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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 901 records · Page 50
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NASA NTRS1987

Hydrogenated amorphous carbon (a:C-H) in the planetary nebula NGC 7027

A spectroscopic identification of the infrared continuum radiation is proposed for the planetary nebula NGC 7027. Hydrogenated amorphous carbon (a:C-H) is shown to account for the undulating spectrum between 5 and 15 microns. The unidentified infrared emission bands lie at the peaks in the a:C-H spectrum, pointing to their association with a carbon polymorph, possibly a:C-H or polycyclic aromatic hydrocarbon molecules. Except for atomic emission lines, all the infrared emission from NGC 7027 comes from one or another polymorph of carbon.

Goebel, John H.↗
NASA NTRS1987

Hot DA white dwarfs seen with Einstein, an ultrahot H- and He-poor white dwarf, and Zanstra temperatures of planetary nebula nuclei

Einstein observations of hot DA white dwarfs are analyzed using model atmospheres. It is found that, for a relatively limited sample of stars, that hot He-poor stars like HZ 43 are indeed the exception, and that there is an apparent correlation between He/H ratio and increasing temperature. The seventh brightest soft X-ray source in the HEAO-A1 catalog, H1504+65, turns out to be an ultrahot white dwarf star, with a temperature of approximately 160,000 K, showing spectral features of the Zanstra method for determining the temperatures of central stars of planetary nebulae demonstrates that the H I Zanstra temperatures are indeed reliable and suggests that a substantial fraction of these central stars have He/H ratios which are considerably less than the solar value.

Shipman, Harry L.↗
NASA NTRS1987

Astrophysics and the solar nebula

The following types of experiments for a proposed Space Station Microgravity Particle Research Facility are described: (1) nucleation of refractory vapors at low pressure/high temperature; (2) coagulation of refractory grains; (3) optical properties of refractory grains; (4) mantle growth on refractory cores; (5) coagulation of core-mantle grains; (6) optical properties of core-mantle grains; (7) lightning strokes in the primitive solar nebula; and (8) separation of dust from a grain/gas mixture that interacts with a meter-sized planetesimal to determine if accretion occurs. The required capabilities and desired hardware for the facility are detailed.

Nuth, Joseph↗
NASA NTRS1988

International ultraviolet explorer spectral atlas of planetary nebulae, central stars, and related objects

The International Ultraviolet Explorer (IUE) archives contain a wealth of information on high quality ultraviolet spectra of approximately 180 planetary nebulae, their central stars, and related objects. Selected are representative low-dispersion IUE spectra in the range 1200 to 3200 A for 177 objects arranged by Right Ascension (RA) for this atlas. For most entries, the combined short wavelength (SWP) (1200to 1900) and long wavelength (LWR) (or LWP, 1900 to 3200 A) regions are shown on 30 cm by 10 cm Calcomp plots on a uniform scale to facilitate intercomparison of the spectra. Each calibrated spectrum is also shown on an expanded vertical scale to bring out some of the weaker features.

Feibelman, Walter A.↗
NASA NTRS1987

Planetary nebulae

The role of the IUE in the study of planetary nebulae (PN) is discussed as well as extinction, the central star and nebular spectra, plasma diagnostics, the chemical compositions of PN, and nebular models. Some information on the spatial distribution of C(2+) or C(3+) ions may be acquired for extended PN from IUE data, making it possible to probe ionization zones different from those observable in the visual region. For high resolution pertaining to filamentary structure and clumpiness, optical region images and data obtained with the Hubble Space Telescope are needed.

Koeppen, J.↗
NASA NTRS1988

Two compact planetary nebulae of moderate excitation - NGC 6565 (3-4.5 deg) and NGC 6644 (8-7.2 deg)

Data obtained with an image-tube scanner at the 3-m Shane telescope are combined with IUE data to obtain plasma diagnostics and chemical compositions for two planetary nebulae of moderately high excitation. Theoretical nebular models were calculated using stellar fluxes given by Husfeld et al. (1984) for T(asterisk) = 85,000 K, log g = 4.72, and elemental abundances were obtained by fitting theoretical to observed line intensities and also by using the model to determine ionization correction factors to be applied to observed ionic concentrations. Although C appears to be about 1.5 times as abundant in NGC 6644 as in NGC 6565, N, O, Ne, S, Cl, and Ar are depleted by factors ranging from 2 to 6 in NGC 6644 as compared to NGC 6565. The high-velocity object, NGC 6644, was evidently made from a less metal-rich mixture than the sun.

Aller, Lawrence H.↗
NASA NTRS1988

R Aquarii - The large-scale optical nebula and the Mira variable position

The R Aquarii symbiotic star system is surrounded by a large-scale optical nebula. Observations of the nebular forbidden O III structure are presented and its morphological significance are discussed in context with previously observed small-scale radio-continuum features, which may be related. It is suggested that a precessing accretion disk may explain the global features of both the large-scale optical emission and the small-scale radio emission. Moreover, an accurate position has been determined of the system's Mira, which suggests that a recent theoretical model, yielding an egg-shaped central H II region for symbiotic systems with certain physical parameters, may apply to R Aquarii. The optical position of the 387 d period Mira variable is consistent with previous findings in the radio, that SiO maser emission is far removed from the Mira photosphere.

Michalitsianos, A. G.↗
NASA NTRS1988

The velocity structure and turbulence at the center of the Orion Nebula

Using the Coude Feed System of the Kitt Peak National Observatory at its highest possible dispersion and a CCD detector with wide dynamic range it was possible to examine in detail the radial and turbulent velocities of the Orion Nebula at the seeing limit. For the first time, multiple components of the spectral lines were resolved and the Gaussian profiles fit to them. Using four different observing runs, two circular areas 2 minutes in diameter were observed, the first centered on the theta sup 1 Orionis and the second centered on theta sup 2 Orionis. The velocity resolution of 1 km/s combined with the angular resolution of 2 seconds allowed observation in detail at line splitting as well as the location and measurement of high internal motions, about -50 km/s with respect to the bulk of ionized matter. Because over 900 different positions were measured, modern statistical techniques were used for the analysis of turbulence, so that the dispersion relation and the structure function could be calculated and compared with predictions of the Kolmogorov theory. The random motions showed correlation with spatial separation, as expected for turbulence; but comparison with predictions of the classical theory for nebular turbulence showed poor agreement. If Kolmogorov theory applies, then the energy must be input at many scales.

Castaneda, Hector O.↗
NASA NTRS1988

Aerodynamic properties of fractal grains - Implications for the primordial solar nebula

Gas-grain and grain-grain interactions have a significant role in the evolution of the primordial solar nebula. There is also compelling evidence for larger grains in dense interstellar clouds than in the diffuse interstellar medium. In the usual treatments of grain collisions, where it is assumed that collisions produce compact bodies with a constant density, the gas drag decreases rapidly with size. The first step toward a quantitative analysis of the aerodynamic behavior of fractal aggregates is reported here, and it is shown that for fractals drag decreases very slowly.

Meakin, P.↗
NASA NTRS1988

The high-latitude F supergiant IRAS 18095 + 2704 - A proto-planetary nebula

The paper reports the discovery of a new high-latitude F supergiant, IRAS 18095 + 2704, which shows a large excess in the far-infrared. Ground-based observations have identified it as a V = 10.4 mag F3 Ib star which displays light and velocity variability. Comparison with the post-asymptotic giant branch (AGB) evolution model of Volk and Kwok suggests that it is an excellent candidate for a protoplanetary nebula. Model fitting of the spectrum of 18095 + 2704 from 0.35 to 100 microns suggests that it evolved from the AGB approximately 265 yr ago and had a mass-loss rate of 0.00003 solar mass/yr at the end of the AGB.

Hrivnak, Bruce J.↗
NASA NTRS1988

Pulsars, X-ray synchrotron nebulae, and guest stars

X-ray observations of supernova remnants and radio pulsars are used to derive luminosities of neutron stars and synchrotron nebulae. Observations of known isolated pulsars are used to develop an empirical relationship between the X-ray luminosity and the rate of loss of rotational energy. This is used to derive the characteristics of pulsars hidden in remnants which show evidence for a central compact object or associated nebular emission, but no clear pulsed signal from the neutron star itself. Possible periods and period derivatives for the hidden pulsars are discussed. Some might have periods as long as 0.5 s, and period derivatives considerably higher than that of PSR 1509 - 58, currently the pulsar with the highest known period derivative.

Seward, Frederick D.↗
NASA NTRS1988

Chondritic meteorites and the solar nebula

Present understanding of the formation of chondritic meteorites in the solar nebula is reviewed, focusing on the nebular history of chondritic material. The stages in the history of chondritic material are summarized. The processing of chondrules and refractory inclusions is considered, and the relatedness of chondrules and refractory inclusions is discussed.

Wood, John A.↗
NASA NTRS1987

The stellar seismology of hot white dwarfs and planetary nebula nuclei

The pulsation properties of hot white dwarfs make it possible to determine their mass, surface composition, rotation, and rate of evolution, and provide constraints on their internal structure. Period spacings are sensitive measures of stellar mass and indicate surface layer structure. Measurement of the rate of period change for these stars provide a way to determine their cooling rates. Attention is also given to how well (or poorly) models of excitation of the pulsations fit within current models of planetary nebula nuclei and hot white dwarfs.

Kawaler, Steven D.↗
NASA NTRS1988

Radiation-driven winds of hot stars. V - Wind models for central stars of planetary nebulae

Wind models using the recent improvements of radiation driven wind theory by Pauldrach et al. (1986) and Pauldrach (1987) are presented for central stars of planetary nebulae. The models are computed along evolutionary tracks evolving with different stellar mass from the Asymptotic Giant Branch. We show that the calculated terminal wind velocities are in agreement with the observations and allow in principle an independent determination of stellar masses and radii. The computed mass-loss rates are in qualitative agreement with the occurrence of spectroscopic stellar wind features as a function of stellar effective temperature and gravity.

Pauldrach, A.↗
NASA NTRS1988

Heterogeneity in the nebula - Evidence from stable isotopes

Meteoritic isotopic evidence, primarily for O, which defines the existence of different nebular reservoirs, is analyzed. The internal isotopic systematics (for chondrules and incisions) is analyzed as well as its role in placing restraints on conditions and processes in the solar nebula. Questions pertaining to the actual origin of these reservoirs are addressed.

Thiemens, Mark H.↗
NASA NTRS1989

The hard X-ray to low-energy gamma-ray spectrum of the Crab Nebula

Data from the UCSD/MIT Hard X-ray and Low-Energy Gamma-Ray Experiment on the first High Energy Astronomy Observatory (HEAO 1) were used to determine the spectrum of the Crab Nebula in the 10 keV-5 MeV energy range. The spectral change in the 100 keV-1 MeV region is characterized. When the combined spectrum is fitted to a broken power-law model, the data require a description with the following distinct spectral indices: a power-law slope of 2.08 in the 1-100 keV region and a slope of about 2.5 above 150 keV.

Jung, G. V.↗
NASA NTRS1989

Collisions of macroscopic fluffy aggregates in the primordial solar nebula and the formation of planetesimals

The characteristics of submicron-size grains lead to the conclusion that, with relative velocities under tens of meters per second deduced for the early stages of accretion in the solar nebula, bodies of meter dimension, were low-density, compressible aggregates. The collision mechanism appropriate to such objects is explored here. The results of representative collisions are summarized which show that the penetration is only a fraction of the radius of the impacting body. Hence, all but near-grazing collisions lead to merger of the two bodies. Aggregate growth occurs without substantially increasing the mean density, but small region with densities about 60 percent of the bulk density will occur in a collision.

Donn, B.↗