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At least 1,189 records · Page 66
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NASA NTRS1982

Environmentally induced discharges on satellites

A discharge process whose trigger conditions are a negative exposed metallic surface surrounded by a less negative dielectric, and a large voltage gradient at a dielectric/metal interface is proposed. Analysis of SCATHA data for a discharge substantiates the postulation. Surface discharges cause a small transient charge transfer to space which results in voltage transients. A method of computing these transients, based on the charge lost through the capacitance to space and a fraction of charge stored in the dielectric at the discharge source was developed. It gives an estimate of the discharge transients at the discharge site, which is used as input for coupling code analysis of structure/system response. The transient computations were applied to a three-axis stabilized, geosynchronous satellite for both sunlight and eclipse charging. The energy of the transient pulses are about 1 mJ for sunlight discharge and 8 mJ for eclipse. Changing of selected coatings on the satellite would relieve the stress.

Stevens, N. J.↗
NASA NTRS1983

A threshold effect for spacecraft charging

The borderline case between no charging and large (kV) negative potentials for eclipse charging events on geosynchronous satellites is investigated, and the dependence of this transition on a threshold energy in the ambient plasma is examined. Data from the Applied Technology Satellite 6 and P78-2 (SCATHA) show that plasma sheet fluxes must extend above 10 keV for these satellites to charge in eclipse. The threshold effect is a result of the shape of the normal secondary yield curve, in particular the high energy crossover, where the secondary yield drops below 1. It is found that a large portion of the ambient electron flux must exceed this energy for a negative current to exist.

Olsen, R. C.↗
NASA NTRS1983

Orbital period changes in Centaurus X-3

Two new times of mid-X-ray eclipse for Cen X-3 are presented on the basis of pulse arrival time analyses of pointed observations with SAS 3. When combined with all other published eclipse times based on Doppler delay measurements, the results show that the 2.1d binary period is decreasing at an average rate of 1.8 x 10 to the -6th/yr. The decrease, however, is seen as having significant fluctuations about a smooth, linear decrease. The changes observed in the orbital period can be accounted for by mass loss from the system through the L2 point, although the rates required are implausibly high. It is also shown that the long-term overall orbital decay can readily be interpreted as the result of torques exerted by the tidally distorted companion star (Krzeminski's star) on the orbiting neutron star. It is noted that the inferred asynchronism between the orbital frequency and the rotation frequency of the companion star may be maintained by mass and angular momentum loss in a stellar wind or by a tidal instability related to the Darwin effect. However, this would not provide a natural explanation for any short-term deviations from a constant rate of orbital decay.

Kelley, R. L.↗
NASA NTRS1983

RW Tauri as a weak W Serpentis star

Observations of the short-period eclipsing binary RW Tau made with the LWR and SWP cameras of the IUE satellite and with the ITS scanner at the 3-m Shane telescope of the Lick Observatory in August and October, 1982, are reported and analyzed. At total eclipse, weak excess continuous radiation in the LWR range and emission lines at Si IV (1), C IV (1), Al III (1), Fe III (34), and Mg II (1) in the SWP range were detected. These findings are similar to those for W Ser-type binaries such as U Cep. The parameters of the system are estimated using a bilinearly interpolated atmospheric model of the type developed by Kurucz (1979): primary-star T(eff) = 11,750 K, log g = 4.2, cool-component spectral type = K0 III, E(B-V) color excess = 0.14 mag, system distance = 270 pc. The implications of these findings for proposed models of the W Ser binaries are discussed, and the observation of a possible distant optical companion star 45 arcsec from RW Tau is reported.

Plavec, M. J.↗
NASA NTRS1983

Studies of highly variable galactic X-ray sources with HEAO-1

Analysis HEAO-Al data on MXB1659-29 revealed a 7.1 hour period from this X-ray burst source. The erratic dips seen in both SAS-3 and HEAO-Al data were concentrated within 1.5 hours (about 25%) of the orbital cycle. There appeared to be a stable dip at the end of the 1.5 h of erratic varibility. It was established that the stable dip was a true eclipse of the central X-ray emitting object. MXB1659-29 is the first X-ray burster to show eclipses and a precise orbital clock.

Cominsky, L.↗
NASA NTRS1984

RCA SATCOM Battery in Orbit Performance Update and Accelerated Life Test Results

No significant degradation of nickel cadmium battery performance in SATCOM F1 and F2 after almost 8 and 7-3/4 years in orbit was shown. Battery minimum discharge voltage data are presented for these spacecraft. In addition, 2 groups of nickel cadmium cells which are representative of those in orbit are undergoing real time eclipse-reduced suntime cycling in the laboratory. These groups of cells, which are being cycled at a maximum of 53% and 62% depth of discharge (based on actual capacity), have completed 14 and 15 eclipse seasons, respectively. Data for these groups of cells are presented and are compared with the in-orbit battery data.

Gaston, S. J.↗
NASA NTRS1984

Management and Performance of APPLE Battery in High Temperature Environment

India's first experimental communication satellite, APPLE, carried a 12 AH Ni-Cd battery for supplying power during eclipse. Failure to deploy one of the two solar panels resulted in the battery operating in a high temperature environment, around 40 C. This also resulted in the battery being used in diurnal cycles rather than just half yearly eclipse seasons. The management and performance of the battery during its life of two years are described. An attempt to identify the probable degradation mechanisms is also made.

Suresh, M. S.↗
NASA NTRS1983

Far-ultraviolet emission lines in U Cephei - Evidence for a hot, turbulent circumstellar envelope

The numerous emission lines in the far-UV spectrum which have been found in the Algol-type eclipsing binary U Cephei are similar to those observed in the W Serpentis stars, although they are weaker and appear only during the primary eclipse. The line-emitting region is studied in both emission and absorption.It surrounds the hotter component and extends to a distance of several of its radii. A model is proposed in which the emission originates in a highly turbulent circumstellar region, at an electron temperature of about 10,000 K. The energy needed for ionization is derived from collisions, and is probably extracted from the accretion energy.

Plavec, M. J.↗
NASA NTRS1984

Physical conditions in the cool parts of spicules

The resonance lines of Na, Sr(+), and the Mg triplet system, together with the middle H Balmer lines, are used to determine the plasma parameters in the cool parts of spicules. Results are presented for the heights 2100 and 2500 km according to the chromospheric line emission data reported by Dunn et al. (1968) for the 1962 eclipse. The low flow-velocity solutions require unsatisfactorily high temperature and microturbulence values (the derived thicknesses are also uncomfortably large). The remaining solutions have flow velocities and thicknesses that are in line with expectations from direct spicule observations, and the electron densities are comparable to those obtained from continuum measurements at the 1952 eclipse. However, contrary to most preconceptions, hydrogen is found to be predominantly neutral in these regions; the total gas pressure is consequently quite large - in fact, exceeding the adjacent interspicular pressure by about a factor of 10.

Landman, D. A.↗
NASA NTRS1985

Epsilon Aurigae

In April 1984, fourth contact ended the two year long eclipse of Epsilon Aurigae. An astrometric study of the study of the system was carried out by Van de kamp (1978) leading to the conclusion that the orbit is seen very close to edge on. The eclipse was monitored by a number of groups from the ground and from spacecraft such as the IUE. Ultraviolet observations of the system from IUE have thrown new light on the nature of the system that led to the conclusion that the secondary object is probably a cold, dusty accretion disk surrounding a star that is completely hidden inside the disk.

Chapman, R. D.↗
NASA NTRS1986

EX-111 Thermal Emission from Hot White Dwarfs: The Suggested He Abundance-Temperature Correlation. EX-112: The Unique Emission Line White Dwarf Star GD 356

Progress in the EXOSAT data analysis program is reported. EXOSAT observations for four white dwarfs (WD1031-115, WD0004+330, WD1615-154, and WD0109-264) were obtained. Counting rates were unexpectedly low, indicating that these objects have a substantial amount of x-ray absorbing matter in their photosheres. In addition, soft x-ray pulsations characterized by a 9.25 minute cycle were discovered in the DA white dwarf V471 Tauri. A residual x-ray flux from the K dwarf companion can be seen during the white dwarf eclipse at orbital phase 0.0. Pronounced dips in the soft x-ray light curve occur at orbital phases 0.15, 0.18, and 0.85. The dips may be correlated with the triangular Lagrangian points of the binary orbit. Smaller dips at phases near the eclipse may be associated with cool loops in the K star corona. Data for the white dwarf H1504+65 was also analyzed. This object is particularly unusual in that its photoshere is devoid of hydrogen and helium. Finally, existing data on the white dwarf Sirius B were analyzed to see what constraints from other data can be placed on the properties of this star. Interrelationships between radius, rotational velocity, and effective temperature were derived.

Shipman, H. L.↗
NASA NTRS1985

Stratification of the extended atmosphere of the Wolf-Rayet component of V444 Cygni

Ultraviolet spectra covering both eclipses of V444 Cygni have been obtained. Longward of 1570 angstroms, the continuum brightness showed essentially the same variation seen in the optical (at 4244 angstroms). Striking atmospheric eclipses of up to 0.7 mag in depth were detected, however, in individual strong lines as well as over large ranges of the continuum shortward of 1500 angstroms. IUE data are used to estimate the degree of interstellar extinction and terminal expansion velocity of the system and to study the phase dependence of the continuum and line absorptions.

Eaton, J. A.↗
NASA NTRS1985

Observational tests for sulfur allotropes on Io

It has been established that, if the Io surface is primarily composed of sulfur allotropes, a change in reflectivity at certain wavelengths should be observable shortly after eclipse reappearance (ER). Four ERs were observed during July and August 1983 with a 61-cm telescope on Mauna Kea, Hawaii, and no posteclipse brightening effects in the filter bands selected for sensitivity to color changes in sulfur were found. The proposed model of the brightness change of elemental sulfur (S8) implies that this material covers less than 50 percent of the Io surface. It is concluded that no significant condensation of optically thick SO2 occurred on the Io surface during the four eclipses.

Hammel, H. B.↗
NASA NTRS1985

Discovery of an accretion shock cone in 22 Vulpeculae

A previous report that 22 Vul is a zeta Aurigae-type eclipsing binary is supported by the discovery of evidence for an accretion shock cone in 22 Vul similar to those found in zeta Aurigae and 32 Cygni. Such systems consist of a cool supergiant star primary and a hot dwarf secondary (G3 Ib-II and B9 in the case of 22 Vul) and undergo atmospheric eclipses.

Ahmad, I. A.↗
NASA NTRS1986

Effect of subcooling on the on-orbit pressurization rate of cryogenic propellant tankage

The SOLA-ECLIPSE code is being developed to enable prediction of the behavior of cryogenic propellants in spacecraft tankage. A brief description of the formulations used for modeling heat transfer and for determining thermodynamic state is presented. Code performance is verified through comparison to experimental data for the self-pressurization of scale model liquid hydrogen tanks. SOLA-ECLIPSE is used to examine the effect of initial subcooling of the liquid phase on the self-pressurization rate of an on-orbit full scale liquid hydrogen tank typical for a chemical propulsion Orbital Transfer Vehicle. The computational predictions show that even small amounts of subcooling will significantly decrease the self-pressurization rate. Further, if the cooling is provided by a Thermodynamic Vent System, it is concluded that small levels of subcooling will maximize propellant conservation.

Hochstein, J. I.↗
NASA NTRS1986

Comment on the three-body theory for period changes in RS CVn systems

In the three-body theory for period variations in RS CVn systems, the timing residuals are interpreted as light-travel time differences as the eclipsing system moves about the barycenter of the triple. These residuals can require a larger orbit than Kepler's law allows, given the time scale of the period variations. For only two of eight systems investigated, SV Cam and V471 Tau, is the theory plausible in that the inferred barycentric motion of the binary is smaller than the orbit of the third body, and the inferred properties of the third body are both reasonable and consistent with its remaining hidden. The theory is thus not a general theory for period changes. Observational testing of the theory is straightforward and may lead to the detection of 'brown dwarfs' associated with eclipsing systems through their kinematic effects.

Van Buren, D.↗
NASA NTRS1986

Measuring interstellar grains from the haloes of binary X-ray sources

Coherent forward scattering of X-rays by interstellar grains creates a halo around the X-ray image of a compact source. The fractional halo brightness at 2 keV is typically of order 10% moderately reddened galactic sources. The angular brightness distribution of the halo, which extends over several arcminutes, indicates the size distribution of the grains, and the spectrum of the halo indicates the composition of the grains. The halo will persist for several hours after the point source vanishes during an eclipse of a binary source; this provides a way to avoid systematic errors in measuring halo brightness due to an extended point response function of the X-ray telescope. Indeed, it is possilbe to infer the size distribution and composition of the grains without an imaging X-ray telescope by observing the time-dependence of the halo spectrum during eclipse.

Xu, Y.↗
NASA NTRS1986

Evidence of large-scale structures in the atmosphere of the active K-dwarf component of V471 Tauri

Contemporaneous IUE ultraviolet spectra and visible photoelectric data of the eclipsing binary V471 Tauri (K2V + DA) between 1979 and 1985 was analyzed. The combined data detail the three-dimensional structure of atmospheric loops and their associated starspots on the K dwarf. The distribution of starspot regions on the surface of the K star was inferred from the visible photometry. When spots are located near the limb of the K dwarf prior to and shortly after the total eclipse of the white dwarf, absorption lines such as C II, C III, c IV, and Si IV appear superimposed on the continuum of the white dwarf. These absorption lines are likely caused by cool coronal loops overlying the spots in the atmosphere of the K dwarf. The loops can extend nearly one stellar radius above the surface of the K2V star.

Guinan, E. F.↗