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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 253 records · Page 14
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NASA NTRS1977

Study of a light curve of Beta Persei at 3428 A

Photometric light-curve data obtained during Copernicus satellite observations of Beta Persei in the Balmer continuum at 3428 A are analyzed and reduced. A small but significant fluctuation with a period of very nearly 0.069167 day in the photometric counts is tentatively attributed to a small perturbation in spacecraft attitude introduced each time the satellite passes from daylight into darkness. The light curve is intensity-rectified, phase-rectified, and solved on the assumption that the limb darkening follows the linear cosine law. The resulting distributions of luminosity among the three components of Beta Persei are plotted, and it is shown that the effective temperature of Algol B may be higher than 5000 K.

Chen, K.-Y.↗
NASA NTRS1977

Lunar gravity determinations and their implications

Mass distributions inferred from lunar gravity data are studied, with emphasis on mascons as mass excesses in topographic lows in all the near-side ringed basins, with best representation as near-surface disks with excess loads of 800 kg per sq cm. The gravity data are derived from earth-based radiometric data on speed variations of orbiting Apollo spacecraft observed from the earth. Mass deficits are found in the case of large 100 km craters (Langrenus, Theophilus, Copernicus), and are consistent with crater volume. Earlier states of the moon are hypothesized on the basis of the small gravitational anomaly associated with the Apennine mountains, and on the basis of lunar isostasy, plasticity, and maria flooding and filling.

Sjogren, W. L.↗
NASA NTRS1977

UV stellar occultation measurements of nighttime equatorial ozone

The ultraviolet spectrometer-telescope on Copernicus was used for stellar occultation measurements of atmospheric ozone. Two sets of observations of the target star Beta-Cen were carried out on 26 July 1975 and 13-14 June 1976 at wavelengths from 2550 A to 3100 A. After unfolding of the data, ozone density profiles near the equator within 3 hours of local midnight were obtained at altitudes from 47 to 114 km. A secondary maximum at 97 km has been observed in both sets of data. The ozone density between 47 and 75 km is a factor of 2 to 3 times as large as current models predict. At the lower boundary, about half the ozone destruction should be caused by NOx and ClOx. Above 55 km, virtually all loss is due to HOx. These results suggest an overestimate of HOx and ClOx loss processes or a serious underestimate of the Ox production rate.

Riegler, G. R.↗
NASA NTRS1976

A survey of mass-loss effects in early-type stars

Intermediate-resolution data obtained with the Copernicus satellite are surveyed in order to define the region in the H-R diagram where mass loss occurs. The survey includes 40 stars, providing good coverage of supergiants from O4 to A2 and main-sequence stars from O4 to B7 as well as spotty coverage of late O giants and intermediate to late B stars. The spectral transitions examined are primarily resonance lines of ions of abundant elements plus some lines arising from excited states (e.g., C III at 1175.7 A and Si IV at 1122.5 A). Observed P Cygni profiles are discussed along with interesting features of some individual profiles. The data are shown to indicate that mass-loss effects occur over a wide portion of the H-R diagram, that mass ejection generally occurs when the holometric magnitude is greater than -6.0, and that the mass-ejection rate is usually high enough to produce P Cygni profile whenever the N V feature at 1240 A is present in a spectrum.

Snow, T. P., Jr.↗
NASA NTRS1977

The abundance of boron in Vega and Sirius

High-resolution (0.05 A) Copernicus observations of the 1362.46-A B II resonance line in Sirius and Vega are analyzed. In Vega, this line is found to be strong, broad, asymmetric, and blended with another line; it is not observed in Sirius. Angular velocities of 18-20 km/s for Vega and 10 km/s for Sirius are determined. A search for subordinate lines of B II reveals no such features in the Sirius spectrum and the possible weak presence of two unblended lines in that of Vega. The cause of the asymmetry of the B II blend in Vega is investigated, line-profile calculations are performed for Sirius, and the B content of Vega is obtained by means of an LTE synthesis of the B II blend as well as a non-LTE computation of the B II line alone. It is concluded that Sirius is deficient in B relative to Vega by a factor of at least 20. The Be abundances of both stars are also examined in relation to the galactic-cosmic-ray spallation theory for the origin of light elements.

Praderie, F.↗
NASA NTRS1977

The continuous UV flux of Alpha Lyrae - Non-LTE results

Non-LTE calculations for the ultraviolet C I and Si I continuous opacity show that LTE results overestimate the importance of these sources of opacity and underestimate the emergent flux in Alpha Lyr. The largest errors occur between 1100 and 1160 A, where the predicted flux in non-LTE is as much as 50 times larger than in LTE, in reasonable accord with Copernicus observations. The discrepancy between LTE models and observations has been interpreted to result from the existence of a chromosphere. Until a self-consistent non-LTE model atmosphere becomes available, such an interpretation is premature.

Snijders, M. A. J.↗
NASA NTRS1976

Degradation of large, period II lunar craters

Nighttime thermal maps of large, young lunar craters show an evolutionary sequence in the gross morphology of thermal patterns associated with the craters. The nature of the sequence is consistent with crater degradation models which predict a filling of large craters with material from the walls. Detailed comparison of a nighttime thermal map of Copernicus with a geologic map demonstrates that a smooth floor unit characterized as impact melt is more likely wall debris. Nighttime thermal maps will be particularly useful for quantifying crater modification which has occurred in the relatively quiescent post-basin period of lunar history.

Mendell, W. W.↗
NASA NTRS1977

A survey of interstellar molecular hydrogen. I

Data from the Copernicus satellite's ultraviolet telescope were used to survey column densities of atomic and molecular hydrogen from a large sample of early-type stars; these data have bearing on an eventual understanding of diffuse and dense interstellar clouds. Column densities are derived by fitting damping profiles to the observed spectra, most of which exhibit strong damping lines in the lower rotational levels surveyed. Plots of dust column density, fractional abundance of molecular hydrogen, and the logarithm of fractional abundance versus total gas column density are given for many of the stars; stars with abnormally large or small hydrogen column densities, as well as some distant stars at high galactic latitudes, are considered. Equilibrium and nonequilibrium theories accounting for the abundance of interstellar hydrogen are compared, and support is found in the data for an account which balances hydrogen formation on interstellar grains with destruction through photodissociation. Overall averages for atomic and molecular hydrogen levels in the galactic plane are also calculated.

Savage, B. D.↗
NASA NTRS1977

Shock models of high-velocity interstellar Si III

Recent Copernicus observations of high-velocity interstellar Si III have been interpreted as evidence for collisionally ionized gas at 30,000 to 80,000 K. In this paper, these observations are summarized, and conductive-interface and shock-heating mechanisms are investigated as sources for the temperatures, column densities, and velocity fields. The conductive mechanism evidently cannot reproduce the observations, but radiatively cooling shocks of 50 to 100 km/s, produced by supernova remnants or stellar winds, may provide a viable explanation. Time-dependent calculations of the cooling and ionization behind such shocks show the importance of initial ionization conditions at 100,000 K and produce results substantially different from those of calculations which start at 1 million K.

Shull, J. M.↗
NASA NTRS1977

A survey of interstellar HI from L alpha absorption measurements 2

The Copernicus satellite surveyed the spectral region near L alpha to obtain column densities of interstellar HI toward 100 stars. The distance to 10 stars exceeds 2 kpc and 34 stars lie beyond 1 kpc. Stars with color excess E(B-V) up to 0.5 mag are observed. The value of the mean ratio of total neutral hydrogen to color excess was found to equal 5.8 x 10 to the 21st power atoms per (sq cm x mag). For stars with accurate E(B-V), the deviations from this mean are generally less than a factor of 1.5. A notable exception is the dark cloud star, rho Oph. A reduction in visual reddening efficiency for the grains that are larger than normal in the rho Oph dark cloud probably explains this result. The conversion of atomic hydrogen into molecular form in dense clouds was observed in the gas to E(B-V) correlation plots. The best estimate for the mean total gas density for clouds and the intercloud medium, as a whole, in the solar neighborhood and in the plane of the galaxy is 1.15 atoms per cu. cm; those for the atomic gas and molecular gas alone are 0.86 atoms per cu cm and 0.143 molecules per cu cm respectively. For the intercloud medium, where molecular hydrogen is a negligible fraction of the total gas, atomic gas density was found to equal 0.16 atoms per cu cm with a Gaussian scale height perpendicular to the plane of about 350 pc, as derived from high latitude stars.

Bohlin, R. C.↗
NASA NTRS1978

A study of mass flow in the massive spectroscopic binary HD47129

HD 47129 is a double-lined spectroscopic binary with a period 14.3961 days and a visual magnitude of V = 6.06. Using the Copernicus U2 spectrometer, observations were obtained of the system on eight occasions, starting March 2, 1977. Some results are presented and discussed.

Barlow, M. J.↗
NASA NTRS1977

Observations of O VI

A useful spectroscopic tracer for a hot phase of interstellar gas is the O VI ion, which reaches its maximum concentration in collisional ionization between 100,000 and 1 million K. Presently, over 70 stars have been observed for O VI absorption by the Copernicus satellite. Nearly all of the stars show broad weak lines, but no evidence favoring a circumstellar origin for the gas can be found. An overall average for the electron density of the hot gas in the galactic plane is of the order of 0.001 per cu cm. The relative volume in space occupied by the hot-gas regions (and hence their internal density) is uncertain, but a filling factor in the range from 0.02 to 0.2 seems most plausible. Fluctuations in radial velocities and column densities suggest there are roughly 6 regions per kpc, each with an O VI column density of approximately 10 trillion per sq cm. The observed rms dispersion of radial velocities for these regions is 26 km/s.

Jenkins, E. B.↗
NASA NTRS1977

Twilight ozone measurement by solar occultation from AE 5

The BUV on AE 5 was used for a solar occultation measurement of atmospheric ozone. An observation was carried out during the morning twilight near 5 deg N, December 17, 1976, at the fixed wavelength of 255.5 nm, and a profile between 49 and 82 km was obtained. The number densities determined by this measurement were 3.7 x 10 to the 10th cu cm at 50 km, 5.1 x 10 to the 9th at 60 km, 3.9 x 10 to the 8th at 70 km, and 3.0 x 10 to the 7th at 80 km. No evidence of a high altitude secondary maximum was found. These concentrations are between a factor of 4 and 20 smaller than those midnight results reported from a Copernicus measurement, and similar to the values from the Krueger-Minzer Mid-latitude Model above 55 km. These values may be as much as a factor of 2 less than the Krueger-Minzer model below 50.

Guenther, B.↗
NASA NTRS1977

Apparent wavelength dependence of v sin i for Zeta Tauri

It was previously reported that the derived projected rotational velocity (v sin i) of the B shell star, Zeta Tau, appeared to depend on the wavelength of the line used in the analysis. This letter documents the apparent wavelength dependence of v sin i for Zeta Tau in order to provide an observational basis against which quantitative explanations can be tested. A value of 300 km/s is adopted for v sin i on the basis of an examination of the visual line spectrum, particularly the lines of He I at 4026 and 4471 A and Mg II at 4481 A. Analysis of the far-UV resonance lines of Si III at 1206 A and Si IV at 1393 and 1463 A in Copernicus spectrograms of Zeta Tau yields a representative value of no more than 150 km/s for v sin i. Gravity darkening of the star's atmosphere and distention of the atmosphere by rapid differential rotation are considered as possible explanations for the discrepancy between the v sin i values determined from the UV and visual lines.

Heap, S. R.↗
NASA NTRS1977

Deuterium and hydrogen in the local interstellar medium

Densities of neutral hydrogen and deuterium are found from observation with the Copernicus satellite of the Ly-alpha line toward two nearby stars. The hydrogen density is 0.03 + or - 0.01 per cu cm toward Alpha Aur (Capella) and 0.20 + or - 0.05 per cu cm in the direction of Alpha Cen A, values indicating that the nearby (less than 14 pc) interstellar medium is inhomogeneous and can be of low density in certain directions. The ratios of deuterium to hydrogen - 3.9 (+5.7, -1.7) by 10 to the -5th power and 0.24 (+0.12, -0.07) by 10 to the -5th power for Alpha Aur and Alpha Cen A, respectively - suggest that variations in the deuterium abundance may exist.

Dupree, A. K.↗
NASA NTRS1977

Search for Jovian auroral hot spots

Auroral emission originating at the foot of the Io-associated flux tube at Jupiter has been detected with a high-resolution spectrometer/telescope on board the Orbiting Astronomical Observatory Copernicus. The emission intensity at Ly-alpha is found to be greater than 100 kR, and the emission is located at zenographic latitudes greater than 65 deg.

Atreya, S. K.↗
NASA NTRS1977

The interstellar medium in the direction of the Crab Nebula - Reconciling soft X-ray and radio observations

The total soft X-ray photoabsorption cross section of the interstellar medium (ISM) in the direction of the Crab Nebula is computed on the basis of a two-phase model of the ISM. This cross section is used to reanalyze Copernicus data on the X-ray spectrum of the Crab between 0.7 and 1.5 keV. The total hydrogen column density along the line of sight to that nebula is found to be approximately 2.6 by 10 to the 21st power H atoms/sq cm. This result is evaluated in light of the two-phase model of the ISM, and the predictions based on the X-ray data are compared with results of radio and UV observations. A discrepancy between the radio and X-ray measurements of the hydrogen column density is resolved by noting that 21-cm absorption measurements sample only the neutral hydrogen in clouds while X-ray measurements are sensitive to all forms of hydrogen in both cloud and intercloud regions. It is suggested that roughly 50% of the hydrogen in the direction of the Crab Nebula is in clouds and that 85% of this hydrogen is neutral and atomic.

Ride, S. K.↗
NASA NTRS1977

Some O I oscillator strengths and the interstellar abundance of oxygen

Calculated and experimental oscillator strengths for the O I intersystem line at 1356 A and for other O I lines of interest in interstellar absorption-line studies are discussed. Attention is given to experimental f-values for the lines at 1302, 1305, and 1306 A, previous work on the f-values for the lines at 1356 and 1359 A, wave-function expansion, and calculations for permitted as well as intercombination lines. Copernicus observations of several interstellar absorption lines due to O I, C II, P II, and Ni II toward Zeta Oph are reported, equivalent widths are determined, and a curve-of-growth analysis is performed for the O I absorption lines. Oscillator strengths are recommended for the far-UV resonance lines of O I, and it is concluded that the oxygen in the interstellar H I regions toward Zeta Oph is depleted by 45% to 69%.

Zeippen, C. J.↗