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At least 1,081 records · Page 60
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NASA NTRS1978

CNO abundances and hydrodynamic studies of the Nova outburst. V - 1.00-solar-mass models with small mass envelopes

The paper reports on an investigation into the consequences of thermonuclear runaways in accreted hydrogen envelopes of 100 millionths of a solar mass on 1-solar-mass white dwarfs. These evolutionary sequences predict that from 10 to 50 millionths of a solar mass will be ejected with speeds from 300 to 3800 km/s (kinetic energies of 10 to the 44th-45th power ergs). Absolute visual magnitudes as high as -8.1 are attained, well within the observed range for fast novae. In addition, the shapes of the theoretical light curves are more reminiscent of an observed fast-nova light curve than those in earlier studies. The ejected material is strongly enhanced in the products of incomplete CNO burning; the most abundant of the ejected nuclei is C-13, followed by N-14 and C-12. The differences from previous studies are attributable to the lower peak temperatures reached in these sequences. These models also produce a large overabundance of Li-7, suggesting that novae may represent significant contributors to the galactic enrichment of this nucleus.

Starrfield, S.↗
NASA NTRS1978

Abundances of isotopes in planetary atmospheres

Carbon and oxygen isotopes show no large anomalies on Venus (no more than 10-15%) or Mars (less than 5%); the high value of N-15/N-14 found on Mars is explained by nonthermal escape of nitrogen. The isotopes of nonradiogenic noble gases in the atmosphere of Mars exhibit abundance patterns similar to those in the primordial component of meteoritic gases and in the earth's atmosphere. This implies that gas fractionation took place in the inner solar nebula prior to planet formation. The relatively high value of Xe-129 on Mars emphasizes its deficiency on earth, implying a difference in accretion histories of volatiles for the two planets. In the outer solar system, normal isotope ratios for nitrogen and carbon on Jupiter, and for carbon on Saturn are found, but precision is low (+ or - 15% at best). Controversy exists about the correct value of D/H, with current estimates ranging from 2.3 plus or minus 1.1 to 5.1 plus or - 0.7 times 10 to the minus 5th. Planetary missions planned for the next few years should add considerably to the quantity and quality of these data.

Owen, T.↗
NASA NTRS1979

On the abundance of interstellar oxygen toward Zeta Ophiuchi

Zeippen, Seaton, and Morton (1977) recently published profiles of interstellar neutral oxygen lines in Zeta Oph. From a reconsideration of the profiles of visual interstellar lines to that star, it follows that the cloud dominating the O I absorption could have a very small velocity spread, approximately 0.5 km/s. Analysis of the profile of the 1302-A line gives a column density of O I in that cloud of 6.0 + or - 5 x 10 to the 17th per sq cm. This implies an upward revision of the abundance of free oxygen by a factor 1.5 and hence that oxygen is only 25% depleted in the gas to Zeta Oph, contrary to the minimum depletion of a factor 1.8 found by Zeippen, Seaton, and Morton. This small depletion of oxygen in interstellar gas is consistent with the widespread absence of absorption by H2O ice mantles on interstellar dust grains.

De Boer, K. S.↗
NASA NTRS1979

Interstellar abundances in the Zeta Ophiuchi clouds

The velocity component structure of UV interstellar absorption line profiles observed toward Zeta Ophiuchi clouds has been analyzed; column densities of Mg II, P II, Mn II and Fe II are found for the clouds at -14.4 km/sec (the principal component) and those at about -30 km/sec (a blend of two components). Little or no depletion is noted in the secondary component, suggesting that the grain mantles have been sputtered off, perhaps by a shock front, in this moderately high-velocity cloud. Residual depletion of calcium and titanium in the shocked cloud provides information on the nature of the grain surfaces. In the low-velocity cloud, the relative abundances of neutral atoms may indicate a differential depletion within the cloud.

Snow, T. P., Jr.↗
NASA NTRS1978

Measurement of the Li/B abundance ratio above 650 MeV/nucleon

A balloon-borne cosmic-ray telescope containing 4 Cerenkov counters and a neon flash tube hodoscope was flown from Sioux City, Iowa in September 1974. The data have been reanalyzed, using information of flashed tubes to reject events due to proton and He interactions. The new event selection has reduced charge overlap between He and Li to an insignificant level, so that abundance ratios involving Li can now be measured with the instrument. Precise correction factors for events lost by inefficiency of the hodoscope have been derived from flight data; they lead to improved accuracy in measurements of light elements.

Byrnak, B.↗
NASA NTRS1978

Abundances of He and CNO ions in the ion pulses observed in the earth's magnetospheric environment

The abundance of He and CNO ions in the ion pulses which are frequently observed in the earth's magnetosheath, magnetotail, and near-by interplanetary space was analyzed and found to be highly variable. Based on the assumption that these ions are solar wind particles accelerated in the magnetospheric environment to the observed energies, it is suggested that this variability reflects the inhomogeneity of the solar atmosphere and/or the physical conditions under which the solar wind particles were energized.

Fan, C. Y.↗
NASA NTRS1979

Solubility of noble gases in serpentine - Implications for meteoritic noble gas abundances

An investigation of the solubilities of the noble gases from synthesis and solubility studies of the sheet silicate mineral serpentine in carbonaceous chondrites is presented. Hydrothermal synthesis and exchange experiments were made at 340C and 1 kbar with noble gas partial pressures from 2 times 10 to the -8th power to 0.1 atm. The measured distribution coefficients for noble gases are not sufficiently high to account for the trapped noble gases in carbonaceous chondrites by exchange in solar nebula if meteoritic minerals have comparable distribution coefficients. Also, serpentine gains and loses noble gases to approach equilibrium values with the terrestrial atmosphere, indicating that this exposure may have influenced the noble gas abundances in phyllosilicate minerals of these chondrites. The dispersion of K-Ar ages of carbonaceous chondrites could be the result of phyllosilicates approaching equilibrium solubility of atmospheric Ar-40.

Zaikowski, A.↗
NASA NTRS1980

A cosmic-ray age based on the abundance of Be-10

The paper discusses a measurement of the isotopic composition of galactic cosmic-ray beryllium made with high resolution aboard the ISEE 3 spacecraft. The isotope fractions of Be-7, Be-9, and Be-10 were determined; the abundance of the radioactive isotope Be-10 is used to deduce the cosmic-ray confinement time of 8,400,000/yr and an interstellar gas density of 0.33 atoms/cu cm in the confinement volume. Errors in the propagation model parameters which result in comparable uncertainties are discussed.

Wiedenbeck, M. E.↗
NASA NTRS1981

Time and energy dependence of heavy ion abundances in solar flare energetic particle events

Examples are presented of the time and energy dependence of the abundances and spectra of the major heavy ions He, C, O and Fe during solar flare events, taken from a survey using the UMD/MPI ULET telescope on IMP-8 during 1973-1977. In some cases, time variations were found in the O/He, O/C and Fe/O ratios which appear to be inconsistent with models based solely on rigidity dependent propagation in the interplanetary medium.

Mason, G. M.↗
NASA NTRS1981

The chromospheric and transition layer emission of stars with different metal abundances

Preliminary results on observations of chromospheric and transition layer emission of stars with different metal abundances are reported. Metal deficient stars generally show reduced emission in the Mg II resonance lines and also in the other chromospheric and transition layer emission lines. This is interpreted as showing that energy fluxes other than acoustic fluxes must at least be coresponsible for the coronal and transition layer heating.

Boehm-Vitense, E.↗
NASA NTRS1981

Interstellar abundance determination using IUE data

Analysis of the silicon interstellar abundances was made for more heavily reddened lines of sight than were accessible to the Copernicus satellite. Silicon rarely had accurate column densities determined from Copernicus data because the available lines all lie on the flat portion of the curve of growth. With International Ultraviolet Explorer (IUE) it is possible to reach color excesses of E(B-V) approximately 0.5-0.7, and in addition obtain data on the weak SiII line at 1808 A, so that a wide range of oscillator strengths is available. The lower resolving power of the IUE causes difficulties in that several of the SiII lines are blended with strong lines of other species. Data on the lines of sight analyzed suggested that some of the absorption lines fall on the damped portion of the curve of growth, implying that silicon may not be as highly depleted as expected.

Joseph, C. L.↗
NASA NTRS1981

Elemental abundances in high-excitation planetary nebulae

The IUE satellite was used to obtain low dispersion spectra of the high excitation planetary nebulae IC 351, IC 2003, NGC 2022, IC 2165, NGC 2440, Hu 1-2, and IC 5217. Numerical modeling was undertaken to determine the chemical composition of these objects with particular emphasis on obtaining elemental carbon and nitrogen abundances. Large variations in the C/N ratio from object to object are suggested.

Marionni, P. A.↗
NASA NTRS1980

On the abundance of carbon monoxide in galaxies - A comparison of spiral and Magellanic irregular galaxies

CO emission has been sought in 52 directions toward six irregular, Magellanic-type galaxies, with null results at a typical detection limit of 0.04 K for a line width of 10 km/s. The positions searched correspond to those of bright OB star clusters, H II regions, H I peaks, and dark clouds. In two cases, one-third of the optical galaxy was covered by the observations. CO has been detected at 2.6 mm toward the nucleus of the low-mass SA(s)dm galaxy NGC 7793. The results imply that the mean surface brightness of CO emission in Magellanic irregulars is less than that of the Galaxy and other spirals which have been studied. Two categories of explanations for this are discussed, including (1) those based on a lower heating rate for interstellar clouds in irregulars, and thus a lower CO temperature, and (2) those based on a lower CO abundance.

Elmegreen, B. G.↗
NASA NTRS1980

Elemental abundances in chondrules from unequilibrated chondrites Evidence for chondrule origin by melting of pre-existing materials

Bulk abundances of Na, Mg, Al, Ca, Sc, V, Cr, Ir, and Au were analyzed by neutron activation of chondrules separated from unequilibrated H, L, and LL chondrites and correlated with petrographic properties. The geometric mean composition of the whole-rock chondrule suites are almost indistinguishable from each other for many elements; chondrules are enriched in lithophile and depleted in siderophile elements in a pattern consistent with chondrule formation by melting of preexistent materials. The porphyritic chondrules are more enriched in refractory and siderophile elements as compared with the nonporphyrytic chondrules, indicating that these two chondrule groups may have formed from different precursors.

Gooding, J. L.↗
NASA NTRS1980

Abundance correlations among comets

Results of narrowband filter photometry are presented for comets 1977g, 1978b, 1978f, 1978j, 1979c, and 1979i. There appears to be a remarkable homogeneity among comets with regard to the relative production rates for those species which are readily measured, specifically C2, CN, C3, and OH. Most comets appear to show identical relative abundances, except for a rather well-defined variation with heliocentric distance. Molecular production rates appear to be unrelated to either the emission-to-continuum (gas-to-dust) ratio or the dynamical age of the comet. The correlation with visual magnitudes is excellent, though the slope is not what could be intuitively expected.

Ahearn, M. F.↗
NASA NTRS1981

H2 abundance in the atmosphere of Venus

The in situ measurements from the Pioneer Venus Orbiter (PVO) Ion Mass Spectrometer have led to the detection of H2(+) ions in the Venus ionosphere. Although H2(+) is a minor ion, its measurement provides the first direct clue for determining the H2 abundance in the upper atmosphere of Venus. A photochemical model using PVO measurements is used to derive an H2 mixing ratio of 10 ppm below 140 km altitude. The presence of this much H2 confirms a previous prediction that the reactions of O(+) with H2 and subsequent recombination of OH(+) provide an important source of nonthermal H observed in the Venus atmosphere. The estimated escape flux for H is 10 to the 8th per sq cm/s.

Kumar, S.↗
NASA NTRS1981

Mean molecular weight and hydrogen abundance of Titan's atmosphere

The 200-600/cm continuum opacity in the troposphere and lower stratosphere of Titan is inferred from thermal emission spectra from the Voyager 1 IR spectrometer (IRIS). The surface temperature and mean molecular weight are between 94 and 97 K and between 28.3 and 29.2 AMU, respectively. The mole fraction of molecular hydrogen is 0.002 + or - 0.001, which is equivalent to an abundance of approximately 0.2 + or - 0.1 km amagat.

Samuelson, R. E.↗
NASA NTRS1980

On krypton isotopic abundances in the sun and in the solar wind

The Kr isotopic systematics in the meteorite Pesyanoe which is known to contain solar-type gases, are reported. Discrepancies in the isotopic data of fractions released at stepwise increasing temperatures cannot be reconciled with spallation Kr components, although spallation effects are significant. Fractionation mechanisms on the parent body and in the solar wind source region are considered and the implications for solar abundances discussed.

Marti, K.↗