulations to obtain a comprehensive understanding on the dynamics and the consequences of tidal interactions in a group environment. The VLA 12-field Mosaic H I observations of 2 x 1.5 deg. region have revealed a vast array of H I filaments which suggests that the severity and extent of tidal disruptions far exceed the previous estimates. A tidal remnant of the former H I disk of M82 extending up to 30 kpc (in projection) is identified for the first time, and the pervasive effects of the tidal disruption are traced into the inner disk by optical and CO observations, including a kinematic trace of a large scale bar potential (Yun, Ho, & Lo 1992). The H I disk of M81 is traced out to 40 kpc in radius, and a large scale (l approx. 20 kpc) velocity anomaly ('High Velocity Trough'), which may be a remnant of a gaseous collision, is found within the disk of M81. The large H I bridge between M81 and NGC 3077 (van der Hulst 1979) is also found to extend approx. greater than 50 kpc further, bending around NGC 3077, toward M82. The total H I detected in this experiment, 5.6 x 10(exp 9) solar mass, represents the majority of the single-dish flux (Appleton, Davies, & Stephenson 1981) and suggests that the bulk of H I found in the region belongs to the three galaxies and the tidal filaments. The impact and details of the tidal interactions have been further examined through the use of numerical techniques. The 'restricted 3-body' approach was used to simulate the observed distribution of tidal H I streamers connecting the three galaxies, and the success of the simulation is further strengthened by the accurate predictions on the gas kinematics.

Yun, M. S.↗