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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
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NASA NTRS1984

Lunar origin: role of giant impacts

The explanation that lunar origin involved giant impacts remains attractive. Large planetesimals are consistent with current accretion models, and may have been widely scattered in the early solar system; their existence is a reasonable, assumption in Moon origin models. Isotopic data require the Moon's formation primarily from local material resembling Earth's upper mantle, not material from elsewhere in the solar system. Giant impacts are stochastic, class predictable events that would provide the required type of ejected Earth mantle material without requiring large moons to form near other planets (a problem with less stochastic processes). Such material may have mixed with incoming meteorites during lunar formation, affecting lunar chemistry.

Hartmann, W. K.↗
NASA NTRS1985

Lunar Origin: Role of Giant Impacts

The explanation that lunar origin involved giant impacts remains attractive. Large planetesimals are consistent with current accretion models, and may have been widely scattered in the early solar systems; their existence is a reasonably, assumption in Moon origin models. Isotopic data require the Moon's formation primarily from local material resembling Earth's upper mantle, not material from elsewhere in the solar system. Giant impacts are stochastic, class predictable events that would provide the required type of ejected Earth mantle material without requiring large moons to form near other planets (a problem with less stochastic processes). Such material may have mixed with incoming meteorites during lunar formation, affecting lunar chemistry.

Hartmann, W. K.↗
NASA NTRS1985

Giant mini-clusters as possible origin of halo phenomena observed in super-families

Among 91 mini-clusters from 30 high energy Chiron-type families in Chacaltaya emulsion chambers, there were observed several extremely large multiplicity clusters in the highest energy range, far beyond the average of ordinary type clusters. Some details of microscopic observation of those giant mini-clusters in nuclear emulsion plates and some phenomenological regularity found in common among them are described. Such giant mini-clusters are possible candidates for the origin of narrow symmetric single halo phenomena in X-ray films which are frequently observed in super-families of visible energy greater than 1,000 TeV.

Source record↗
NASA NTRS1984

Consequences of a chromospheric temperature gradient on the width of H-alpha in late-type giants

An analytic expression for the integrated H alpha optical depth profile is derived for a one dimensional slab geometry model chromosphere, with electron temperature increasing as a power law with height. The formula predicts H alpha opacity and profile width to be sensitive functions of the thermal gradient. Application of the model to observation reveals that broad H alpha absorption widths in G and K giant stars are consistent with a mean H alpha chromospheric optical depth of 50, while narrower widths in M stars indicate slightly lower opacities. It is proposed that differences in H alpha width between late-type giants of similar spectral type may be due, in part, to differences in their chromospheric thermal gradient, and associated H alpha opacity.

Zarro, D. M.↗
NASA NTRS1984

X-rays from accretion of red giant winds

X-ray observations of the late-type red giants Mira and R Aqr obtained with the Einstein Observatory are presented, and the general problems of white dwarf accretion from late-type giant winds is considered. The extremely low measured luminosities obtained for the two systems leads to the conclusion that the companions of Mira and R Aqr are most likely low-mass main sequence objects rather than white dwarfs as is usually assumed. The expected X-ray luminosities of true red giant/white dwarf systems are considered, and it is concluded that far too few have been detected if the canonical accretion scenario is adopted. A possible explanation of this situation in terms of grain-dominated Eddington-limited accretion is proposed.

Jura, M.↗
NASA NTRS1985

Bipolar nebulae and mass loss from red giant stars

Observations of several bipolar nebulae are used to learn something of the nature of mass loss from the probable red-giant progenitors of these nebulae. Phenomena discussed are: (1) probable GL 2688's optical molecular emissions; (2) newly discovered very high velocity knots along the axis of OH 0739 - 14, which reveal evidence for mass ejections of + or 300 km/s from the M9 III star embedded in this nebula; (3) the bipolar structure of three extreme carbon stars, and the evidence for periodic mass ejection in IRC + 30219, also at high speed (about 80 km/s); and (4) the curious cool TiO-rich region above Parsamian 13, which may represent the very recent shedding of photospheric material from a cool, oxygen-rich giant. Several general key questions about bipolar nebulae that relate to the process of mass loss from their progenitor stars are raised.

Cohen, M.↗
NASA NTRS1986

Rapidly rotating single late-type giants: New FK Comae stars?

A group of rapidly rotating single late-type giants was found from surveys of chromospherically active stars. These stars have V sin I's ranging from 6 to 46 km/sec, modest ultraviolet emission line fluxes, and strong H alpha absorption lines. Although certainly chromospherically active, their characteristics are much less extreme than those of FK Com and one or two other similar systems. One possible explanation for the newly identified systems is that they have evolved from stars similar to FK Com. The chromospheric activity and rotation of single giant stars like FK Com would be expected to decrease with time as they do in single dwarfs. Alternatively, this newly identified group may have evolved from single rapidly rotating A, or early F stars.

Fekel, Francis C.↗
NASA NTRS1987

Star formation in Carina OB1: Observations of a giant molecular cloud associated with the eta Carinae Nebula

A giant molecular cloud associated with the eta Carinae nebula was fully mapped in CO with the Columbia Millimeter-Wave Telescope at Cerro Tololo. The cloud comples has a mass of roughly 700,000 solar mass and extends about 140 pc along the Galactic plane, with the giant Carina HII region situated at one end of the complex. Clear evidence of interaction between the HII region and the molecular cloud is found in the relative motions of the ionized gas, the molecular gas, and the dust; simple energy and momentum considerations suggest that the HII region is responsible for the observed motion of a cloud fragment. The molecular cloud complex appears to be the parent material of the entire Car OB1 Association which, in addition to the young clusters in the Carine nebula, includes the generally older cluster NGC 3325, NGC 3293, and IC 2581. The overall star formation efficiency in the cloud complex is estimated to be approximately 0.02.

Grabelsky, D. A.↗
NASA NTRS1986

Microwave continuum measurements and estimates of mass loss rates for cool giants and supergiants

Attention is given to the results of a sensitive, 6-cm radio continuum survey conducted with the NRAO VLA of 39 of the nearest single cool giants and supergiants of G0-M5 spectral types; the survey was conducted in order to obtain accurate measurements of the mass loss rates of ionized gas for a representative sample of such stars, in order to furnish constraints for, and a better understanding of, the total mass loss rates. The inferred angular diameters for the cool giant sources are noted to be twice as large as photospheric angular diameters, implying that these stars are surrounded by extended chromospheres containing warm partially ionized gas.

Drake, S. A.↗
NASA NTRS1986

Planetesimal dissolution in the envelopes of the forming, giant planets

An evaluation is made of the capacity of planetesimals to penetrate the envelopes of giant planets during their growth phase, by means of a core instability mechanism in which the growing core becomes gradually more adept in the gravitational concentration of gas from its solar nebula environment, until a runaway gas accretion occurs. If most of the accreted mass is contained in planetesimals larger that about 1 km, the critical core mass for runaway accretion will not significantly change when planetesimal dissolution is taken into account; it is accordingly suggested that giant planet envelopes should contain above-solar proportions of virtually all elements, relative to hydrogen.

Pollack, J. B.↗
NASA NTRS1986

Mass loss from giant and supergiant stars

The 12 m telescope of the National Radio Astronomy Observatory has been used at the J = 2-1 transition of CO to increase the known list of giant and supergiant stars with observable circumstellar envelopes. The candidate objects were generally M-type giants and supergiants, chosen for their strong infrared luminosities. Of the 35 objects which were previously undetected, or only marginally detected, 10 were found to produce detectable CO emission. Physical parameters of the envelopes are derived by source modeling. Mass-loss rates vary from 10 to the -7th to 4 x 10 to the -5th solar mass/yr.

Wannier, P. G.↗
NASA NTRS1987

The fate of the earth in the red giant envelope of the sun

The effect on the earth of entering the red giant envelope of the future sun is studied. Employing a 30-zone red giant model, the earth orbital decay timescale, neglecting ablation/vaporization, is determined to be of the order of 200 years, rendering earth survival impossible. The effects of ablation/vaporization processes are found to increase the ballistic coefficient of earth, thereby setting the 200-year decay timescale as an upper limit.

Goldstein, J.↗
NASA NTRS1987

Abundances in red giant stars - Carbon and oxygen isotopes in carbon-rich molecular envelopes

Millimeter-wave observations have been made of isotopically substituted CO toward the envelopes of 11 carbon-rich stars. In every case, C-13O was detected and model calculations were used to estimate the C-12/C-13 abundance ratio. C-17O was detected toward three, and possibly four, envelopes, with sensitive upper limits for two others. The CO-18 variant was detected in two envelopes. New results include determinations of oxygen isotopic ratios in the two carbon-rich protoplanetary nebulae CRL 26688 and CRL 618. As with other classes of red giant stars, the carbon-rich giants seem to be significantly, though variably, enriched in O-17. These results, in combination with observations in interstellar molecular clouds, indicate that current knowledge of stellar production of the CNO nuclides is far from satisfactory.

Wannier, P. G.↗
NASA NTRS1987

A radio-continuum survey of the coolest M and C giants

A VLA continuum survey of 22 cool M and C type giants and supergiants has been conducted in order to investigate the physical properties of the partially ionized, expanding chromospheres of the coolest luminous star. The results indicate small upper limits for the ionized-mass-loss rates and 2-6 cm spectral indices that are much steeper than the 0.6 value predicted by the standard stellar-wind model. It is suggested that hydrogen is predominantly neutral and that carbon and other metals may not be completely ionized in many of the late M giants. For several of the stars the brightness-temperature upper limits imply that the winds and chromospheres are optically thin at 6 cm, setting useful constraints on the chromospheric emission for these stars.

Drake, Stephen A.↗
NASA NTRS1987

Europa, tidally heated oceans, and habitable zones around giant planets

Tidal dissipation in the satellites of a giant planet may provide sufficient heating to maintain an environment favorable to life on the satellite surface or just below a thin ice layer. Europa could have a liquid ocean which may occasionally receive sunlight through cracks in the overlying ice shell. In such a case, sufficient solar energy could reach liquid water that organisms similar to those found under Antarctic ice could grow. In other solar systems, larger satellites with more significant heat flow could represent environments that are stable over an order of eons and in which life could perhaps evolve. A zone around a giant planet is defined in which such satellites could exist as a tidally-heated habitable zone. This zone can be compared to the habitable zone which results from heating due to the radiation of a central star. In this solar system, this radiatively-heated habitable zone contains the earth.

Reynolds, Ray T.↗
NASA NTRS1988

The outer atmospheres of the coolest M giants - Ultraviolet spectra of Rho Persei, 2 Centauri, g Herculis, Theta Apodis, and R Lyrae

New IUE observations of five M giants cooler than most studied previously have been obtained in order to study the variation of chromospheric properties with effective temperature. The ultraviolet spectrum lines of one of these stars, Rho Persei, are identified. The properties of the spectra of of the five stars are examined in detail, including the line widths and turbulence, the radial velocities of groups of lines, the structure in the Mg II resonance lines, and the fluorescent lines. The physical properties of the M giants' chromospheres are considered, including the formation of Fe II, the electron density, the degree of ionization of iron, magnesium, silicon, and carbon, the hydrogen ionization and energy balance, and the creation and destruction of photons.

Eaton, Joel A.↗
NASA NTRS1988

4 Draconis - A unique triple system containing an M3 giant and a cataclysmic binary

The M3 III giant 4 Dra has a UV bright companion discovered with IUE as a probable cataclysmic variable (CV). The orbit of the M giant around the center of gravity of the system that it forms with the cataclysmic binary is determined. 4 Dra B shows spectrum variations like a CV; the photometric period is 3 h 58.5 m + or - 1 m is determined. 4 Dra B is the first CV for which an independent distance estimate can be given from the spectroscopic parallax of the companion. Properties of the CV, which is probably of the magnetic type (AM Her), are briefly discussed. The age of the CV may be less than 100 million yr.

Reimers, D.↗
NASA NTRS1988

Companions to peculiar red giants: HR 363 and HR 1105

Recent IUE observations of two Tc-deficient S-type peculiar red giants that are also spectroscopic binaries, HR 363 and HR 1105 are reported. A 675 min SWP exposure of HR 363 shows emission lines of O I 1304 and Si II 1812 and a trace of continuum. Compared to the M giants, the far UV flux may be relatively larger, indicating a possible contribution from a white dwarf companion, but no high temperature emission lines are seen to indicate that this is an interacting system where mass-transfer recently occurred. However, HR 1105 appears to have a highly variable UV companion. In 1982, no UV flux was discerned for this system, but by 1986 C IV was strong, increasing by a factor of 3 in 1987 with prominent lines of Si III, C III, O III, Si IV, and N V. Using orbital parameters, these observations are consistent with high activity occuring when the side of the S-star primary illuminated by the companion faces the Earth, but since the IUE data were taken over 3 orbits, a secular change in the UV component cannot be excluded.

Ake, Thomas B., III↗