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At least 1,459 records · Page 81
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NASA NTRS1988

The X-ray spectra of the extragalactic sources in the Einstein extended medium sensitivity survey

The X-ray energy distribution of a large sample of faint extragalactic X-ray sources is studied. Two independent methods are used to determine that the average spectral index for the sources is about 0.95. The intrinsic dispersion of the spectral index distribution is about 0.36. The sources are characterized by a range of power law slopes in the soft X-ray band with a mean of 1.03 + 0.05 or - 0.06 and an intrinsic dispersion of about 0.36. The present analysis extends knowledge of the AGN spectral properties to sources which are about three times more distant and more than 10 times fainter than those previously studied. There is no evidence in the data for any change in the mean spectral index of either extragalactic sources as a whole or AGN in particular, as a function of the X-ray flux in the flux range 10 to the -11th to 10 to the -13th ergs/sq cm/s.

Maccacaro, Tommaso↗
NASA NTRS1988

Gamma-ray limits on Na-22 production in novae

Data accumulated from 1980 to 1987 by the gamma-ray spectrometer aboard the SMM are analyzed to search for annual increases at 1.275 MeV as evidence of Na-22 decay in the Galactic center region and in the ejecta of recent individual novae. No evidence of any 1.275 MeV emission of celestial origin is found. A limit of 3 x 10 to the -6th solar mass is placed on the accumulated Na-22 from many novae occurring near the Galactic center, and a limit of 7 x 10 to the -7th solar mass is placed on the mass of Na-22 ejected by the closest of the recent neon-rich novae.

Leising, Mark D.↗
NASA NTRS1988

Large molecules in diffuse interstellar clouds

The effects of the presence of a substantial component of large molecules on the chemistry of diffuse molecular clouds are explored, and detailed models of the zeta Persei and zeta Ophiuchi clouds are constructed. The major consequence is a reduction in the abundances of singly charged atomic species. The long-standing discrepancy between cloud densities inferred from rotational and fine-structure level populations and from the ionization balance can be resolved by postulating a fractional abundance of large molecules of 1 x 10 to the -7th for zeta Persei and 6 x 10 to the -7th for zeta Ophiuchi. If the large molecules are polycyclic aromatic hydrocarbons (PAH) containing about 50 carbon atoms, they contain 1 percent of the carbon in zeta Persei and 7 percent in zeta Ophiuchi. Other consequences of the possible presence of PAH molecules are discussed.

Lepp, S.↗
NASA NTRS1988

Noise tube sources for the far IR and millimeter region

The radiant output of a noise tube designed for the 90-140-GHz (3.3-2.1-mm) frequency range has been compared with that from mercury lamps over the wavelength region from 0.4 to about 6 mm. Lamellar grating and Michelson Fourier transform spectrometers were used in conjunction with He cooled bolometers of NEP from 10 to the -12th to 10 to the -14th W/sq rt H2 to measure relative spectral irradiance. With this instrumental arrangement, the radiant power emitted by the noise tube was observed to be less than that from a mercury lamp, at least to a 3-mm wavelength, but it produced less source noise than an ac operated mercury lamp. When the noise tube operating current was reduced, the spectral irradiance peak shifted to longer wavelengths.

Moller, K. D.↗
NASA NTRS1988

Kinetics of the reaction OH + HO2 yields H2O + O2 from 254 to 382 K

The discharge-flow resonance fluorescence technique has been used to determine the absolute rate constant for the reaction OH + HO2 yields H2O + O2 from 254 to 382 K at a total pressure of 1 Torr. Pseudo-=first-order conditions were used with HO2 in large excess over OH. The rate constant was obtained directly from observed decays of OH and measured concentrations of HO2. Since the observed rate constant was found to be very sensitive to small background concentrations of O and H atoms, NO2 was used to remove both atom species from the system. With added NO2 the result at 299 K is (1.1 + or - 0.3) x 10 to the -10th cu cm/molecule where the error limits are one standard deviation and include an estimate of overall experimental uncertainty. The temperature dependence expressed in Arrhenius form is (4.8 + or - 0.8) x 10 to the -11th exp (250 + or - 50)/T. The results are independent of the type of reactor surface and the precursor used to produce OH and HO2. The present results agree well with earlier measurements near 1-atm total pressure and suggest that this rate constant exhibits little or no pressure dependence between 1 and 1000 Torr.

Keyser, Leon F.↗
NASA NTRS1988

Extremely low nonalloyed and alloyed contact resistance using an InAs cap layer on InGaAs by molecular-beam epitaxy

Extremely low alloyed and nonalloyed ohmic contact resistances have been formed on n-type InAs/In(0.53)Ga(0.47)As/In(0.52)Al(0.48)As structures grown on InP(Fe) by molecular-beam epitaxy. To insure the accuracy of the small contact resistances measured, an extended transmission line model was used to extrapolate contact resistances from test patterns with multiple gap spacings varying from 1 to 20 microns. For a 150-A-thick InAs layer doped to 2 x 10 to the 18th/cu cm and a 0.1-micron-thick InGaAs layer doped to 1 x 10 to the 18th/cu cm, a specific contact resistance of 2.6 x 10 to the -8th ohm-asterisk sq cm was measured for the nonalloyed contact, while a resistance less than 1.7 x 10 to the -8th ohm-asterisk sq cm is reported for the alloyed contact. Conventional Au-Ge/Ni/Au was used for the ohmic metal contact and alloying was performed at 500 C for 50 s in flowing H2. Using a thermionic field emission model, the barrier height at the InAs/InGaAs interface was calculated to be 20 meV.

Peng, C. K.↗
NASA NTRS1988

JPL pulsar timing observations. V - Macro and microjumps in the Vela pulsar 0833-45

In this paper, 14.5 yr of JPL timing data are analyzed to study the spindown of the Vela pulsar. Discontinuities in the spin frequency and its derivatives have bimodal amplitude distributions. Macrojumps are characterized by relative changes in spin rate of about 10 to the -6th with a derivative of 0.01, while microjumps have relative changes in spin rate of less than about 10 to the -9th with derivative of less than about 0.0001. Macrojumps display a (+,-) signature with a well-defined ratio of the amplitudes. Microjumps, however, show all possible signs of events and a larger range in signature. Six macrojumps and their associated transients are quantified using a 10-parameter model for each jump. Amplitude and decay time parameters vary by a factor of about five over the set of jumps. The results are discussed in terms of superfluid vortex models. The macrojumps are consistent with a model in which angular momentum is suddenly transferred to the crust from a more rapidly rotating superfluid core, followed by relaxation of the spin-down rate.

Cordes, J. M.↗
NASA NTRS1987

Substorm-associated lower hybrid waves in the plasma sheet observed by ISEE 1

Observations of the electric field at frequencies from 2-128 Hz, using the burst mode of the spherical double probe on ISEE 1, have been examined for a time period previously identified as containing the traversal of a near-earth neutral line past the satellite. Intense waves (3 to over 30 mV/m) at approximately half the lower hybrid frequency were observed throughout the plasma sheet from the neutral sheet to the boundary, but only during the period of the large dc electric field and E x B velocity associated with the substorm neutral line. The wave number deduced from linear fits of the data was comparable to the inverse electron gyroradius. These results are consistent with the lower-hybrid drift instability. Although peaks between the ion and electron plasma frequency were sometimes observed simultaneously, the integrated power below 100 Hz was usually at least one to two orders of magnitude greater than that above 100 Hz. Although theoretical work has suggested that the instability would be suppressed at the neutral sheet, the largest waves observed occurred right at the neutral sheet when the southward component of the magnetic field was 6 gamma. The observed waves could provide an anomalous resistivity of about (3-1000) x 10 to the -7th S (compared to the classical value of 1 x 10 to the -18th S).

Cattell, C. A.↗
NASA NTRS1987

A spectral radiance comparison of a noise tube and a HgXe arc lamp between 60 GHz and 600 GHz

The relative spectral radiance of a noise tube, model TN-167, designed for the frequency range 90-140 GHz (3.3 mm to 2.1 mm) was compared to that from a 200-watt high pressure HgXe arc lamp over the wavelength region from 0.5 to about 5 mm. A Michelson Fourier transform spectrometer and a lamellar grating instrument were used in conjunction with liquid helium-cooled bolometers of NEP 10 to the -12th to 10 to the -14th watt/(Hz) exp 1/2 to measure relative spectral radiant power. With this instrumental arrangement, the noise tube exhibited a very sharp low frequency cutoff at about 2.2/cm. The HgXe arc lamp emitted more radiant power than the noise tube in the wavelength region below 3 mm (100 GHz) down to 0.5 mm. Above 3 mm, the noise tube had a stronger output. The noise tube spectral radiance shifted to lower frequencies when the input current was lowered from 125 mA to 50 mA.

Heaney, J. B.↗
NASA NTRS1988

Extremely low resistance nonalloyed ohmic contacts on GaAs using InAs/InGaAs and InAs/GaAs strained-layer superlattices

Employing a structure consisting of n(+)-InAs/InGaAs and InAs/GaAs strained-layer superlattices (SLSs) grown by molecular beam epitaxy on GaAs films, nonalloyed contact resistances less than 8.5 x 10 to the -8th ohm sq cm have been obtained. Self-consistent simulations show that these extremely small nonalloyed contact resistances are due to the suppression of the depletion depth in the GaAs channel and tunneling through the SLS layer. Similar structures on InGaAs channels have led to nonalloyed specific contact resistances of about 1.5 x 10 to the -8th ohm sq cm. These results represent the smallest figures reported for these important material systems.

Peng, C. K.↗
NASA NTRS1988

HEAO 1 hard X-ray observation of clusters of galaxies and intracluster magnetic fields

The results of HEAO 1 hard X-ray measurements of three clusters of galaxies, Abell 401, Abell 2255, and Abell 2256 are reported. Nonthermal components were not detected above the level of 10 to the -5th photons/sq cm/s/keV. Comparison of the flux upper limits with theoretical predictions yields lower limits of about 10 to the -7th gauss on the mean value of the intracluster magnetic fields in the central regions of these clusters.

Rephaeli, Yoel↗
NASA NTRS1988

Europium mass balance in polymict samples and implications for plutonic rocks of the lunar crust

The mean concentrations of Sm and Eu in the lunar surface crust were analyzed by correlating the Sm concentration and the Sm/Eu ratio with Th concentration obtained from published data on a large number of polymict samples from various locations in the lunar highlands, and using the value of 0.91 microg/g for the mean Th concentration of the highlands surface crust obtained by the orbiting gamma-ray experiments. The mean concentration of Sm in the lunar surface crust was found to be between 2 and 3 microg/g, and that of Eu between 0.7 and 1.2 microg/g. The results indicate that there is no significant enrichment or depletion of Eu, compared to Sm, relative to chondritic abundances; i.e., there is no significant 'Eu anomaly' in average upper crust, contrary to predictions by some earlier investigators.

Korotev, Randy L.↗
NASA NTRS1989

Time-resolved simplified chemical kinetics modelling using computational singular perturbation

A CO-CH4-air reaction system is used to demonstrate the computational singular perturbation (CSP) method for deriving time-resolved simplified chemical kinetics models. CSP provides a programmable algorithm to group the given collection of elementary reactions into reaction groups which are ordered according to their speed. The concept of Importance Index k(m)exp s is introduced: k(m)exp s is defined to be a number between 0 and 1 which measures the importance of the m-th reaction group to the s-th reactant and can readily be computed from data generated by CSP. It is suggested that the robustness of the solutions of the reaction system can be qualitatively assessed by inspecting the Importance Index data.

Lam, S. H.↗
NASA NTRS1989

Star-formation rates and forbidden O II emission in blue galaxies

Observations of forbidden O II 3727 A and H-beta emission-line strengths are presented from large-aperture spectrophotometric observations of nearby (z less than 0.1) optically blue galaxies covering a wide range in surface brightness and luminosity. The forbidden O II emission-line equivalent widths in these galaxies primarily lie between 30 and 100 A. Emission-line strengths in nearby blue galaxies thus resemble those found in faint galaxies at moderate-to-high redshifts. The most actively star-forming nearby galaxies are characterized by surface brightness in the forbidden O II line of about 10 to the -15th to 10 to the -14th ergs/s per sq cm per sq arcsec over spatial distances of more than 1 kpc. The fluxes of the forbidden O II and H-beta emission lines are well correlated, and thus forbidden O II luminosities can be used to derive approximate star-formation rates in blue galaxies.

Gallagher, J. S.↗
NASA NTRS1989

Significant variation from a 1/R-squared potential in the cumulative flux determined from the Giotto Comet Halley Dust Impact Experiment

The Dust Impact Detection System (DIDSY) aboard the Giotto spacecraft provided the information on the dust flux, mass spectrum, and cumulative mass distribution flux in the coma of Comet Halley. Analysis of discrete pulse height data of cometary particles for the mass range of particles between 4.0 x 10 to the -10th g and 6.0 x 10 to the -6th g registered by the Giotto DIDSY detectors 2, 3, and 4 has been completed, and a cumulative flux has been determined for this size range of particles. Inside the cometopause, anomalous peaks have been identified as deviation from a 1/R-squared curve in both pre- and postencounter measurements.

Alexander, W. M.↗
NASA NTRS1989

Infrared fluorescence efficiencies for the nu1 and nu5 bands of formaldehyde in the solar radiation field

Formaldehyde scattering strengths have been determined for equilibrium distributions of 100 K, 50 K, 20 K, and for the the non-LTE case of an essentially fully relaxed distribution. Integrated band g factors of 2.89 x 10 to the -4th photons/s per molecule for nu1 and 3.83 x 10 to the -4th for nu 5 are obtained. The results indicate that the most promising regions to search for cometary H2CO are at about 2782/cm, at 2794.5/cm, and at about 2835/cm.

Reuter, D. C.↗
NASA NTRS1989

The composition of comets

The present discussion of recent studies concerning cometary composition gives attention to the results obtained by in situ measurements of Comet Halley's dust composition by Vega 1's impact mass spectrometer, which discovered a mineral fraction that appears to be CI chondritic, as well as an organic fraction consisting of highly unsaturated hydrocarbons. The mineral fraction of comets appears to form a core that is embedded in essentially organic material; the spectroscopic invisibility of carbon is due to its presence in the cometary dust. The mass of most dust particles is found to be in the 10 to the -12th to 10 to the -14th g range. A considerable fraction of the dust grains serves as an extended source of gas in the inner coma.

Jessberger, E. K.↗
NASA NTRS1989

Analytical expressions for the Rosseland-mean opacity and electron scattering in stellar atmospheres

Simple analytical approximations for the calculations of the electron scattering coefficient, the Rosseland-mean opacity and the equation of state in the temperature range of 4000 to 50,000 K and the density range of 10 to the -13th to 10 to the -7th g/cu cm. These approximations are valid for a gas with 1, 0.1, and 0.01 times the solar metal abundances in LTE. The accuracy of the approximations is better than 5 percent over most of this temperature and density range.

Burger, P.↗