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At least 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

A discussion of Cepheid masses

Masses and compositions of Cepheids are essential to map the places in the Hertzsprung-Russell diagram where various radial pulsation modes occur. Luminosity observations and stellar evolution theory give masses for Cepheids which range from 10 percent to a factor of four more than those given by pulsation theory. Combining the evolution and pulsation theories, a theoretical mass was determined using only the period and an approximate surface effective temperature. The ratio of the theoretical to evolutionary masses averaged 0.99 + or - 0.07 for 16 Cepheids.

Cox, A. N.↗
NASA NTRS1980

Dependence of red edge on eddy viscosity model parameters

The dependence of the red edge location on the two fundamental free parameters in the eddy viscosity treatment was extensively studied. It is found that the convective flux is rather insensitive to any reasonable or allowed value of the two free parameters of the treatment. This must be due in part to the fact that the convective flux is determined more by the properties of the hydrogen ionization region than by differences in convective structure. The changes in the effective temperature of the red edge of the RR Lyrae gap resulting from these parameter variations is quite small (approximately 150 K). This is true both because the parameter variation causes only small changes and because large changes in the convective flux are required to produce any significant change in red edge location. The possible changes found are substantially less than the approximately 600 K required to change the predicted helium abundance mass fraction from 0.3 to 0.2.

Deupree, R. G.↗
NASA NTRS1980

Theoretical mean colors for RR Lyrae variables

A hydrodynamically pulsating 0.6 solar mass model of a typical RR Lyrae variable was studied with a radiation transport-hydrodynamic computer program to predict theoretical T sub 3 and colors at many phases and to find the proper methods for getting mean colors and the consequent mean effective temperatures. The variable Eddington radiation approximation method was used with gray and with multifrequency absorption coefficients to represent the radiation flow in the outer optically thin layers. Comparison between observed and computed B-V colors indicate that these low Z population 2 models are reasonably accurate using King 1A composition opacities. The well known Oke, Giver, and Searle relation between B-V and T sub e reproduced. Mean colors were found by four different averaging methods. The method that gives a mean color and the mean T sub e closest to the nonpulsating model was the separate intensity means of B and V.

Davis, C. G.↗
NASA NTRS1980

The physics of heterodyne detection in the far-infrared: Transition from electric-field to photon-absorption detection in a simple system

The history of heterodyne detection is reviewed from the radiowave to the optical regions of the electromagnetic spectrum with emphasion the submillimeter/far infrared. The transition from electric field to photon absorption detection in a simple system is investigated. The response of an isolated two level detector to a coherent source of incident radiation is calculated for both heterodyne and video detection. When the processes of photon absorption and photon emission cannot be distinguished, the relative detected power at double- and sum-frequencies is found to be multiplied by a coefficient, which is less than or equal to unity, and which depends on the incident photon energy and on the effective temperature of the system.

Teich, M. C.↗
NASA NTRS1980

Application Explorer Mission-A heat capacity mapping mission

The author has identified the following significant results. Fine orogenic sediments, not detectable on the images recorded within the visible wavelength region, can be clearly recognized on all night IR-images. The area of glaciation in the northern foreland of the Alps can be partially identified on the basis of night-IR data. The terminal moraine of various glacierization phases can be mapped and the relative dry and unsorted moraine material consisting of crystalline components can be located through the vegetation cover. The eastern part of the glaciation area can be traced because of reverse temperature effect. In addition to the upper Bavarian lakes, the night-IR images clearly indicate the distribution of high soil moisture areas such as swamps and bogs. The geological border of the upper fresh water molasse in the northern part of the basin north of Munich is identifiable on night IR-imagery. Drift peat areas can be located on day-IR images of the dry season of the year.

Source record↗
NASA NTRS1980

A study on the electrolysis of sulfur dioxide and water for the sulfur cycle hydrogen production process

Experimental electrolysis cells using various platinum catalyzed carbon electrodes were tested. When operated at 200 mA/sq cm current density using 50 w/o acid at 50 C and 1 atm, a reference cell required 1.22 volts and degraded rapidly. After several improvements were incorporated into electrodes and the test cell configuration, a later cell required only 0.77 volts under identical operating conditions. At a lower current density, 100 mA/sq cm, the cell required only 0.63 volts. Kinetic studies on metal electrodes, measurements of temperature effects on electrode kinetics, investigations of electrocatalytic activities of metal electrodes over a wide range of acid concentrations, cyclic voltametric studies and evaluation of alternate catalysts were also conducted. From diffusivity experiments, a cation exchange membrane material, P-4010, exhibited an excellent diffusion coefficient, more than two orders of magnitude lower than that of rubber. Ionic resistivity measurements of eight materials showed that microporous rubber had the lowest resistivity.

Source record↗
NASA NTRS1980

Development and test results of a flight management algorithm for fuel conservative descents in a time-based metered traffic environment

A simple flight management descent algorithm designed to improve the accuracy of delivering an airplane in a fuel-conservative manner to a metering fix at a time designated by air traffic control was developed and flight tested. This algorithm provides a three dimensional path with terminal area time constraints (four dimensional) for an airplane to make an idle thrust, clean configured (landing gear up, flaps zero, and speed brakes retracted) descent to arrive at the metering fix at a predetermined time, altitude, and airspeed. The descent path was calculated for a constant Mach/airspeed schedule from linear approximations of airplane performance with considerations given for gross weight, wind, and nonstandard pressure and temperature effects. The flight management descent algorithm is described. The results of the flight tests flown with the Terminal Configured Vehicle airplane are presented.

Knox, C. E.↗
NASA NTRS1979

Stellar evolution at high mass including the effect of a stellar wind

The effect of a stellar wind on the evolution of stars in the mass range from 15 to 120 solar masses is investigated. All the stellar models are constructed with the use of Cox-Stewart opacities. Four possible cases of mass loss are considered: (1) no mass loss at all; (2) substantial mass loss from stars in all stages of evolution; (3) heavy mass loss from red supergiants only; and (4) sudden and very heavy mass loss from luminous yellow supergiants. The assumption of mass loss during the main-sequence phase of evolution is found to lead to a lowering of the luminosity and, unless the mass loss is extremely heavy, of the effective temperature as well. A comparison of the adopted mass-loss rates with observed rates suggests that stellar winds are probably not an important factor in the evolution of main-sequence stars and supergiants unless the initial masses are greater than about 30 solar masses.

Stothers, R.↗
NASA NTRS1979

C IV resonance line profiles in O stars

Graphs of normalized fluxes derived from objective-prism spectra (from the S-019 experiment on Skylab), and C IV profiles are presented. The C IV resonance profile, when studied with respect to bolometric magnitude and effective temperature, reveals mass loss conditions without requiring precise mass loss rates. There is a clear trend in the C IV feature toward more fully developed P Cygni profiles at higher luminosities, and a significant velocity shift in the C IV absorption down to bolometric magnitude approximately equal to -7.7.

Parsons, S. B.↗
NASA NTRS1979

Winds in hot, subluminous stars

Ultraviolet spectra over the wavelength range 1150 to 1750 A of the nuclei of two planetary nebulae are used to derive information concerning their winds. The nucleus of NGC 6826 is a 10th magnitude star with an O3f-type spectrum, an apparent terminal wind velocity of 2000 km/s, and a general level of ionization similar to that in Zeta Pup. The nucleus of Abell 78 is a 13th magnitude star having an OVI-type spectrum, an effective temperature between 100,000 and 150,000 K, and a gravity greater than 100,000 cm/second squared. Its apparent terminal wind velocity is 3400 km/s, and it has a very high general level of ionization. It is suggested that a luminosity-to-mass ratio (in solar units) greater than about 1600 is now an applicable mass-loss criterion for all hot stars, including planetary nuclei.

Heap, S. R.↗
NASA NTRS1980

Pioneer Saturn infrared radiometer - Preliminary results

Preliminary results of the infrared radiometer experiment on Pioneer Saturn are reported. The instrument made use of two broadband channels centered at 20 and 45 microns which scan at a fixed 75-deg angle with respect to the spacecraft spin axis to acquire 10,000 image pairs of Saturn and its rings in the 2.5 h before closest approach, as well as several observations of Titan. The intensities of radiation observed in both bands indicate an effective temperature of 94.4 + or - 3 K for the planet, implying a total emission greater than twice the absorbed sunlight. Infrared data also indicates a molecular abundance of 0.85 for H2 relative to H2 + He, which can be improved by comparing the derived temperature profiles and radio occultation data. Planetary temperatures are found to range from a minimum of 83 to 140 K at the 1 bar level, with differences of 2.5 K between belts and zones up to the 0.06-bar level, while ring temperatures range from 60 to 70 K on the illuminated side and from less than 60 to 67 K in the planet's shadow and average 55 K on the unilluminated side. Preliminary estimates indicate a 45-micron brightness temperature of 80 + or - 10 K for Titan.

Ingersoll, A. P.↗
NASA NTRS1979

X-ray evidence for electron-ion equilibrium and ionization nonequilibrium in Young supernova remnants

X-ray emission up to 25 keV from the supernova remnants Cas A and Tycho has been detected with the A-2 spectroscopy experiment on HEAO 1. The spectra must include continuum components with effective temperature of about 10 to the 8th K which could arise from optically thin plasmas in the collisionless shock fronts. This is the first indication of electron-ion temperature equilibrium in the expanding shell of young remnants. The equivalent widths of the K-alpha and K-beta iron line blends in Cas A have also been measured and it is found that their ratio is not compatible with the measured X-ray temperature in the collisional ionization equilibrium model. Finally, an unsuccessful search has been conducted for hard X-ray pulsars in both remnants.

Pravdo, S. H.↗
NASA NTRS1979

Chemical abundances in Hg-Mn stars

An abundance analysis has been conducted of 21 elements in 21 Hg-Mn, two Si-Cr, and six normal stars using model atmospheres and high-dispersion spectroscopy in the visible and UV. Manganese line strengths imply abundances that correlate well with stellar effective temperature. Within the studied sample of Hg-Mn stars there appears to be no correlation of abundances of any element with projected rotational velocity. Abundances in several Hg-Mn stars show patterns that are probably consistent with diffusion but difficult to reconcile with equilibrium nucleosynthesis. In general, no combination of gross stellar physical parameters is sufficient to characterize the patterns of line strengths observed in Hg-Mb Hg-Mn stars.

Heacox, W. D.↗
NASA NTRS1980

Solar wind ions accelerated to 40 keV by shock wave disturbances

Observations in the solar wind with the LASL/MPI fast plasma experiment on ISEE 1 and 2 reveal the common presence of ions with energies extending from 100 eV up to at least 40 keV in a broad region, typically 10 million kilometers wide, following interplanetary shocks. Peak differential fluxes up to 5000/sq cm s sr keV at 28 keV are observed either at the shock or within the first 1.5 hours following shock passage. In the solar wind frame the distribution function of these ions is roughly isotropic, peaks near zero velocity, and above 5 keV can adequately be characterized as power law in energy with a spectral index of 2.7. The effective 'temperature' of these ions generally exceeds 100 million K. These suprathermal interplanetary ions are almost certainly solar wind ions which have been accelerated by some mechanism associated with the shock wave disturbance. Present evidence leads the authors to favor stochastic particle acceleration involving electrostatic and/or electromagnetic turbulence in the postshock flow.

Gosling, J. T.↗
NASA NTRS1980

A process of stellar nucleosynthesis which mimicks mass fractionation in P-xenon

The combination of the O-shell theory of Heymann and Dziczkaniec and the supernova theory of Woosley and Howard clearly identifies the astrophysical sites for the formation of the anomalous light Xe component in carbonaceous chondrites. These sites are the O- and Ne-shells, and possibly C-shell of a massive star. Most of the Xe-124 and Xe-126 are formed in the O-shell during hydrostatic core silicon-burning, when a seed of heavy nuclei is exposed to an effective temperature near T9 = 2.0. Xe-128 is formed via Ba-128 in the O-shell, but the amounts appear too small to satisfy the deduced Xe-128/Xe-124 and Xe-128/Xe-126 yield ratios from the chondrites. However, substantial amounts of Xe-128 can be formed in the adjacent Ne- and C-shells during the explosion. The formation of Ba-128 in the O-shell would increase if the (gamma, alpha) photodisintegration rate in Ba-128 is actually smaller than calculated by Woosley and Howard. Lewis et al. have proposed that the anomalous light Xe component is mass-fractionated normal Xe. It is in this sense that the process of stellar nucleosynthesis of the present paper mimicks mass-fractionation.

Heymann, D.↗
NASA NTRS1980

Stellar winds and coronae in cool stars

Recent observational and theoretical results are reviewed that pertain to the presence and characteristics of stellar coronae and winds in late-type stars. It is found that stars - principally dwarfs exist with 'hot' coronae similar to the sun with thermally driven winds. For stars, at the lowest effective temperatures, and gravities characteristic of supergiant and giant stars, high temperature (approximately 100,000 deg K) atmospheres are absent (or if present are substantially weaker than in the dwarf stars), and massive winds are present. There also exist 'hybrid' examples - luminous stars possessing both a 'hot' corona and a supersonic stellar wind. Constraints for theoretical models are discussed.

Dupree, A. K.↗
NASA NTRS1980

Laboratory measurements of the O+/2D/ + N2 and O+/2D/ + O2 reaction rate coefficients and their ionospheric implications

Rate coefficients which have been measured at thermal energies for the charge transfer reactions of metastable O+/2D/ ions with N2 and O2 are reported. It is found that at an effective temperature of about 550 K, k(n2) = (8 + or - 2) x 10 to the -10 cu cm/sec and k(O2) = (7 + or - 2) x 10 to the -10 cu cm/sec. Drift tube-mass spectrometer measurements employ the reaction He(+) + O2 as the source of metastable O+ ions, showing that the ions produced in this manner are in the 2D state rather than the 2P state, a possible alternative identification. Finally, consideration is given to the ionospheric implications of the laboratory measurements.

Johnsen, R.↗
NASA NTRS1980

Synthetic line profiles in early-type stellar winds. I - H and He/+/

Statistical equilibrium calculations for the first 10 levels of H and He(+) were carried out for several model atmospheres of a star with an effective temperature of 40,000 deg K and log g equal to 3.5. Atomic level populations and line profiles were computed for a wind model (Klein and Castor, 1978) with a mass-loss rate of about 1.2 x 10 to the -5th solar mass per yr. A comoving frame method was applied and the radiative transfer problem in the expanding atmosphere is treated. Two variants of the Klein-Castor model produce He II at 4686 A in emission with 3203 A in absorption, as is commonly observed in Of spectra.

Kunasz, P. B.↗