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At least 955 records · Page 53
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Nonlinear calculation of a radial mode of oscillation of a hot white dwarf

A linearly unstable first harmonic radial mode of oscillation (period 0.86 s) of a hot white dwarf with a hydrogen envelope is analyzed in the nonlinear regime by application of the Stellingwerf technique to a nonlinear hydrocode. The mass and equilibrium luminosity of the hot white dwarf are 1.3 solar masses, and 1.6 x 10 to the 37th erg/s, respectively. A stable nonlinear mode is found to exist, in which appreciable shock damping occurs at and near the surface - the shock damping being the dominant stabilizing effect. This enhances the possibility that such an object can produce a pulsating X-ray source.

Degregoria, A. J.↗
NASA NTRS1979

Soft X-ray observations of degenerate dwarfs and cataclysmic variables

Observations in outburst of two dwarf novae, SS Cyg and U Gem, reveal soft, pulsed X-ray emission greatly enhanced over the emission present during the quiescent periods. In order to characterize the nature of the X-ray pulsations a random phase analysis was carried out, which indicated the phase of SS Cyg behaved like a random variable with zero mean and an r.m.s. value that increased with the square root of the time. Attention is also given to the properties of the detected dwarf novae during optical quiescences, to isolated white dwarfs, and to optical observations of AM Her. It is concluded that the dwarf novae, which occur in binary systems, appear to be controlled by the varying accretion of matter generated by the main sequence companion, and that the X-ray emission during outburst appears to consist of two separate components: a periodically varying (on about a 10 s timescale) soft component, together with a more slowly randomly varying hard component.

Garmire, G.↗
NASA NTRS1980

Phase variability in the rapid optical oscillations of SS Cygni

High-speed photometry of SS Cygni on four nights during the 1978 September eruption revealed a small-amplitude optical oscillation with period 8.23 s and mean semiamplitude 0.0005 mag on September 21 and period 8.50 s and mean semiamplitude 0.0006 mag on September 23. If the observed periods present orbital periods near the surface of the white dwarf, then the white dwarf mass must exceed 0.92 solar mass. Phase variability of the oscillation is analyzed quantitatively and found to be less than that observed in 1978 June for soft X-ray pulsations from SS Cygni.

Horne, K.↗
NASA NTRS1980

Hydromagnetics and period changes in RR Lyrae stars

Balazs-Detre and Detre have drawn an interesting parallel between the observed time scales of variability in the sun and in RR Lyrae stars. Additional information is presented here to support their conjecture that an analog of the solar magnetic cycle is operating in RR Lyrae stars. Rough considerations of the expected changes of photospheric radius and of magnetic energy content during a magnetic cycle suggest that the pulsation periods of these stars should also change in time. Within the large observational and theoretical uncertainties, the predicted period changes are compatible with those observed.

Stothers, R.↗
NASA NTRS1982

Nozzle Modification Suppresses Flow Transients

Proposal for steadying flow from rocket nozzle on Space Shuttle main engine could be applied to other large-area-ratio contoured nozzles. Oscillations and pulsations in boundary-layer flow would be reduced by flaring nozzle exit. Transient side loads on nozzle would be suppressed. Large pressure gradients and eddies that can lead to fluctuations are suppressed. Exact radius of curvature of corner would depend on shear-layer thickness.

Rao, G. V. R.↗
NASA NTRS1980

An alternating-direction-implicit algorithm for the unsteady potential equation in conservation form

An implicit finite difference scheme for an efficient computation of unsteady potential flow about airfoils is presented. The formulation uses density and velocity potential as dependent variables, and is cast in conservation form to assure the theoretically correct determination of shockwave location and speed. To enable boundary conditions to be imposed directly on the airfoil surface, a time varying sheared rectilinear coordinate transformation is employed. Calculated time history solutions on a pulsating airfoil are compared with the results of another unsteady transonic code. It is concluded that the method has excellent numerical stability and gives accurate solutions with sharply resolved shocks.

Chipman, R. R.↗
NASA NTRS1981

Visible-band and IUE observations of mu Sagittarii

H alpha and U band photometry and IUE spectra of the binary system mu Sagittarii are discussed. An estimate of mass-loss is made from the observed P Cygni profiles. There are indications of pulsation in the supergiant B8 component.

Dorren, J. D.↗
NASA NTRS1980

Structural variability of the Sun

Changes in solar radius and luminosity are discussed as a function of age. Surface and satellite observations showing departures from mean solar irradiance on a time scale of a few days are summarized. The most prominent depressions in these data appear to be correlated with sunspot number, indicating that the activity modulation of solar luminosity may be larger than expected. High activity is correlated with low luminosity, but during the Maunder minimum the mini ice age (presumably low luminosity) was accompanied by a near absence of solar activity. Pulsations driven by sources deep within the Sun are thought to be an unlikely cause of luminosity changes, but causes may be sought in the unstable convective zone. Experiments designed to measure the solar radius are mentioned, along with modelling efforts.

Sofia, S.↗
NASA NTRS1978

Two-dimensional dynamic stall as simulated in a varying freestream

A low speed wind tunnel equipped with a axial gust generator to simulate the aerodynamic environment of a helicopter rotor was used to study the dynamic stall of a pitching blade in an effort to ascertain to what extent harmonic velocity perturbations in the freestream affect dynamic stall. The aerodynamic moment on a two dimensional, pitching blade model in both constant and pulsating airstream was measured. An operational analog computer was used to perform on-line data reduction and plots of moment versus angle of attack and work done by the moment were obtained. The data taken in the varying freestream were then compared to constant freestream data and to the results of two analytical methods. These comparisons show that the velocity perturbations have a significant effect on the pitching moment which can not be consistently predicted by the analytical methods, but had no drastic effect on the blade stability.

Pierce, G. A.↗
NASA NTRS1980

Some features of inverted-V events as seen from simulated double layers

Results from a numerical simulation of a double layer show some features similar to those of inverted-V events. The strong heating of thermal and precipitating electrons is observed along with extremely low frequency fluctuations found during inverted-V events. It is suggested that after the acceleration of auroral electrons by the double layer, the precipitating free electrons are heated by the nonlinear effects of the electron beam plasma instability. Fluctuations and pulsations of auroral electron fluxes during auroral events are caused by a relaxation type of oscillation. The finite extent of one dimensional plasma is simulated by solving the Vlasov and Poisson equations as an initial and boundary value problem.

Singh, N.↗
NASA NTRS1981

An alternating-direction implicit algorithm for unsteady potential flow

An implicit finite-difference scheme is presented for the efficient computation of unsteady potential flow about airfoils. The formulation uses density and the velocity potential as dependent variables and is cast in conservation form to assure the theoretically correct determination of shockwave location and speed. To enable boundary conditions to be imposed directly on the airfoil surface, a time-varying sheared-rectilinear coordinate transformation is employed. Calculated time-history solutions on a pulsating airfoil are compared with the results of other unsteady transonic codes, including a previous method of the authors. The present method is demonstrated to be unconditionally stable and to give accurate solutions with sharply resolved shocks.

Chipman, R.↗
NASA NTRS1981

Kelvin-Helmholtz instability in the magnetopause-boundary layer region

The Kelvin-Helmholtz instability in the magnetopause-boundary layer region is studied on the basis of an idealized model which consists of three uniform plasma regions: the magnetosheath, the boundary layer, and the magnetosphere. There are two unstable modes in the magnetopause-boundary layer region: one is excited at the magnetopause (the magnetopause mode) and the other is excited at the inner surface of the boundary layer (the inner mode). The inner mode is found to be unstable most of the time, while the excitation of the magnetopause mode depends on the magnetic field in the magnetosheath. The observed variation of the boundary layer thickness can be attributed to the unstable inner mode. Possible relationships between the Pc 3-5 geomagnetic pulsations and the surface waves excited on the magnetospheric boundary are also discussed.

Lee, L. C.↗
NASA NTRS1981

Further determination of the characteristics of magnetospheric plasma vortices with Isee 1 and 2

Further studies of the vortices in magnetospheric plasma flow with the Los Alamos Scientific Laboratory/Max-Planck-Institut (LASL/MPI) fast plasma experiment on Isee 1 and 2 have revealed that the pattern of vortical flow has a wavelength of approximately 20-40 earth radii and moves tailward through the magnetosphere at speeds of several hundred kilometers per second. The tendency toward vorticity pervades the total breadth of the plasma sheet tailward of the dawn-dusk meridian. The sense of rotation of the plasma flow (as viewed from above the ecliptic plane) is clockwise in the morning side of the plasma sheet and counterclockwise in the evening side. The sense of rotation in the morning and evening boundary layers is reversed from that in the contiguous regions of the plasma sheet. The occurrence of vortical flow is independent of the level of geomagnetic activity but is associated with long-period geomagnetic pulsations.

Hones, E. W., Jr.↗
NASA NTRS1981

SAS 3 observations of GX 1 + 4

GX 1 + 4 is one of the brightest celestial sources of high-energy X-rays. It is a pulsar with a period of approximately 2 min (perhaps a multiple of 2 min), decreasing at a variable rate which, since 1971, has averaged approximately 2% per year, but which can be larger than 5% per year. This is the largest rate of decrease observed for any pulsar. The rate of decrease appears to be correlated with the luminosity, in support of the idea that the period decrease is produced by accretion torques acting upon a neutron star. No evidence is seen for a Doppler shift due to motion of the pulsar in a binary orbit; this is consistent with the results of optical observations which suggest that any orbital period is fairly long (months to years). The spectrum of GX 1 + 4 is measured as a function of pulse phase, as well as the phase-averaged total spectrum, and the average spectrum of the pulses alone. The shape of the average pulsed spectrum suggests that the pulsations may be produced by 'hot spots' which are a few hundred meters in extent, with temperatures of approximately 10 to the 8th K (kT being approximately equal to 8 keV).

Doty, J. P.↗
NASA NTRS1981

Rapid oscillations in cataclysmic variables. V - H2252-035, a single-sideband X-ray and optical pulsar

Photometric and spectroscopic observations of the optical counterpart of the X-ray source H2252-035 reveal the existence of stable periodicities of 3.59 hr and 14.31 min. Both radial velocity and photometric variations occur at the longer period, suggesting that it is the true orbital period of the underlying cataclysmic binary. The shorter period appears as a 10% modulation in the light curve, superposed on the 3.59 hr variation. The frequency is the lower orbital sideband of the X-ray pulsation frequency, suggesting that the optical pulses arise from reprocessing in the atmosphere of the secondary. Unlike the majority of X-ray pulsars, H2252-035 appears to contain a white dwarf rather than a neutron star.

Patterson, J.↗
NASA NTRS1981

A UBV photometric study of the 5.2 hour X-ray binary 4U 2129+47

The paper presents results derived from UBV photometric observations of the 5.2 X-ray binary 4U 2129+47 which were made on 11 nights between 1978 October and 1979 August. The light curves observed during this period are relatively stable in shape and amplitude (Delta B = 1.5 mag) and are a surprisingly close match to the time-averaged light curves observed for HZ Her. There is a significant asymmetry in their shape which is independent of color and varies in amplitude by a factor of approximately 2. The UBV colors at minimum light suggest a considerably earlier spectral type (G or earlier) than is inferred by assuming that the star fills its Roche lobe (late K or early M). Optical pulsations were studied, and a 3 sigma limit of 0.5% was set on the fraction of the flux which is pulsed for periods between 60 ms and 300 s. Assuming that the optical companion is a normal main-sequence star filling its Roche lobe, its mass is approximately 0.65 solar mass. Assuming also that the compact object has a mass of 1.3 solar masses and that the orbital inclination of the system is approximately 90 deg, it is found that the orbital separation is approximately 1.9 solar radii and K-opt is approximately 300 km/s.

Mcclintock, J. E.↗
NASA NTRS1981

Observations of SMC X-1 with the monitor proportional counter on the Einstein /HEAO 2/ X-ray Observatory - The pulse period and its history

The results of the sixth determination of the pulse period of the X-ray source SMC X-1 are presented. The observations were made in April 1979. The pulse period at this epoch was found to be (0.713683 + or - 0.000032) s referred to phase zero of the binary motion. This result together with all available historical data is utilized to quantify the observed trend toward spin-up. It is found that simple secular trends are not applicable. This finding is interpreted as evidence for additional variations in the pulse period. The obtained results are discussed in the context of present theoretical models for period variations in the binary pulsating X-ray sources. The apparent absence of spin-down episodes in the period history of SMC X-1 is also considered.

Darbro, W.↗
NASA NTRS1981

Polarization characteristics of hydromagnetic waves at low geomagnetic latitudes

A field campaign, using three magnetometer stations spaced in latitude around the equatorial magnetic field distance L approximately 1.9, was conducted in early 1979 to investigate the polarization characteristics of hydromagnetic waves at low geomagnetic latitudes. The magnetic pulsations are observed to have periods in the range approximately 20-25 s and to occur primarily in the local morning hours. Statistically, the polarizations at all three stations were predominantly left handed in the local morning hours and right handed in the local afternoon. At the highest-latitude station (L approximately 2) the orientation of the major axis of the polarization ellipse changed from a predominantly NW-SE direction in the local morning to a mixed NW-SE/NE-SW direction in the afternoon. These two statistical results are consistent with the excitation of the waves by the Kelvin-Helmholtz instability at the magnetopause. However, frequent changes in phase are often observed in the magnetic variations, which result in polarization variations on the time scale of minutes, a situation not readily reconcilable with the Kelvin-Helmholtz instability. The existence of the waves at very low latitudes, furthermore, places constraints on the damping rate of externally excited surface waves inside the magnetosphere. We conclude that present theories for hydromagnetic waves in the geomagnetosphere cannot readily incorporate all of these low-latitude results.

Lanzerotti, L. J.↗