SEARCH · Search NASA

Results for “giant”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18

All sources NASA NTRS DOE OSTI

NASA NTRS1987

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

The different roles played by orbital dynamics and dissipative cloud-cloud collisions in the formation of giant molecular clouds (GMCs) in a global spiral structure are investigated. The interstellar medium (ISM) is simulated by a system of particles, representing clouds, which orbit in a spiral-perturbed, galactic gravitational field. The overall magnitude and width of the global cloud density distribution in spiral arms is very similar in the collisional and collisionless simulations. The results suggest that the assumed number density and size distribution of clouds and the details of individual cloud-cloud collisions have relatively little effect on these features. Dissipative cloud-cloud collisions play an important steadying role for the cloud system's global spiral structure. Dissipative cloud-cloud collisions also damp the relative velocity dispersion of clouds in massive associations and thereby aid in the effective assembling of GMC-like complexes.

Roberts, W. W., Jr.↗

NASA NTRS1986

Far-ultraviolet fluorescence of carbon monoxide in the red giant Arcturus. II - Analysis of high-dispersion IUE spectra

Faint, diffuse emissions near 1380 A in deeply exposed IUE spectrograms of the red giant Arcturus very likely are associated with bands of the A-X fourth-positive system of carbon monoxide, fluoresced by multiplet UV2 of neutral oxygen near 1305 A. Numerical simulations indicate that the strength of the CO bands is exceedingly sensitive, in the best available one-dimensional model of the chromosphere of Arcturus, to a delicate balance between the rapid inward attenuation of the oxygen radiation field and the rapid outward decline of the molecular absorptivity. The fortuitous character of the overlap region in the single-component model argues that one should also consider the possibility that the pumping occurs in a highly inhomogeneous chromosphere, of the type proposed in previous studies of Arcturus based on observations of the infrared absorption bands of CO.

Ayres, T. R.↗

NASA NTRS1986

Calculations of the accretion and evolution of giant planets The effects of solid cores

The present calculation of giant planet evolution proceeds under the hypothesis that the solid cores formed by small particle accretion later attracted their gaseous envelopes from the solar nebula gravitationally. Evolutionary calculations are presented for the beginning of gaseous envelope formation around the core mass; the growth of core and envelope to a critical core mass; the derivation of high luminosity from the envelope mass due to gravitational contraction, and the termination of both core and envelope accretion on a time-scale of 5 Gyr.

Bodenheimer, P.↗

NASA NTRS1986

RV Tauri stars as post-asymptotic giant branch objects

RV Tau stars are rare, luminous pulsators of spectral types F, G, and K. Analysis of the IRAS data shows that the mass-loss rates from RV Tau stars have apparently significantly decreased during the past 500 yr. It seems likely that these stars have just evolved from the phase of rapid mass loss, characteristic of the last stages of the asymptotic giant branch (AGB). The birthrate of RV Tau stars in the solar neighborhood is about a tenth of the birthrate of all planetary nebulae, and this is consistent with the view that we are witnessing the subset of stars undergoing post-AGB evolution that are low mass and at least in some cases of low metallicity.

Jura, M.↗

NASA NTRS1986

Physical conditions near red giant and supergiant stars - An interpretation of SiO VLBI maps

Understanding the dynamical structure of circumstellar envelopes around cool giant and supergiant stars depends critically on the knowledge of what happens in the 'near zone' of the envelope, within a few stellar radii of the star. One probe with adequate angular resolution to study the near zone is VLBI observation of the SiO masers. It is shown that VLBI maps of VX Sgr establish that the particle density in the SiO masers is very high (about 10 to the 12th/cu cm), indicating that the masers form in dense cloudlets and not in a spherically expanding wind. The implications of these results for the mechanism of mass loss are discussed.

Alcock, Charles↗

NASA NTRS1987

Ground-based observations of IRAS candidates for late asymptotic giant branch stars

Ground-based IR photometric observations were obtained for 16 of the IRAS Circular No. 9 sources which have been suggested to be candidates of OH/IR stars. Five of these sources are confirmed to be OH/IR stars and three others as carbon stars. Two are young objects in an early stage of star formation and one, possibly two, are planetary nebulae. Two sources of very low color temperatures (about 180 K) are also found, and may represent objects in transition between the asymptotic giant branch and planetary nebula phases.

Kwok, Sun↗

NASA NTRS1987

Hydromagnetic constraints on deep zonal flow in the giant planets

A simple model of the equatorial zonal jet in the giant planets is studied in which the flow is assumed uniform on cylinders concentric with the spin axis, and viscous and magnetic torques on each cylinder are balanced. This 'Taylor constraint' is solved simultaneously with the dynamo equation to obtain the velocity and magnetic field in the equatorial plane. The model is used to reproduce the widely differing jet widths of Jupiter and Saturn eddy viscosity of 2500 sq cm/s. For Saturn, the model has a large magnetic Reynolds number where the Chandrasekhar number Q = 1, and hence exhibits substantial axisymmetrization of the field in the equatorial plane.

Kirk, R. L.↗

NASA NTRS1987

SiO cooling instability in the envelopes of cool giant stars

In the outer atmospheres of oxygen-rich giant and supergiant stars, the formation of the silicon monoxide molecule not only represents a first step toward the production of dust but may also produce an efficient radiative cooling agent. Molecular cooling occurs as a thermal instability in many cases with an onset at 2000 to 3500 K, rapidly reducing the temperature by as much as 1000 K. This process may occur in the upper photosphere-lower chromosphere of such stars and can have profound dynamical consequences. The process occurs far from LTE, on a time scale controlled by the reaction rates for SiO formation. SiO masers may be indicative of this process. The molecule CS which may play a similar role in carbon stars and serve as the basis for carbonaceous dust production.

Muchmore, David O.↗

NASA NTRS1987

The red/infrared evolution in galaxies - Effect of the stars on the asymptotic giant branch

The effect of including the asymptotic giant branch (AGB) population in a spectral synthesis model of galaxy evolution is examined. Stars on the AGB are luminous enough and also evolve rapidly enough to affect the evolution of red and infrared colors in galaxies. The validity of using infrared colors as distance indicators to galaxies is then investigated in detail. It is found that for z of 1 or less infrared colors of model galaxies behave linearly with redshift.

Chokshi, Arati↗

NASA NTRS1987

An IUE survey of activity in red giants and supergiants

Chromospheric and transition region line activity is examined in apparently single red giants and supergiants using the IUE archives. Low-resolution, large-aperture spectra (mostly short-wavelength) were used to search for variations of emission-line fluxes in time. A series of automatic processing procedures were implemented in order to uniformly calibrate a large number of spectra, fit continua to each of them, determine the fluxes of as many as 18 emission lines, and compare them at different epochs. A method is offered to compute the overall error in the integrated flux, a critical measure of activity, independent of the observing and processing details. This processing was applied to above 120 images of 26 stars taken over a period of 7 yr (1978-1984). Four stars showed UV emission-line flux variations. Alpha Aqr, Beta Peg, and Sigma Oph showed a single enhanced-emission event in all detectable emission lines. Gamma Aql exhibited an increase in the flux level of the O I (1641 A) line in mid-1981 with no comparable change in any other lines. These four stars lie in a region of the H-R diagram in which time-dependent circumstellar absorption lines appear.

Oznovich, I.↗

NASA NTRS1987

1300 micron continuum and C(O-18) line mapping of the giant molecular cloud cores in Orion, W49, and W51

Observations of the 1300-micron continuum emission and the C(O-18) spectral-line emission from three well-studied giant molecular cloud cores: Orion, W49, and W51 are presented. The observations were obtained at the Five College Radio Astronomy Observatory, and they provide a means to examine the consistency of these two methods to trace the column density structure of molecular clouds. A good general correlation is found between the 1300-micron continuum, which traces the column density of dust, and the C(O-18) J = 2-1 line emission, which traces the column density of molecular gas, when the effects of source temperature are taken into consideration. Moreover, nominal values for the gas and dust abundances and the dust properties reproduce the observed continuum-to-line ratios. Thus, no strong C(O-18) abundance gradients within sources has been found, and it appears that either the C(O-18) emission lines or the submillimeter dust emission may used to derive the mass column density within molecular clouds accurately.

Schloerb, F. Peter↗

NASA NTRS1988

Terrestrial versus giant planet formation

Given a solar nebular surrounding the early protosun, containing dust grains that have already undergone growth through collisions to about centimeter-size, the question of the formation of the terrestrial and giant planets is considered. In contrast to the usual approach of emphasizing how well a problem is understood, the uncertainties and areas where more work needs to be done will be accentuated. Also, the emphasis will be on the dynamics of planetary formation, because profound problems still exist in this area, and because it seems most logical to concentrate first on the dynamical questions involved with assembling the planets before putting too much effort into the detailed chemical and geological consequences of certain formation mechanisms.

Boss, Alan P.↗

NASA NTRS1987

Ground-based infrared observations of variable IRAS sources as candidates for late asymptotic giant branch stars

Analysis of the color distribution of OH/IR stars and IRAS low-resolution spectra class 30 objects suggests the presence of a well-defined evolutionary sequence which is populated by late asymptotic giant branch (LAGB) stars. The paper reports ground-based identification and infrared photometry of 10 candidates of news LAGB stars. None of the selected sources are found to have optical counterparts, and eight of the 10 show a strong 10-micron silicate absorption feature. It is suggested that these stars represent an invisible extension of extreme Mira variables and are some of the most evolved stars observed to date.

Kwok, Sun↗

NASA NTRS1988

Redshifts of high-temperature emission lines in the far-ultraviolet spectra of late-type stars. II - New, precise measurements of dwarfs and giants

Results are presented from an IUE SWP camera investigation of the occurrence of gasdynamic flows, analogous to the downdrafts of 10 to the 5th K material observed over magnetic active regions of the sun, among stars of late spectral type. The SWP calibration spectra study conducted documents the existence of local, small, persistent distortions of the echelle wavelength scales that are of unknown origin. Attention is given to the enormous widths of the stellar high-excitation emission lines in both the dwarfs and the giants, with respect to the comparatively small, subsonic Doppler shifts; the widths are typically an order of magnitude greater than the redshifts.

Ayres, Thomas R.↗

NASA NTRS1988

A white dwarf companion to the main-sequence star 4 Omicron(1) Orionis and the binary hypothesis for the origin of peculiar red giants

Ultraviolet spectra of the peculiar red giants (PRGs) called MS stars are investigated, and the discovery of a white dwarf (WD) companion to the MS star 4 Omicron(1) Orionis is reported. The observations and data analysis are discussed and compared with those for field WDs in order to derive parameters for the WD and the luminosity of the primary. Detection limits for the other MS stars investigated are derived, and the binary hypothesis for PRGs is reviewed.

Ake, Thomas B.↗

NASA NTRS1988

The velocity dispersion of the giant molecular clouds - A viscous origin

The energy source is proposed and the details of the acceleration mechanism for the random motion of the giant molecular clouds (GMCs) in the Galaxy are discussed. Specifically, it is proposed that gravitational scattering of the massive clouds off each other in the differentially rotating Galactic disk constitutes an effective gravitational viscosity, which causes an increase in the random kinetic energy of the GMCs at the expense of their ordered, rotational kinetic energy in the Galactic disk. The energy input due to this gravitational viscosity is calculated, and the other terms contributing to the energy balance of the GMCs are computed. These results are used to determine the steady state cloud velocity dispersion and its dependence on the cloud mass and radius and on the galactocentric radial distance.

Jog, Chanda J.↗

NASA NTRS1988

Is supernova 1987A a stripped asymptotic-branch giant in a binary system?

It is proposed that the progenitor of supernova 1987A was a previously undetected red star in orbit about a blue supergiant. The progenitor was the remnant of an asymptotic-branch giant that had lost most of its hydrogen-rich envelope to its blue companion by type C mass transfer. A detailed evolutionary model strongly supports the feasibility of this proposition. It is found that the original mass of the supernova precursor was 10-15 solar (unless a large fraction of the mass was ejected from the binary sytem), and its final mass, just before the supernova event, was 3-6 solar. The system remained bound, with a new orbital period of 3-10 yr and an eccentricity of 0.1-0.4. This picture can provide plausible qualitative explanations for several anomalies in the observational properties of this supernova.

Joss, P. C.↗

NASA NTRS1988

Cloud microphysics of the giant planets

The characteristic time constants for the atmospheres of the giant planets were compared for the purpose of finding analogs for clouds in the atmospheres of these planets among the several types of cloud systems found on earth. The NH3 clouds on Jupiter and Saturn were found to be weakly precipitating systems, similar to the thicker cirrus clouds on earth, while the H2O clouds were found to be much more massive than the water clouds on earth. The CH4 and H2O clouds on Uranus and Neptune are completely unlike any cloud on earth, being 10 to 100 times more massive than the densest water clouds on earth. It is concluded that, while the microphysical characteristics of the clouds on Jupiter and Saturn are comparable to those of terrestrial stratus and cumulus clouds, the massive clouds on Uranus and Neptune reach conditions unlike anything on earth.

Carlson, Barbara E.↗