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NASA NTRS1994

Vega: A rapidly rotating pole-on star

High-dispersion (2.4 A/mm), ultrahigh signal-to-noise ratio (3000:1) Reticon spectra of Vega revealed two distinct types of profiles. The strong lines exhibit classical rotational profiles with enhanced wings, but the weak lines have distinctly different, flat-bottomed profiles. Using ATLAS9 model atmopheres and SYNTHE synthetic spectra, Vega has been modeled as a rapidly rotating, pole-on star with a gradient in temperature and gravity over the photosphere. By fitting to the flat-bottomed line profiles of Fe 1 lambda 4528 and Ti 2 lambda 4529, we find least-squares fit values of V sin i = 21.8 plus or minus 0.2 km/sec polar T(sub eff) = 9695 plus or minus 25 K, polar log(base 10)g = 3.75 plus or minus 0.02 dex, V(sub eq) = 245 plus or minus 15 km/sec, and inclination 5 deg .1 plus or minus 0 deg .3. The variations in T(sub eff) and log(base 10)g over the photosphere total 390 K and 0.08 dex, respectively. Assuming V sin i = 21.8 km/sec, an independent fit to the observed continuous flux from 1200 to 10,500 A produced a similar set of values with polar T(sub eff) = 9595 plus or minus 20 K, polar log(base 10)g = 3.80 plus or minus 0.03 dex, and inclination 6 deg .0 plus or minus 0 deg .7.

Gulliver, Austin F.↗

NASA NTRS1994

Optical, IUE, and ROSAT observations of the eclipsing nova-like variable V347 Puppis (LB 1800)

Using time-resolved optical spectroscopy and UBVRI and high-speed photometry obtained at Mount Stromlo Observatory, Mount John University Observatory, and the South African Astronomical Observatory; International Ultraviolet Explorer (IUE) ultraviolet spectroscopy; and Roentgen Satellite (ROSAT) survey X-ray fluxes, we present a study of the accretion disk, hot spot, and emission line regions in the bright eclipsing nova-like variable V347 Pup (LB 1800). In the optical and UV, V347 Pup is a strong emission line source with a continuum spectrum which is remarkably red for a high-M cataclysmic variable. Consistent with its high inclination, we interpret the continuum spectrum as the superposition of the spectrum of the cool (T(sub eff) approximately 7000 K) outer edge and the hot (T(sub eff) approximately 100,000 K) inner regions of a self-eclipsed accretion disk. For the assumed parameters, the model matches the level and shape of the observed spectrum for an inclination of approximately 88 and a distance of approximately 300 pc. The prominent hump in the optical and UV light curves just before eclipse manifests the presence of the hot spot where the accretion stream strikes the edge of the disk. The wavelength dependence of the amplitude of the hump is best modeled by a spot having an effective temperature of approximately 25,000 K and an area of approximately 3 x 10(exp 18) sq cm if the spot radiates like a blackbody, or an effective temperatue of approximately 14,000 K and an area of approximately 3 x 10(exp 19) sq cm if it radiates with a stellar spectrum. In either case, the hot spot produces only one-tenth of the predicted luminosity for the assumed mass-transfer rate of 10(exp -8) solar mass/yr. Either the hot spot is 'buried' in the edge of the accretion disk, or a significant fraction of its luminosity is radiated away in lines. The difference in azimuth between the peak of the hump and the dynamically expected location of the hot spot suggests that the spot's emitting surface is rotated forward by approximately 36 deg relative to the edge of the disk.

Mauche, Christopher W.↗

NASA NTRS1994

The hard X-ray emission spectra from accretion columns in intermediate polars

We consider the hard (greater than 2 keV) X-ray emission from accretion columns in an intermediate polar system, GK Per, using a simple settling solution. The rate of photon emission per logarithmic energy interval can be fitted with a power law, E(exp -gamma), with gamma approximately 2.0, in agreement with observations. This index is only weakly dependent on the mass accretion rate, dot-M, for dot-M in the range of a few times 10(exp 16-18) g/s. The peak energy of the photon spectra (after photoelectric absorption) is expected to be E(sub p) approximately (5 keV) gamma(exp -1/3) (N(sub H)/10(exp 23)/sq cm)(exp 1/3) where N(sub H) is the hydrogen column density along the line of sight. The observed spectra of GK Per and possibly of V1223 Sgr suggest N(sub H) approximately 10(exp 23)/sq cm. This large N(sub H) may be due to partially ionized preshock column material. Alternatively, we also consider absorption by the cool outer parts of an accretion disk. In this case the photoelectric absorption depth in the disk is a sensitive function of inclination. For GK Per the required inclination is approximately 83 deg. For mass accretion rates larger than a critical rate of approximately 10(exp 18) g/s, X-ray emission from the column accretion is significantly affected by radiation drag. Although the mass accretion rate increases dramatically during outbursts, the observed hard (greater than 2 keV) X-ray luminosity will not rise proportionately. The slope and peak energy of the outburst spectra are only weakly affected. We conclude that the observed X-ray spectra can be explained by this simple analytic solution and that the production of hard X-rays from the accretion shock at the magnetic poles in the intermediate polars is in general agreement with the observations. However, since the X-ray emission and absorption depend on the mass accretion rate in a complicated manner, observed hard X-ray luminosities (greater than 2 keV) are not a good indicator of the mass accretion rate in the X-ray emitting regions.

Yi, Insu↗

NASA NTRS1995

HST observations of the ring around SN 1987A

Hubble Space Telescope observations of SN 1987A show that the circumstellar material near the supernova forms a clumpy elliptical ring. We analyze six epochs of images made with the Faint Object Camera (FOC) imaging the lines of (O III) and H beta over the interval 1990 August-1993 October, and three epochs of observations with the Planetary Camera in the continuum and (N II) line over the interval 1992 April-1993 May. The elliptical ring in the (O III) images has a semimajor axis of 858 +/- 11 milliarcsec and a semiminor axis of 621 +/- 11 milliarsce (6.4 x 10(exp 17) and 4.6 x 10(exp 17) +/- 8.1 x 10(exp 15) cm, respectively, for a distance of 50 kpc), which implies an inclination angle of 44 +/- 1, if the ring is a circle inclined to our line of sight. The apparent width of the ring in (O III) is 121 +/- 22 milliarcsec (9.0 x 10(exp 16) +/- 1.6 x 10(exp 16) cm) based on our deconvolved image, which represents an upper limit to the width of the of the ring. The position angle of the major axis on the sky is 89 deg +/- 3 deg. One of the 'clumps' on the ring is a superposed star, whose energy distribution is consistent with an A-type dwarf (V = 20.2 mag) in the Large Magellanic Cloud (LMC) The radial profile of the ring is not symmetric, with the inner boundary sharper than the outer. Based on the radial profiles and the flux interior to the ring, we conclude that the ring is indeed a tilted ring, and not a spheroidal shell. While the observed width of the ring is approximately 10(exp 17) cm, we find that the average path length through the emitting gas is approximately 4 x 10(exp 15) cm, so the filling factor is small. The ring has faded by a factor of approximately equals 3.7 in (O III) from 1990 August to 1993 October. The characteristic electron density from the (O III) lambda 5007 emission line averages 1.2 x 10exp 4)/cu cm, based on recombination models of the emission. Because the fading rate is uncorrelated with the surface brightness, the density appears to be independent of the brightness, which implies significant path length differences through the ring. By assuming the current structure observed in optical lines applies to the early structure in ultraviolet lines, we construct light curve models to compare with early IUE observations. The clumpy structure of the ring has relatively little effect on the light curves. The (O III) images show faint arclike nebulosity probably associated with the bipolar nebula around SN 1987A and projected inside the ring, which may have affected previous observations of the ring. Finally, we find that the fading rate provides evidence, though weak, favoring an expanding ring over a contracting one.

Plait, Philip C.↗

NASA NTRS1994

Quasi-periodic oscillations of the magnetopause during northward sheath magnetic field

The Geotail satellite quasi-periodically crossed the dawn flank of magnetopause more than ten times during an interval of 1.5 hours on November 4, 1992. Magnetopause crossings were characterized by quasi-periodic pulses of a sawtooth wave form in the magnetic field and the plasma flow components tangential to the magnetopause. The magnetic field strength in the magnetosheath was larger than that in the magnetosphere. The direction of magnetic field outside the magnetopause current layer was northward with antisunward tilt, indicating the draping of magnetic field on the magnetopause. Boundary normals of wavy magnetopause systematically incline sunward on the upstream side, while they tend to incline antisunward with considerable deviation on the downstream side. Comparison with other multiple crossing events suggests that the November 4 event exhibits wavy structure of the dawn flank magnetopause associated with the northward interplanetary magnetic field (IMF).

Kokubun, S.↗

NASA NTRS1994

Baroclinic instability in the interiors of the giant planets: A cooling history of Uranus?

We propose a quasigeostrophic, baroclinic model for heat transport within the interior of a stably stratified Jovian planet, based on motion in thin cylindrical annuli. Density decreases from the center outward and is zero at the surface of the planet. In the homogeneous case (no core), we find instability for the poles hotter than the equator, but not for the reverse. If the motion is bounded by an impenetrable core, instability occurs for both cases. Much of the behavior can be explained by analogy to conventional baroclinic instability theory. Motivated by our results, we explore a possible connection between the highly inclined rotation axis of Uranus and its anomalously low surface heat flux. We assume that the planets formed hot. Our conjecture is that heat was efficiently convected outwards by baroclinic instability in Uranus (with the poles hotter than the equator), but not in the other three Jovian planets. The surface temperature was higher for the stably stratified case (Uranus), leading to a higher rate of infrared emission and faster cooling. Therefore, we propose that Uranus lost its internal heat sooner than Neptune because baroclinic motions, permitted by its inclination to the sun, were able to extract its internal heat while the surface was still warm.

Holme, Richard↗

NASA NTRS1995

Asteroid orbit evolution due to thermal drag

Thermal drag, a variant of the Yarkovsky effect, may act on small asteroids with sizes from a few meters to a few tens of meters. Yarkovsky thermal drag comes from an asteroid's absorbing sunlight in the visible and reradiating it in the infrared. Since the infrared photons have momentum, by action-reaction, they kick the asteroid when they leave its surface. The reradiation, which is asymmetric in latitude over the asteroid, gives a net force along the asteroid's pole. Due to the asteroid's thermal inertia, averaging this force over one orbital period produces a net drag if the spin axis has a component in the orbital plane. Thermal drag tends to circularize orbits. It can increase or decrease orbital inclinations. An object whose spin axis points in random directions over its lifetime displays little change in orbital inclination. Thermal drag appears to have little to do with the delivery of chondrites from the asteroid belt; the thermal drag timescale (10(exp 8) years for meter-sizzed objects) is long compared with their cosmic ray exposure ages, and aphelia in the asteroid belt are not expected for mature thermal drag orbits. However, Yarkovsky thermal drag may act on the recently discovered near-Earth asteroids, which have radii of 10-30 m. Asteroid 1992 DA, for instance, might have its orbit shrunk by 0.1 AU in 3 x 10(exp 7) years, removing it from an Earth-crossing orbit. The near-Earth asteroids also tend to have small to moderate orbital eccentricities, as expected for highly evolved thermal drag objects. However, the time needed to bring them in from the asteroid belt (about 10(exp 9) years) is long compared with the collisonal and dynamical lifetimes (both about 10(exp 8) years) for Earth-crossing objects, arguing against their emplacement by thermal drag.

Rubincam, David Parry↗

NASA NTRS1995

Geometry and pulse profiles of x-ray pulsars: Asymmetric relativistic fits to 4U1538 - 52 and vela x-1

Model calculation of inhomogenuous magnetized neutron star atmospheres are used to analyze the phase-dependent spectra of 4U 1538-52 and Vela X-1 taking into account general relativistic effects. A chi (exp 2) fitting procedure is used on the Ginga data for these objects to determine the geometry and the magnetic structure of the polar caps, as well as their size and location on the surface of the stars. Other parameters determined by the fit are the masses and radii of neutron stars, the accretion rates, and the rotation and magnetic inclination angles including an azimuthal offset to reproduce the asymmetry of the pulse shapes. We find that general relativistic effects play a significant role in determining the derived cap sizes, the fluxes, and the observational appearance of the pulses. There is evidence that the polar caps are unequal and nonantipodal, suggesting either an off-center or bent magnetic axis, or a strong nondipole component. The magnetic field structure is approximated by a two-component model. The rotation axes are inferred to be moderately close to the mean magnetic axes and seen at large inclinations with respect to the line of sight.

Bulik, T.↗

NASA NTRS1995

Emission line signatures of anisotropic turbulence in accretion disks

We investigate observable effects of anisotropic turbulence on the velocity profiles and eclipse behavior of emission lines from accretion disks. Turbulence expands the local line broadening profile, enhancing the surface brightness of saturated emission lines. Anisotropic turbulence produces anisotropic emission in such lines. The effects become observable when the turbulence exceeds the thermal velocity. Each term in the velocity-velocity correlation matrix produces a distinctive azimuthal pattern of enhanced emission-line surface brightness on the face of the accretion disk. These patterns express themselves as changes in the observable shapes of the disk's emission lines. The best place to look for turbulence effects is in saturated emission lines of heavy elements such as Ca, Mg, and Fe, which have a smaller thermal velocity at a given sound speed and at moderate inclination (60-70 degrees), since the Keplerian shear broadening dominates at higher inclinations.

Horne, Keith↗

NASA NTRS1995

Evaporation of binary mixtures in microgravity

The motivation of this research is to obtain a better understanding of phase-change heat transfer within single and binary liquid meniscii, both in 1-g and 0-g environments. During phase 1 and part of phase 2, in a glass test cell with an inclined heated plate, 1-6 experiments on pentane with additions of decane up to 3% were conducted to determine the optimum concentration that will exhibit the maximum heat transfer and stability. During phase 2 emphasis was given to explore fundamental research issues and to ultimately develop a reliable capillary pumped loop (CPL) device for low gravity. In related experimental work, it was found that thermocapillary stresses near the contract line could result in a degraded wettability which ultimately could explain the observed failure of CPL devices in zero-gravity environment. Therefore, the current experimental effort investigates the effect of adding binary constituents in improving the thermocapillary characteristics near the contact line within the loop configuration. Achievements during second phase include: (1) Further enhancement of Central State University's Microgravity Laboratory by adding or improving upon capabilities of photography, video imaging, fluid visualization, and general experimental testing capabilities; (2) Experimental results for the inclined plate cell; (3) Modeling effort with a detailed scaling analysis; (4) Additional testing with a tube loop configuration to extend experimental work by Dickens, et al.; (5) Fabrication of a capillary loop to be tested using binary fluid (pentane/decane). The device that has been recently completed will be set up horizontally so that the effect of gravity on the performance is negligible. Testing will cover a wide range of parameters such as decane/pentane concentration, heat input value, heat input location (below or above meniscus), and loop temperature.

Girgis, Morris↗

NASA NTRS1995

Reusable LH2 tank technology demonstration through ground test

The paper presents the project plan to demonstrate, by March 1997, the reusability of an integrated composite LH2 tank structure, cryogenic insulation, and thermal protection system (TPS). The plan includes establishment of design requirements and a comprehensive trade study to select the most suitable Reusable Hydrogen Composite Tank system (RHCTS) within the most suitable of 4 candidate structural configurations. The 4 vehicles are winged body with the capability to deliver 25,000 lbs of payload to a circular 220 nm, 51.6 degree inclined orbit (also 40,000 lbs to a 28.5 inclined 150 nm orbit). A prototype design of the selected RHCTS is established to identify the construction, fabrication, and stress simulation and test requirements necessary in an 8 foot diameter tank structure/insulation/TPS test article. A comprehensive development test program supports the 8 foot test article development and involves the composite tank itself, cryogenic insulation, and integrated tank/insulation/TPS designs. The 8 foot diameter tank will contain the integrated cryogenic insulation and TPS designs resulting from this development and that of the concurrent lightweight durable TPS program. Tank ground testing will include 330 cycles of LH2 filling, pressurization, body loading, depressurization, draining, and entry heating.

Bianca, C.↗

NASA NTRS1995

Analysis of the heliospheric current sheet at Earth's orbit and model comparisons

IMP 8 magnetic field data for the first half of the year 1994, i.e., for about 6 solar rotations, are analyzed around regions of sector boundary crossings with the purpose of obtaining both gross- and fine-scale characteristics of the related heliospheric current sheets separating the observed sectors. For purposes of estimating the attitudes of the normals to the sector boundaries. analysis intervals (sometimes 30 min or more in length) allowing the field to fully complete an excursion of about 180 deg were used in the study, which consisted of variance analyses of the field within those intervals. The resulting boundary normals were analyzed and compared to known (generic) models of projected heliospheric current sheets and to a coronal field model for the same time period. One of the most outstanding features of the resulting ensemble of estimated boundary normals for this period is that they strongly prefer low inclinations, indicating that the observations do not support a 1 AU model that predicts a current sheet whose surface is approximately parallel with the sun's equator, such as the 'sombrero' model. They instead support a model that predicts a relatively high inclination current sheet at 1 AU. Also the normals assume a surprisingly large range of longitudes, somewhat favoring those consistent with a Parker model (45 deg and 225 deg) and/or radial alignment (0 deg and 180 deg). These boundary structures, as defined, are shown typically to be as broad as several hundred proton gyroradii, but having embedded within them very thin structures associated with stronger currents. Such thin structures have normals usually differing markedly from the gross boundary. For some crossings there are indications of a wave-like structure in the current sheet as it passed the spacecraft.

Lepping, R. P.↗

NASA NTRS1995

System and antenna design considerations for highly elliptical orbits as applied to the proposed Archimedes Constellation

The paper discusses various aspects of the system design for a satellite in a highly elliptical inclined orbit, and presents a number of antenna design options for the proposed Archimedes mission. A satellite constellation was studied for the provision of multi media communication services in the L and S Band for northern latitudes. The inclined elliptical orbit would allow coverage of Europe, America, and East Asia. Using Canada and North America as the baseline coverage area, this paper addresses system considerations such as the satellite configuration and pointing, beam configuration, and requirements for antennas. A trade-off is performed among several antenna candidates including a direct radiating array, a focal-fed reflector, and a single reflector imaging system. Antenna geometry, performance, and beam forming methods are described. The impact of the designs on the antenna deployment is discussed.

Paynter, C.↗

NASA NTRS1995

Key design issues for user terminals operating with HEO satellites

Coverage of high-latitude areas in the northern hemisphere using geostationary satellites can not be achieved with good elevation angles, and highly inclined elliptical orbits (HEO) present one possible way to overcome this problem. The high elevation angles they achieve and the relative motion of the satellites across the sky, including satellite hand-over, present a specific set of requirements for the design of user terminals, be they for communication or broadcast-reception purposed. This paper reviews key design issues for user terminals operating with satellites in highly inclined elliptical orbits. While all aspects of user terminal design are broadly covered, special emphasis is given to the antenna and RF front-end design. The types of terminals discussed include communications terminals: the hand-held, the notebook, and the vehicle-mounted types, as well as the vehicle-mounted digital audio broadcasting (DAB) receiver terminal. A number of solutions are outlined, based on tradeoffs with respect to the most significant system interface parameters such as G/T and equivalent isotropically radiated power (EIRP). Candidate antenna and front-end configurations were studied, in particular patch and helical antennas with the principal aim of deriving low cost, yet effective solutions. Issues related to achieving best EIRP for the communications terminals are presented, taking into account radiation safety limits. The differences in the design of receive only versus receive/transmit RF front-ends for HEO system terminals are highlighted and solutions for both proposed. This paper summarizes the results of internal ESTEC studies and external ESA-sponsored work concerning major user terminal parameters and design aspects performed for Archimedes -- a HEO satellite constellation for multimedia broadcasting and communications.

Stojkovic, I.↗

NASA NTRS1994

Experiment K-7-41: Radiation Experiments on Cosmos 2044

The Cosmos 2044 biosatellite mission offered the opportunity for radiation measurements under conditions which are seldom available (an inclination of 82.3 degrees and altitude of 294 x 216 km). Measurements were made on the outside of the spacecraft under near-zero shielding conditions. Also, this mission was the first in which active temperature recorders (the ATR-4) were flown to record the temperature profiles of detector stacks. Measurements made on this mission provide a comparison and test for modeling of depth doses and LET spectra for orbital parameters previously unavailable. Tissue absorbed doses from 3480 rad (252 rad/d) down to 0.115 rad (8.33 mrad/d) were measured at different depths (0.0146 and 3.20 g/sq. cm, respectively) with averaged TLD readings. The LET spectra yielded maximum and minimum values of integral flux of 27.3 x 10-4 and 3.05 x 10(exp -4) cm(exp -2).s(exp -1).sr(exp -4) of dose rate of 7.01 and 1.20 mrad/d, and of dose equivalent rate of 53.8 and 11.6 mrem/d, for LET(infinity).H2O is greater than or equal to 4 keV/micro-m. Neutron measurements yielded 0.018 mremld in the thermal region, 0.25 mrem/d in the resonance region and 3.3 mrem/d in the high energy region. The TLD depth dose and LET spectra have been compared with calculations from the modeling codes. The agreement is good but some further refinements are in order. In comparing measurements on Cosmos 2044 with those from previous Cosmos missions (orbital inclinations of 62.8 degrees) there is a greater spread (maximum to minimum) in depth doses and an increased contribution from GCR's, and higher LET particles, in the heavy particle fluxes.

Benton, E. V.↗

NASA NTRS1997

Static Investigation of a Multiaxis Thrust-Vectoring Nozzle With Variable Internal Contouring Ability

The thrust efficiency and vectoring performance of a convergent-divergent nozzle were investigated at static conditions in the model preparation area of the Langley 16-Foot Transonic Tunnel. The diamond-shaped nozzle was capable of varying the internal contour of each quadrant individually by using cam mechanisms and retractable drawers to produce pitch and yaw thrust vectoring. Pitch thrust vectoring was achieved by either retracting the lower drawers to incline the throat or varying the internal flow-path contours to incline the throat. Yaw thrust vectoring was achieved by reducing flow area left of the nozzle centerline and increasing flow area right of the nozzle centerline; a skewed throat deflected the flow in the lateral direction.

Wing, David J.↗

NASA NTRS1998

OORT-Cloud and Kuiper-Belt Comets

This paper follows the broadly accepted theory that Oort-Cloud Comets originated in the Solar Nebula in the general region where the major planets, Jupiter and Saturn, were formed while the Kuiper-Belt Comets originated farther out where the temperatures were lower. The Oort-Cloud Comets are identified orbitally by long periods and random inclinations and, including the Halley-type comets, comets with a Tisserand Criterion less than 2.0. Kuiper-Belt comets are identified by short periods, usually much less than 200 years, and small inclinations to the ecliptic. Here two criteria for comet activity are found to separate the two classes of comets. These quantities NG1 and NG2, were intended to measure theoretical nongravitaional effects on comet orbits. They are only, mildly successful in correlations with observed cases of measured non-gravitational forces. But, in fact, their variations with perihelion distance separate the two classes of comets. The results are consistent with the theory that the activity or intrinsic brightness of Oort-Cloud Comets fall off faster with increasing perihelion distance that does the intrinsic brightness of short-period Kuiper-Belt Comets.

Whipple, Fred L.↗

NASA NTRS1961

The Effect of Lateral- and Longitudinal- Range Control on Allowable Entry Conditions for a Point Return from Space

The problem of return to a specified landing point on the earth from flight in space is considered by studying the interaction between an assumed control over the lateral and longitudinal range and the initial conditions of approach to the earth, given by orbital-plane inclination, vacuum perigee location, and time of arrival. The maneuvering capability in the atmosphere permits a point return for a range of entry conditions. A lateral-range capability of +/- 500 miles from the center line of an entry trajectory can allow a variation in the time of arrival of over 3.5 hours. Variation in the orbital-plane inclination angle can be as much as +/- 13 deg.

Boissevain, Alfred G.↗