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At least 1,243 records · Page 69
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NASA NTRS1976

Search for X-ray emission from Nova Cygni 1975

A search for X-rays from Nova Cygni 1975 was carried out before, during, and after the time of optical maximum. No X-rays were detected over the spectral range 0.1-50 keV. On the basis of these results a strong upper limit of .0001 has been placed on the ratio of X-ray to optical luminosity for this nova, consistent with effective temperatures of about 10,000 K. If Nova Cygni 1975 is a virgin nova, its low mass exchange rate would imply that any associated X-ray emission would not be detectable by present techniques.

Hoffman, J. A.↗
NASA NTRS1976

On the white dwarf HZ 43 as an extreme-ultraviolet source

It is confirmed by atmospheric modeling that both the visual continuum and the XUV spectrum (60 to 600 A) of HZ 43 can be fitted self-consistently with the emergent flux from a stellar atmosphere having an effective temperature of about 125,000 K and log g of at least 7. An examination of theoretical white-dwarf cooling sequences and estimates of the stellar luminosity indicate that the parameters of HZ 43 are consistent with a hot white dwarf of 0.6 to 1.2 solar masses. The age is estimated to be as short as 100,000 years, raising the possibility that the star may have ejected a planetary nebula or may be surrounded by an expanding H II region. It is concluded that the detection of a planetary nebula shell or an H II region around HZ 43 may provide an astrophysical test of the direct neutrino-electron interaction.

Durisen, R. H.↗
NASA NTRS1976

On the total energy output of the nova outburst

It is shown that the postmaximum decline in optical light during the outburst of a normal classical nova is caused by a shift in the wavelength of maximum energy to the ultraviolet. The constant-luminosity phase which was directly observed in Nova FH Ser 1970 may, therefore, be common to all novae, regardless of speed class. This implies that all speed classes of novae have similar energy budgets and that it is the total energy involved in the outburst which produces the variation in the observed characteristics of the novae. Recent thermonuclear runaway models for the outburst also show an extended period of high luminosity and high effective temperature following the initial outburst.

Gallagher, J. S.↗
NASA NTRS1976

High intensity solar irradiation testing of UV optics

The Orbiting Solar Observatory-I (OSO-8 in orbit) incorporates two high resolution solar pointing spectrometers operating from 1000 A to 4000 A. Energy from the sun enters a Cassegrainian telescope and is focused on a slit while the solar disk is scanned to one arc-second resolution. The stability of the secondary mirrors reflectance was of concern since they would be exposed to intense focused solar energy up to 27 suns. A test program was initiated to simulate this energy input on sample UV mirrors of the MgF2 and LiF types and to evaluate their performance after irradiation. Tests were conducted to simulate the solar spectrum at high intensities (25 suns) and at a single wavelength near Lyman-alpha, but with twenty times the solar intensity at Lyman-alpha. Post-test measurements after every exposure were made at wavelengths from 1025 A to 1849 A. After 75 simulated 'orbits', reflectance changes due to temperature effects were noted to be less than 10%. Reductions in reflectance under high intensity solar radiation were generally greater than 10%. Polymerization of surface contaminations on the LiF mirrors reduced reflectances at short wavelengths by 40%.

Greyerbiehl, J. M.↗
NASA NTRS1977

Tough strong iron alloys for cryogenic service

Series of alloys with minor additions of reactive metals possesses outstanding strength and toughness at cryogenic temperatures. Effective metal additons include aluminum, niobium, titanium, and vanadium. Strengthening of series is achieved by thermomechanical processing and by precipitate strengthening while maintaining high level of toughness. Possible applications include liquefied natural-gas storage and transmission, structural members in superconducting machinery, and welding rod for other alloys such as nine nickel steels.

Stephens, J. R.↗
NASA NTRS1976

Spectrophotometry of the X-ray binary HZ Herculis

An extensive set of absolute spectrophotometric observations of HZ Her over the wavelength range from 3300 to 10,000 A is presented and analyzed to determine to what extent the observations can be interpreted in terms of a model for the system. The specific model to be tested consists of a large star (HZ Her) with a hot cap beneath the X-ray source, an accretion disk around the X-ray source, and a gas stream flowing between the primary star and the disk. It is found that the model is compatible to a large extent with the detailed observations and that the physical characteristics of the various components can be derived. The mass, luminosity, and radius of the primary star, the mass of the X-ray object, and the orbital dimensions are obtained by analyzing the energy distribution at minimum light and X-ray eclipse data. The angle and effective temperature of the hot cap on the primary are also determined along with the energy distribution, diameter, and thickness of the accretion disk. It is concluded that HZ Her fills its Roche lobe and is an A7 V star, that the logarithm of its surface gravity is about 3.6, and that the mass of the X-ray object is 1.95 solar masses.

Oke, J. B.↗
NASA NTRS1976

Stellar model chromospheres. V - Alpha Centauri A /G2 V/ and Alpha Centauri B /K1 V/

Models for the upper photospheres and lower chromospheres of Alpha Centauri A and B are derived from high-dispersion spectrograms of the Ca II K-line emission cores and damping wings. Effective temperatures, surface gravities, and ages consistent with the measured broadband colors, metallicities, and absolute magnitudes are estimated for the two stars. The spectrograms are calibrated by fitting the far-wing K profiles with synthetic fluxes based on radiative-equilibrium models, and the model atmospheres are obtained through a partial-redistribution analysis of the line cores and damping wings. These models are compared with those previously proposed for Procyon, Arcturus, and the sun. Many features in all the models are found to be quite similar, and some evidence suggests that Alpha Cen A may be significantly older and more evolved than the sun, even though both stars exhibit similar chromospheric properties.

Ayres, T. R.↗
NASA NTRS1976

Theoretical models of Beta Cephei stars constructed with new radiative opacities

Equilibrium models for the main-sequence and early post-main-sequence phases of evolution are constructed for stars of 10.9 and 15 solar masses with two different initial hydrogen and metals abundances. Carson's (1976) radiative opacities are used in the calculations. It is found that the models which lie off the zero-age main sequence have cooler effective temperatures than corresponding models based on other opacities and cross the observed Beta Cephei strip in the H-R diagram only during the main phase of core hydrogen burning. The stability of radial and nonradial pulsations is studied by applying linear nonadiabatic perturbations to the models. The results show that all the modes examined are stable and that the margin of stability decreases with advancing evolution. It is suggested that the closeness of the periods of the two lowest nonradial quadrupole p-modes may be responsible for the 'beat' phenomenon observed in many Beta Cephei stars, if these modes are somehow excited. Possible instability mechanisms are evaluated, particularly the ultimate ionization of CNO elements.

Stothers, R.↗
NASA NTRS1976

An intense extreme-ultraviolet source in Cetus

The detection of a second EUV (100-1000 A) probable white dwarf star with the EUV telescope aboard the Apollo Soyuz mission is reported. The positional error box includes Feige 24, a binary system consisting of a very blue white dwarf and a dMe star. A grid of LTE pure-hydrogen, high-surface-gravity model atmospheres, is used, in combination with the EUV data and optical spectrophotometry, to derive the stellar parameters, yielding an effective temperature of 60,000 K, a radius of 17,000 km at 100 pc, and a luminosity of 7(d/100 pc) squared solar luminosities. The Feige 24 models appear to rule out white dwarfs with pure hydrogen atmospheres if the interstellar helium abundance and ionization are not anomalous.

Margon, B.↗
NASA NTRS1976

Feige 24 - A very hot white dwarf

OAO-2 ultraviolet photometry of the composite spectrum white dwarf Feige 24 is compared with model atmospheres to derive an effective temperature of 70,000 (+20,000, -15,000) K. The distance to this star is estimated to be 90 (+55, -31) pc if the cool component is assumed to be a main-sequence star with a red and near-infrared energy distribution that is normal for its spectral type. The derived characteristics of Feige 24 are compatible with its identification as the extreme-ultraviolet source observed by Margon et al.

Holm, A. V.↗
NASA NTRS1977

Does the upper main sequence extend across the whole H-R diagram

The effect of using Carson's (1976) radiative opacities in evolutionary sequences of stellar models has been studied over the mass range from 7 to 60 solar masses. The opacities are very large in the outer part of the envelope and induce such enormous radii for masses greater than about 30 solar masses for a heavy-element fraction of 0.02 or about 20 solar masses for a heavy-element fraction of 0.04 that the evolutionary tracks during the phase of core hydrogen burning extend across the whole H-R diagram. The choice of the Schwarschild or Ledoux criterion for convection makes very little difference for the behavior of the tracks. Evolution through the effective-temperature range (in logarithms) of 3.6 to 4.0 occurs in all cases on a rapid (secular) time scale. Core helium burning takes place exclusively in the red-supergiant configuration for stellar masses exceeding 8 solar masses (heavy-element fraction of 0.02) or 6 solar masses (heavy-element fraction of 0.04). These stellar models seem to be in significantly better agreement with the observed distribution of bright stars on the H-R diagram than are the older models based on the Cox-Stewart opacities. It can be inferred that a large envelope opacity (e.g., Carson's) exists and that substantial mass loss takes place in very massive late-type supergiants.

Stothers, R.↗
NASA NTRS1977

A search for thermal extreme ultraviolet radiation from nearby pulsars

Results are presented for extreme ultraviolet (100-1000 A) observations of radio pulsars. Using an EUV telescope carried aboard the Apollo-Soyuz mission, data were acquired on the nearby pulsars PSR 1133 + 16, 1451 - 68, and 1929 + 10. None of the objects was detected, with typical upper limits of 6 by 10 to the -26th power erg/sq cm/sec per Hz in the 170-620-A band set on flux incident at earth. The data are interpreted to set limits on the effective temperatures of the neutron stars, yielding values of less than 160,000 K in the best cases, and the limits compared with theoretical predictions.

Greenstein, G.↗
NASA NTRS1977

Apollo-16 far-ultraviolet spectra in the Large Magellanic Cloud

Spectra in the wavelength ranges from 900 to 1600 A and 1050 to 1600 A of some OB associations in the Large Magellanic Cloud were obtained from the lunar surface by the Apollo-16 far-ultraviolet camera/spectrograph on April 22, 1972. The observed spectral distributions appear consistent with a stellar model having an effective temperature of 30,000 K, reddened by E(B-V) = 0.3, and characterized by the average far-ultraviolet extinction curve of Bless and Savage (1972). However, the absolute intensity of the far-ultraviolet spectrum of the associations NGC 2050 and 2055 seems somewhat too bright in comparison with ground-based photometry.

Carruthers, G. R.↗
NASA NTRS1977

The Jovian surface condition and cooling rate

A theory which is almost fully analytic is used to investigate Jupiter's cooling rate. A simple model of contraction with adiabatic interior structure gives a total cooling time to the present in good agreement with the age of the solar system. The interplay between the surface condition and the cooling rate is discussed. The current rate of change of the effective temperature is calculated to be -1 K per 0.145 billion yr. Discrepancies with fully numerical investigations of the Jovian age and cooling rate are noted.

Hubbard, W. B.↗
NASA NTRS1977

Analytical prediction of moisture absorption/desorption in resin matrix composites exposed to aircraft environments

The moisture absorption/desorption behavior of resin matrix composites was mathematically modeled by classical diffusion theory using an effective diffusion coefficient. Good agreement was found between calculated moisture content and published data for T300/5208 graphite fiber reinforced epoxy matrix composite. Weather Bureau data for Langley Air Force Base and Norfolk, Va., were used to calculate the amount of moisture a T300/5208 composite panel would contain if exposed outdoors. Results obtained by using average monthly weather data for several high aircraft usage locations around the world suggest that, except for desert areas, geographical locations should have only minimal effect on the moisture absorption level reached in composites. Solar radiation data together with cloud and wind information were included in the analysis to estimate an effective temperature of the composite panel during ground exposure.

Unnam, J.↗
NASA NTRS1977

The ultraviolet spectra of Alpha Aquilae and Alpha Canis Minoris

Scans of Alpha Aql (A7 IV, V) and Alpha CMi (F5 IV-V) obtained with the Copernicus satellite spectrometer over the wavelength range from 2100 to 3200 A are presented along with a spectrum of the integrated solar disk over the same range procured during a calibrated rocket flight. About 1500 fairly strong absorption lines in the Alpha CMi spectrum between 2400 and 2961 A are identified by comparison with a solar atlas and by using a theoretical spectrum synthesized from a blanketed LTE model with an effective temperature of 6500 K and a surface gravity of 10,000 cm/sec per sec. The Mg II resonance doublet at 2795.528 and 2802.704 A is found to be present in all three stars together with a discontinuity at 2635 A due to Fe II, Fe I, Cr I, and Mn II. It is concluded that the Mg II resonance lines and the 2635-A continuum break would be the best spectral features for estimating the redshift of a galaxy observed at low resolution provided the redshift is not less than about 0.75.

Morton, D. C.↗
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 NTRS1976

Latitudinal variations in the solar wind

There are two classes of latitudinal solar wind variations - those which are an imprint of coronal processes, and those which are internally generated within the solar wind. An axisymmetric description of the latitudinal variations is presented. As an example, a large polar coronal hole is discussed, with attention to directed meridional flow in the corona out to five solar radii, temperature and density characteristics in a coronal hole and beyond, temperature effects in the interplanetary medium, internally generated meridional flow in the interplanetary medium, and the detection of polar hole material in the ecliptic at 1 AU.

Suess, S. T.↗