Search NASA⌕ Search

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.

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

At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Evolution of oxygen-rich and carbon stars on the asymptotic giant branch

The transition from oxygen-rich (M) stars to S stars and then to C stars is examined using data on the chemical properties of the stars. The photospheric and circumstellar spectral characteristics of M and C stars are summarized. Consideration is given to the color distributions of carbon stars, visual carbon stars as transition objects, and radio observations of visual carbon stars. The chemical characteristics of S stars, the evolution of oxygen-rich stars on the AGB, and the transition between AGB stars and planetary nebulae are discussed. IRAS data are used to construct an evolutionary scenario for AGB stars, in which some mass-losing M stars remain oxygen rich, while others become carbon rich.

Kwok, Sun↗
NASA NTRS1989

Chromospheres of chemically peculiar giant stars

The chromospheres of evolved stars with peculiar chemical abundances are reviewed, emphasizing the dependence of chromospheric properties on the evolutionary status of the stars. A sample of intermediate mass stars observed in the radio, IR and UV wavelength regions is compiled. The chromospheres of MS, S, and C stars are compared with one another. The sample is used to study the relationship between stellar parameters and chromospheric heating. The results are used to construct a scenario for AGB evolution.

Judge, P. G.↗
NASA NTRS1990

Mass loss from red giant stars. II - Carbon stars

A millimeter-wave survey has been made of bright relatively unobscured, carbon stars, chosen on the basis of their optical properties. Out of 26 program objects, (J = 2-1)CO emission is detected from 15. Most of these had not been previously detected. There are many differences among the observed objects, but one rather interesting trend emerges: a positive correlation, at moderate IR excesses, between the IR dust emission and the expansion velocity of the dense wind. A similar, positive correlation with the mass-loss rate implies that stars with larger mass fluxes also accelerate them to larger velocities. At high-IR excesses, both correlations break down, and the momentum rate may be limited by the momentum rate of the stellar radiation. All these effects could be ascribed to differences in the gas-to-dust ratio, assuming that radiation pressure initiates and accelerates the wind.

Wannier, P. G.↗
NASA NTRS1990

Unusual infrared line profiles in the post-asymptotic giant branch star HD 56126

Brackett series lines with very broad inverted P Cygni and shell-like profiles have been detected toward the post-AGB star HD 56126. These profiles suggest that an active phase of mass loss has begun recently in this star. The star has had at least one previous episode of intense mass loss, attested to by its strong far-infrared emission. HD 56126 is also known for having a broad emission feature at 21 microns. The unidentified emission bands at 3.3 and 3.4 microns are also present in the spectrum of HD 56126, but the near-infrared spectrum provides little help in establishing the carrier of the 21 microns band.

Kwok, S.↗
NASA NTRS1990

IUE and Einstein survey of late-type giant and supergiant stars and the dividing line

Results are presented on an IUE UV survey of 255 late-type G, K, and M stars, complementing the Maggio et al. (1990) Einstein X-ray survey of 380 late-type stars. The large data sample of X-ray and UV detections make it possible to examine the activity relationship between the X-ray and the UV emissions. The results confirm previous finding of a trend involving a steeply-dropping upper envelope of the transition region line fluxes, f(line)/f(V), as the dividing line is approached. This suggests that a sharp decrease in maximum activity accompanies the advancing spectral type, with the dividing line corresponding to this steep gradient region. The results confirm the rotation-activity connection for stars in this region of the H-R diagram.

Haisch, Bernhard M.↗
NASA NTRS1990

H I in the giant elliptical galaxy NGC 4406

The Arecibo 305-m radio telescope was used to make 21-cm measurements of NGC 4406, an elliptical galaxy containing 10 to the 10th solar mass of hot diffuse gas. The H I decreases by a factor of 3 one beamwidth from the center and by an order of magnitude two beam widths from the center. Excess H I emission is seen in the X-ray tail, but not in dust-rich regions. The H I measurements do not support the suggestion that the X-ray tail is hot gas being stripped from the galaxy.

Bregman, Joel N.↗
NASA NTRS1990

p-mode absorption in the giant active region of 10 March, 1989

The interaction of solar p-mode oscillations with solar active regions has been studied in the case of the velocity oscillations observed in the vicinity of NOAA region 5359 on March 10, 1989. A maximum of nearly 70 percent of the power of incident high-degree modes is noted to be absorbed by this exceptionally large sunspot group. The occurrence of a major flare within this region during the observing sequence furnished a unique opportunity for testing the hypothesis that flares can excite photospheric acoustic waves. A search for acoustic 'pulses' emanating from the flare leads no detection of pulses, to a level of about 20 percent of the background power.

Braun, D. C.↗
NASA NTRS1991

Chromospherically active stars. IX - HD 33798 = V390 Auirigae - A lithium-rich rapidly rotating single giant

Results are presented of spectroscopic observations of HD 33798 obtained to determine if this star is a short-period binary and to examine its evolutionary status. Analysis of 40 radial velocities indicates no periodic velocity variations, suggesting that the star is single, so its rapid rotation is highly unusual. This rotation is inconsistent with the rotational brake hypothesis advanced by Gray (1989). It is proposed that HD 33798 is in a post-main-sequence phase of evolution. Its space motion is similar to FK Com, suggesting that it is a coalesced binary in the process of spinning down. Scenarios are presented to explain its large lithium abundance. A version in which material is transferred from a rapidly rotating core is suggested as the most likely.

Fekel, Francis C.↗
NASA NTRS1991

Millimeter continuum observations of Galactic center giant molecular cloud cores

Results are presented of observations of 1.3- and 0.8-mm continuum emission toward the cores of three Galactic center molecular clouds with ongoing massive star formation, Sagittarius B2, C, and D, which were made in order to study possible variations in the high-mass star formation rate per unit mass between the Galactic center and the disk. The luminosity-to-mass ratio, based on the mass estimates derived from the millimeter continuum emission, is used as a tracer of the high-mass star formation rate in GMC cores. The magnitude of errors involved in using millimeter continuum emission for determining the core mass is estimated through radiative transfer modeling. It is inferred from the present millimeter data, along with previously published far-infrared data, that the Sgr C and D cores are very similar in terms of mean dust optical depth and temperature. The luminosity-to-mass ratios derived for the Sgr C and D cores are found to be consistent with those of typical disk GMC cores with comparable far-infrared luminosities.

Lis, D. C.↗
NASA NTRS1991

Abundances in red giant stars - Nitrogen isotopes in carbon-rich molecular envelopes

Results are presented of millimeter- and submillimeter-wave observations of HCN and HCCCN that were made of the circmustellar envelopes of eight carbon stars, including the two protoplanetary nebulae CRL 618 and CRL 2688. The observations yield a measure of the double ratio (N-14)(C-13)/(N-15)(C-12). Measured C-12/C-13 ratios are used to estimate the N-14/N-15 abundance ratio, with the resulting lower limits in all eight envelopes and possible direct determinations in two envelopes. The two determinations and four of the remaining six lower limits are found to be in excess of the terrestrial value of N-14/N-15 = 272, indicating an evolution of the nitrogen isotope ratio, which is consistent with stellar CNO processing. Observations of thermal SiO (v = 0, J = 2-1) emission show that the Si-29/Si-28 ratio can be determined in carbon stars, and further observations are indicated.

Wannier, P. G.↗
NASA NTRS1992

Giant and large impacts in the context of planetary formation theory

Of particular relevance to the subject of this workshop is the expected size and velocity distribution of the interplanetary projectiles during the late stages of planet formation. This will in turn be determined by the evolution of these distributions throughout the planet forming process. Even at this early stage of understanding, it seems clear that assumption of simple power-law mass distributions are certain to lead to incorrect conclusions. Furthermore, the lunar cratering evidence, valuable as it is for relatively late (i.e., less than 4.2 b.y.) events, cannot provide information regarding stochastic events of potentially moon-destroying energy expected to be experienced by the early Earth. The early stages of terrestrial planet formation are discussed in more detail.

Wetherill, G. W.↗
NASA NTRS1992

Measurement capabilities of giant lidars for middle and upper atmospheric applications

The development and refinement of sophisticated remote sensing techniques during the past three decades have contributed enormously to our knowledge of the atmosphere. Lidar technologies have developed rapidly since the invention of the laser in 1961. Today, sophisticated systems are used to probe composition and structure throughout the atmosphere from the troposphere into the lower thermosphere and are making important contributions to several global change studies. While the recent advances in lidar technology have been impressive, the accuracy, resolution, and sensitivity of many systems are still limited by signal levels. We review the scientific rationale for developing a major new lidar facility to study the chemistry and dynamics of the Earth's atmosphere. The centerpiece of the facility is to be a 10-meter telescope which serves as the receiving system for several very large lidar systems. We discuss the observational capabilities of the proposed facility with particular emphasis on measurements of temperature, winds, water vapor, and ozone.

Gardner, Chester S.↗
NASA NTRS1991

Post-asymptotic giant branch nonradial instability strips

Stability analyses are performed for nonradial g(+)-mode pulsations of postasymptotic AGB stellar models to determine the location of their pulsational instability strips in the Hertzsprung-Russell diagram. Stellar models are analyzed that are assumed to have undergone a major mass loss event either near the tip of the AGB or shortly thereafter and, therefore, consist of a 50-percent carbon 50-percent oxygen core. Their mass is 0.6 solar mass, which is in good agreement with the peak of the observed distribution of white dwarf masses. The results are compared both to the observed planetary nebula nuclei variables and the hot pulsating DO variables. An analysis is also presented of the stability of DB white dwarfs, which have much deeper and stronger convection zones, and an instability strip is found between about 18,000 and 26,000 K approximately as observed for the known variable stars.

Stanghellini, Letizia↗
NASA NTRS1992

Ambiguities in the identification of giant molecular cloud complexes from longitude-velocity diagrams

Techniques which use longitude-velocity diagrams to identify molecular cloud complexes in the disk of the Galaxy are investigated by means of model Galactic disks generated from N-body cloud-particle simulations. A procedure similar to the method used to reduce the low-level emission in Galactic l-v diagrams is employed to isolate complexes of emission in the model l-v diagram (LVCs) from the 'background'clouds. The LVCs produced in this manner yield a size-line-width relationship with a slope of 0.58 and a mass spectrum with a slope of 1.55, consistent with Galactic observations. It is demonstrated that associations identified as LVCs are often chance superpositions of clouds spread out along the line of sight in the disk of the model system. This indicates that the l-v diagram cannot be used to unambiguously determine the location of molecular cloud complexes in the model Galactic disk. The modeling results also indicate that the existence of a size-line-width relationship is not a reliable indicator of the physical nature of cloud complexes, in particular, whether the complexes are gravitationally bound objects.

Adler, David S.↗
NASA NTRS1992

The molecular interstellar medium in two giant H II regions in M33 - NGC 604 and NGC 595

This study reports single-dish and interferometric CO observations for the two largest H II regions in M33, NGC 604, and NGC 595. Six individual molecular clouds are identified in the H II regions. Molecular and virial masses for three resolved clouds in NGC 604 agree very well, from which it is inferred that the CO-to-H2 conversion factor in this region is the same as it is in the inner disk of M33 and in the Galaxy, despite a metallicity in NGC 604 that is three times below solar. The total gas mass of the molecular complex associated with NGC 604 estimated from single-dish data is dissimilar to those of molecular complexes in the inner disk of M33, while the molecular gas mass of NGC 505 is roughly an order or magnitude smaller. Both regions contain a large mass fraction of atomic hydrogen, which suggests that photodissociation of molecular hydrogen is an important process in these energetic H II regions.

Wilson, Christine D.↗