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At least 649 records · Page 36
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NASA NTRS1990

Photometry of variable AFGL sources

Results of infrared photometric observations of 63 AFGL sources over the past 9 years are presented. Using these data together with previous measurements of these stars, pulsation periods and mean photometric characteristics are determined. These stars are found to lie midway between optically identified Mira variables and the radio-luminous OH/IR stars in their period distribution and photometric properties. For the sample as a whole, there is no evidence for sudden or transient behavior such as a switch in pulsation mode. Rather, these stars suggest rapid, but continuous, evolution from shorter period Miras with weak mass loss to longer periods and larger mass-loss rates. The carbon-rich stars in the sample have the same period distribution as the oxygen-rich stars. None of the carbon stars have periods as long as those of the very long period radio-luminous OH/IR stars.

Jones, Terry Jay↗
NASA NTRS1991

Classical Cepheid luminosities from binary companions

Luminosities for the classical Cepheids Eta Aql, W Sgr, and SU Cas are determined from IUE spectra of their binary companions. Spectral types of the companions are determined from the spectra by comparison with the spectra of standard stars. The absolute magnitude inferred from these spectral types is used to determine the absolute magnitude of the Cepheid, either directly or from the magnitude difference between the two stars. For the temperature range of the companions (A0 V), distinctions of a quarter of a spectral subclass can be made in the comparison between the companions and standard stars. The absolute magnitudes for Eta Aql and W Sgr agree well with the period-luminosity-color relation of Feast and Walker (1987). Random errors are estimated to be 0.3 mag. SU Cas, however, is overluminous for pulsation in the fundamental mode, implying that it is pulsating in an overtone.

Evans, Nancy Remage↗
NASA NTRS1990

The contribution of the preparation of the Hipparcos mission to solving the evolutionary problems of LPV's

The pulsation modes, the long-term behavior, and the evolutionary status of some long-period variable stars are discussed with reference to the results of investigations carried out as part of the preparation for the Hipparcos mission. In particular, attention is given to three different methods used to predict the brightness variations of large-amplitude (greater than 2 mag) LPVs. The discussion also covers statistical results relating to the evolutionary problems of LPVs and preliminary data on the pulsation mode.

Barthes, D.↗
NASA NTRS1991

The unusual X-ray pulse timing of AE Aquarii

X-ray data for AE Aqr are reanalyzed by using a refined analysis technique and different approaches. The results obtained indicate that X-rays are produced at the same site as the optical, and there is no evidence for the direct X-ray beam from the white dwarf. The optical period was found to be the most significant X-ray period. The direct X-ray beam from the white dwarf is at least four times weaker than the pulsed X-ray signal at the optical period. Given the above-mentioned correlations with the optical, it is inferred that the X-ray pulsations must originate at the same target in the orbit responsible for the optical pulsations. There is a possible orbital modulation of the optical pulse-timing and emission-line orbits. It is shown that the X-ray pulse has slipped at least twice in phase by more than 30 percent in two years' time.

De Jager, O. C.↗
NASA NTRS1992

The 144 second periodic flux variations during x ray turn-on of Hercules X-1

Hercules X-1 is a well known bright binary X ray pulsator. It has a 1.70 day orbital period, a pulsation period of 1.24 second, and a 35 day semiperiodic variability. The discovery is reported of a new 144 second periodicity in the X ray emission from Her X-1. The periodicity is seen in X ray observations of Her X-1 by the LAC instrument onboard the Ginga satellite during Aug. to Sep. 1988. The periodic flux variations occur during the time of X ray turnon at the beginning of a high state of Her X-1, in the same time that a pre-eclipse dip also occurs. An analysis of the LAC spectra of Her X-1 during this period is also presented. Large changes in spectral shape occur associated with the dip.

Leahy, D. A.↗
NASA NTRS1991

Electromagnetic ion cyclotron waves observed in the plasma depletion layer

Observations from AMPTE/CCE in the earth's magnetosheath on October 5, 1984 are presented to illustrate 0.1 - 4.0 Hz magnetic field pulsations in the subsolar plasma depletion layer (PDL) for northward sheath field during a magnetospheric compression. The PDL is unambiguously identified by comparing CCE data with data from IRM in the upstream solar wind. Pulsations in the PDL are dominated by transverse waves with F/F(H+) 1.0 or less and a slot in spectral power at F/F(H+) = 0.5. The upper branch is left hand polarized while the lower branch is linearly polarized. In the sheath the proton temperature anisotropy is about 0.6 but it is about 1.7 in the PDL during wave occurrence. The properties and correlation of waves with increased anisotropy indicate that they are electromagnetic ion cyclotron waves.

Anderson, B. J.↗
NASA NTRS1992

The dynamic cusp

Results of a study of satellite and ground-based data acquired near the dayside cusp during a period when the IMF was inferred to have a strong northward component are presented. The presence of Pc 1 bursts, Pc 4-5 pulsations, and a tailward traveling twin vortex pattern of ionospheric convection suggests that the magnetosphere may have been temporarily compressed. Magnetic field data acquired at a synchronous altitude from GOES 5 and on the ground from Huanacayo support this suggestion. Plasma with ion dispersion characteristics associated with a cusp during southward IMF was detected by Viking over a 3.5-deg range of latitude. The presence of standing Alfven waves and ring current ions suggests that this 'cusplike' plasma was observed on closed geomagnetic field lines. It is suggested that the magnetosphere, during a northward IMF, is temporarily compressed by a solar wind pressure enhancement that produces the Pc 1 bursts, Pc 4-5 pulsations, and ionospheric vortices.

Potemra, T. A.↗
NASA NTRS1990

Observations of filamentary field-aligned current coupling between the magnetospheric boundary layer and the ionosphere

A distinct class of dayside high-latitude magnetic pulsations can be identified from the spatial characteristics of the disturbance field. These pulsations exhibit traveling radial patterns such as would result from moving filaments of field-aligned current interacting with the ionosphere to produce cells of Hall current and vortexlike plasma flow. Time intervals containing a series of continuous multiple vortices are investigated here. The vortices occur on the boundary between sunward and antisunward ionospheric plasma convection. Low altitude DMSP satellite particle measurements indicate that the vortices are on magnetic field lines which map to the inner edge of the magnetospheric low latitude boundary layer. No repetitive solar wind disturbance (e.g., pressure variations) appears to be associated with the events, suggesting that the vortices are related to a local magnetospheric instability. No strong correlation between interplanetary field conditions and the detection of vortices is found.

Clauer, C. R.↗
NASA NTRS1992

Rosat sky survey observations of the eclipsing binary V471 Tauri

Rosat observations of the DA white dwarf + K2V binary system V471 Tauri, obtained during the sky survey phase of the mission, are presented. A lower amplitude shorter time-scale variability is seen in both the soft X-ray and EUV bands. This is associated with the white dwarf pulsations previously discovered by Exosat and also observed at optical wavelengths. The minimum in the EUV light curve is found to coincide with the maximum in the optical. This direct comparison of the phases of the optical and EUV pulses confirms the prediction made by an earlier indirect comparison and shows conclusively that the V471 Tau oscillations cannot arise from nonradial g-mode pulsations in the white dwarf. They are argued to be caused by rotation of the white dwarf with accretion-darkened magnetic poles. On the basis of the EUV and optical pulse shapes, the accretion geometry is studied, and it is estimated that the rate of accretion onto the white dwarf is about (4-11) x 10 exp -13 solar mass/yr.

Barstow, M. A.↗
NASA NTRS1992

The binary Cepheid BP Cir - A test of evolutionary tracks

IUE spectra of the system containing the Cepheid BP Cir are used to derive the magnitude difference between the two stars (Delta V = 1.88 +/- 0.3 mag) and the spectral type of the companion (B6.0V). This is the first system yet encountered for which the standard Seaton reddening law is inappropriate. For BP Cir, a stronger 2200-A bump than the standard law is required. However, this does not affect the determination of the temperature of the hot star or the magnitude difference between the two stars, because those use the wavelength regions shorter than 1600 A and longer than 2500 A. The stars are very similar in mass. Possible pulsation in the first overtone mode implies that BP Cir B is evolved beyond the ZAMS but is still in the main sequence band. The best match to BP Cir A and B is with isochrones by Stothers and Chin (1991) with Inglesias and Rogers (1991) opacities and no core convective overshoot and Cepheid pulsation in the first overtone.

Evans, Nancy R.↗
NASA NTRS1992

Whole Earth Telescope observations of V471 Tauri - The nature of the white dwarf variations

Time-series photometric observations of the binary star V471 Tauri were conducted using the Whole Earth Telescope observing network. The purpose was to determine the mechanism responsible for causing the 555 and 277 s periodic luminosity variations exhibited by the white dwarf in this binary. Previous observers have proposed that either g-mode pulsations or rotation of an accreting magnetic white dwarf could cause the variations, but were unable to decide which was the correct model. The present observations have answered this question. Learning the cause of the white dwarf variations has been possible because of the discovery of a periodic signal at 562 s in the Johnson U-band flux of the binary. By identifying this signal as reprocessed radiation and using its phase to infer the phase of the shorter wavelength radiation which produces it, made it possible to compare the phase of the 555 s U-band variations to the phase of the X-ray variations. It was found that U-band maximum coincides with X-ray minimum. From this result it was concluded that the magnetic rotator model accurately describes the variations observed, but that models involving g-mode pulsations do not.

Clemens, J. C.↗
NASA NTRS1992

Cheap System Supplies Liquid At Steady High Pressure

High-pressure pumps and pulsation-damping devices not needed. Compressed gas provides pressure in liquid-supply system. Proposed to use system to inject alcohol in fuel-additive tests. Principal advantage of system is elimination of costs of high-pressure pumps and of accompanying costs of pulsation-damping devices.

Morse, C. Robert↗
NASA NTRS1992

Ultraviolet studies of Cepheids

We discuss whether with new evolutionary tracks we still have a problem fitting the Cepheids and their evolved companions on the appropriate evolutionary tracks. We find that with the Bertelli et al. tracks with convective overshoot by one pressure scale height the problem is essentially removed, though somewhat more mixing would give a better fit. By using the results of recent nonlinear hydrodynamic calculations, we find that we also have no problem matching the observed pulsation periods of the Cepheids with those expected from their new evolutionary masses, provided that Cepheids with periods less than 9 days are overtone pulsators. We investigate possible mass loss of Cepheids from UV studies of the companion spectrum of S Mus and from the ultraviolet spectra of the long period Cepheid l Carinae. For S Mus with a period of 9.6 days we derive an upper limit for the mass loss of M less than 10(exp -9) solar mass, if a standard velocity law is assumed for the wind. For l Carinae with a period of 35.5 days we find a probable mass loss of M is approximately 10(exp -5+/-2) solar mass.

Boehm-Vitense, Erika↗
NASA NTRS1991

Liquid oxygen/metal gelled monopropellants

The objectives of this program were to establish the feasibility of metallized/liquid oxygen monopropellants and select the best monopropellant formulation for continued study. The metal powders mixed with the liquid oxygen were aluminum/magnesium (80/20), silicon and iron (Iron was only tested for burning properties). The formulations were first evaluated on whether they detonated when ignited or burned. The formulations only burned when ignited. The viscosity for the formulations ranged from 900 cps to 100 cps at shear rates up to 300 seconds(sup -1). Two percent (by weight) of Cab-O-Sil was added to the aluminum and aluminum/magnesium formulations for gelling while the silicon formulation used three percent. Within a seven hour period, settling was suggested only in the 29 percent aluminum and 29 percent aluminum/magnesium formulations. The monopropellants were burned in a cylinder submerged in a liquid nitrogen bath. Experimental data at ambient pressure indicated that the monopropellants were extinguished when the flame front reached regions submerged under the liquid nitrogen. The burn rate increased dramatically when burned in a cylinder enclosure with less heat sink available to the monopropellant. The test results were inconclusive as to whether the increased burn rate was due to the lower heat sink capacity or the small amount of pressure (2 psi) generated during the burning of the monopropellant. The burning of the aluminum and aluminum/magnesium resulted in a brilliant white flame similar to that of an arc welder. These monopropellants burned in a pulsating manner with the aluminum/magnesium appearing to have less pulsating combustion. The silicon monopropellant burned with an orange glow. No sparks or energetic burning was apparent as with the aluminum or aluminum/magnesium.

Wickman, John H.↗
NASA NTRS1992

The infrared counterpart of the eclipsing X-ray binary HO253 + 193

We report the identification of the infrared counterpart of the pulsating X-ray source HO253 + 193. It is a highly reddened star varying in K light with a period near 3 hr, but an apparent even-odd effect in the light curve implies that the true period is 6.06 hr. Together with the recent report of X-ray eclipses at the latter period, this establishes the close binary nature of the source. Infrared minimum occurs at X-ray minimum, certifying that the infrared variability arises from the tidal distortion of the lobe-filling secondary. The absence of a point source at radio wavelengths, plus the distance derived from the infrared data, suggests that the binary system is accidentally located behind the dense core of the molecular cloud Lynds 1457. The eclipses and pulsations in the X-ray light curve, coupled with the hard X-ray spectrum and low luminosity, demonstrate that HO253 + 193 contains an accreting magnetic white dwarf, and hence belongs to the 'DQ Herculis' class of cataclysmic variables.

Zuckerman, B.↗
NASA NTRS1992

Pc3 activity at low geomagnetic latitudes - A comparison with solar wind observations

On an hourly time-scale the different roles of the solar wind and interplanetary magnetic field (IMF) parameters on ground micropulsation activity can be better investigated than at longer time-scales. A long-term comparison between ground measurements made at L'Aquila and IMP 8 observations confirms the solar wind speed as the key parameter for the onset of pulsations even at low latitudes, although additional control of the energy transfer from the interplanetary medium to the earth's magnetosphere is clearly exerted by the cone angle. Above about 20 mHz the frequency of pulsations is confirmed to be closely related to the IMF magnitude while, in agreement with model predictions, the IMF magnitude is related to the amplitude of the local fundamental resonant mode. We provide an interesting example in which high resolution measurements simultaneously obtained in the foreshock region and on the ground show that external transversal fluctuations do not penetrate deep into the low latitude magnetosphere.

Villante, U.↗
NASA NTRS1993

Nonlinear evolution of electromagnetic ion beam instabilities

A comparative study of the ion/ion right-hand resonant instability and the ion/ion nonresonant instability is carried out to investigate and contrast their properties. Linear analysis demonstrates that the nonresonant instability becomes resonant if the density of the ion beam is sufficiently high. Hybrid simulations show that both the resonant and nonresonant instabilities result in the formation of nonlinear pulses, called pulsations, but with distinct features. For example, the pulsations generated by the resonant instability have a positive correlation with the ion density, while those generated by the nonresonant instability are likely to have a relatively weak negative correlation. The waves generated by the nonresonant instability are subject to a parametric decay instability and tend to form a state of condensate where the turbulence becomes nearly monochromatic.

Akimoto, K.↗