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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The Cepheid mass problem and Cepheid binaries

Existing mass determinations for Cepheids with different periods are examined. Wesselink masses are independent of the adopted distance scale. For short periods (less than 6 days) they follow the sequence of evolutionary masses. For periods longer than 10 days they are lower by up to a factor of 2. The lower mass branch joins up with the bump masses. The new pulsational masses agree with the Wesselink masses for periods longer than 6 days. Cepheid masses determined by means of their giant companions also agree with the Wesselink masses and the new pulsational masses. While the error bars are large, the derived dynamical masses determined for S Mus and V636 Sco also agree with the low Wesselink and giant companion masses.

Bohm-Vitense, E.↗
NASA NTRS1986

Theoretical studies on rapid fluctuations in solar flares

Rapid fluctuations in the emission of solar bursts may have many different origins e.g., the acceleration process can have a pulsating structure, the propagation of energetic electrons and ions can be interrupted from plasma instabilities and finally the electromagnetic radiation produced by the interaction of electrostatic and electromagnetic waves may have a pulsating behavior in time. In two separate studies the conditions for rapid fluctuations in solar flare driven emission were analyzed.

Vlahos, Loukas↗
NASA NTRS1986

RR Lyrae stars - A theoretical study of Bailey type c variables

New theoretical models of stars pulsating in the first overtone have been constructed to simulate RR Lyr variables of Bailey type c. Despite the use of different opacities, these new models agree very well with earlier models built by Christy and Stellingwerf. Quantitative comparisons using empirical light curves and velocity curves of metal-poor type c variables confirm the validity of the models. Masses of 0.55-0.65 solar mass and luminosities of 40-50 solar luminosities derived here for the type c variables, are consistent with previous results obtained for type ab variables. A Christy echo of the kind normally associated with fundamental-mode pulsators was detected in the interior velocity structure of one first-overtone model that happens to have a large velocity amplitude.

Hubickyj, Olenka↗
NASA NTRS1987

Geomagnetic field-line resonant harmonics measured by the Viking and AMPTE/CCE magnetic field experiments

The first simultaneous observations of multiple harmonic, azimuthally polarized, ULF pulsations at two points along a geomagnetic flux tube in space are reported. In March 1986, the elliptically orbiting equatorial AMPTE/CCE satellite was oriented with the apogee near 0830 h MLT, and the orbital plane of the polar-orbiting Viking satellite was at 1000 MLT. The satellites were situated within approximately the same flux tube but with an effective separation of approximately 10 R(e) near L = 8 on the inbound pass of the AMPTE/CCE orbit. Structured harmonic pulsations were observed by the magnetic field experiments on both spacecraft, and they appeared to turn off and on simultaneously at both locations. Both the observations and the relative amplitudes along the magnetic field lines support recent ideas of multiple field-line resonances of Alfven waves.

Zanetti, L. J.↗
NASA NTRS1987

Periodic photospheric and chromospheric modulation in Alpha Orionis (Betelgeuse)

The bright cool supergiant Alpha Orionis has been monitored spectroscopically and photometrically over the past three years (1984-1986) in the optical and the ultraviolet wavelength regions. A 420-day periodic modulation of the flux is observed in the optical and ultaviolet continua, and in the Mg II line emission cores. Periodic photospheric pulsations are the most likely explanation of these observations. This identification is based on the large amplitude of the variation, the correlation of the continuum and chromospheric fluxes, and the length of the observed period. Pulsation may heat and extend the atmosphere of Alpha Ori and initiate the mass flow from the star.

Dupree, A. K.↗
NASA NTRS1987

Intrinsically variable stars

The characteristics of intrinsically variable stars are examined, reviewing the results of observations obtained with the IUE satellite since its launch in 1978. Selected data on both medium-spectral-class pulsating stars (Delta Cep stars, W Vir stars, and related groups) and late-type variables (M, S, and C giants and supergiants) are presented in spectra, graphs, and tables and described in detail. Topics addressed include the calibration of the the period-luminosity relation, Cepheid distance determination, checking stellar evolution theory by the giant companions of Cepheids, Cepheid masses, the importance of the hydrogen convection zone in Cepheids, temperature and abundance estimates for Population II pulsating stars, mass loss in Population II Cepheids, SWP and LWP images of cold giants and supergiants, temporal variations in the UV lines of cold stars, C-rich cold stars, and cold stars with highly ionized emission lines.

Bohm-Vitense, Erika↗
NASA NTRS1988

Mass loss from S stars

The mass-loss process in S stars is studied using 65 S stars from the listing of Wing and Yorka (1977). The role of pulsations in the mass-loss process is examined. It is detected that stars with larger mass-loss rates have a greater amplitude of pulsations. The dust-to-gas ratio for the S stars is estimated as 0.002 and the average mass-loss rate is about 6 x 10 to the -8th solar masses/yr. Some of the properties of the S stars, such as scale height, surface density, and lifetime, are measured. It is determined that scale height is 200 pc; the total duration of the S star phase is greater than or equal to 30,000 yr; and the stars inject 3 x 10 to the -6th solar masses/sq kpc yr into the interstellar medium.

Jura, M.↗
NASA NTRS1987

Far-ultraviolet observations of the Delta-Scuti variable Beta Cassiopeiae

FUV (115-200-nm) observations of Beta Cas, obtained at resolutions 5 and 0.15 A by the IUE during September 1984, are reported and analyzed. The data are presented graphically and compared with results for Procyon, a sharp-lined F star similar in temperature to Beta Cas. A sharp factor-of-two increase in H I Ly-alpha flux on a time scale comparable to its Delta Sct pulsation period was observed, but the data were insufficient to determine whether the increase was flarelike or pulsation-related. The outer edges of the Beta Cas emission are shown to be significantly wider than those of Procyon.

Ayres, Thomas R.↗
NASA NTRS1987

A possible solution to the Cepheid mass problem?

With new, smaller distances of the Cepheids, as determined recently by Schmidt (1984) and by Bohm-Vitense (1985), smaller pulsational masses are obtained than previously. Giant companions of Cepheids show that the luminosities of the Cepheids are too large in comparison with those of the giants. If increased mixing is responsible for this, then an increase in the luminosity of the Cepheids of a given mass by approximately a factor of 4 (as compared to conventional evolution calculations) is expected. Taking into account both of these effects good agreement is found between the corrected evolutionary masses, the pulsational masses, the dynamical masses, the giant companion masses, and the Wesselink masses. The bump masses are only slightly smaller than the other masses.

Bohm-Vitense, Erika↗
NASA NTRS1988

Evolutionary period changes in the rapidly oscillating Ap stars

Preliminary computations of the asymptotic p-mode frequency spacings, and their time derivatives, are presented for a grid of evolutionary models of the rapidly oscillating Ap ('roAp') stars. It is shown how the frequency spacings depend on stellar mass, metallicity, and evolutionary stage. The frequency spacings observed in two of the roAp stars indicate that they either are main-sequence objects or have evolved off of the main sequence, depending on how the pulsation spectrum is interpreted. A way to remove this ambiguity is suggested: secular evolutionary changes in the pulsation frequencies may be measurable if the roAp stars have evolved off of the main sequence. In principle, this determination may be made for the roAp star HR 1217 with existing data.

Heller, Clayton H.↗
NASA NTRS1987

The stellar seismology of hot white dwarfs and planetary nebula nuclei

The pulsation properties of hot white dwarfs make it possible to determine their mass, surface composition, rotation, and rate of evolution, and provide constraints on their internal structure. Period spacings are sensitive measures of stellar mass and indicate surface layer structure. Measurement of the rate of period change for these stars provide a way to determine their cooling rates. Attention is also given to how well (or poorly) models of excitation of the pulsations fit within current models of planetary nebula nuclei and hot white dwarfs.

Kawaler, Steven D.↗
NASA NTRS1989

Distortion effects in spacecraft observations of MHD toroidal standing waves - Theory and observations

This paper considers the distortion effect of the simultaneous occurrence of locally resonant toroidal pulsations over a range of L shells on the interpretation of spacecraft observations of these waves. A theoretical model is developed which yields quantitative predictions of the phase shear effect that causes a shift from the true pulsation frequency measured from a spacecraft moving radially across L shells. Calculations with this model indicate that, due to this effect, the AMPTE/CCE satellite should observe 8-30 percent lower frequencies on outbound passes than on inbound passes. A comparison of fundamental mode frequency measurements made on 10 inbound and 9 outbound passes of the AMPTE/CCE satellite in the morning sector confirms the predictions of the model.

Anderson, B. J.↗
NASA NTRS1989

SC- and SI-associated ULF and HF-Doppler oscillations during the great magnetic storm of February 1986

Results are presented of an investigation of SC- and SI-associated ULF and HF-Doppler pulsations observed during the great geomagnetic storm of February 1986, which began with a sudden commmencement on February 6 at about 13:12 UT, developed slowly over the next two days, and, after a rapid intensification late on February 8, reached a minimum. It is shown that these ULF and geomagnetic pulsations can be explained by the dynamo-motor mechanism of ionospheric electric fields and by global compressional oscillations in the magnetosphere and ionosphere, respectively.

Yumoto, K.↗
NASA NTRS1990

Toroidal standing waves excited by a storm sudden commencement - DE 1 observations

A 74-nT sudden commencement on July 13, 1982, was observed in the magnetosphere, with instruments on the Dynamics Explorer 1 satellite. Inbound, near L = 4.5, the satellite was located at 1524 magnetic local time and 20 deg magnetic latitude. The sudden commmencement established a strong, east-west oscillation, with 100-s period, which was observed in the magnetic field, the electric field, and the plasma flow velocity records. There was also a compressional component of this 100-s oscillation and a rapidly damped 300-s compressional pulsation. The compressional oscillations may be an evidence of cavity resonances, excited by the sudden commencement. The cavity waves may, in turn, couple to toroidal waves in field line resonance at the satellite location. In addition, the sudden commencement caused the onset of waves with frequencies from 0.1 up to at least 0.5 Hz. The observations are compared with similar reports from earlier pulsations related to sudden commencements.

Cahill, L. J., Jr.↗
NASA NTRS1990

Observations of the magnetic fluctuation enhancement in the earth's foreshock region

Upstream waves have been postulated to be a major source of energy for the dayside magnetic pulsations within the magnetosphere. Thus, it is of interest to determine over what frequency range in the ion foreshock the power of fluctuations in the solar wind is enhanced. The magnetic field data from pairs of spacecraft, when they stay on either side of the ion foreshock boundary, were examined. It was found that the power of magnetic fluctuations is enhanced only at periods less than about two minutes, not at longer periods. Thus the upstream waves may contribute to Pc 3 and Pc 4 pulsations in the dayside magnetosphere, but they cannot be directly responsible for the longer-period waves.

Le, G.↗
NASA NTRS1990

Relationship of solar wind parameters to continuous, dayside, high latitude traveling ionospheric convection vortices

The results of a statistical study of the occurrence of steady traveling ionospheric convection vortices are presented. The upstream solar wind parameters observed by the IMP 8 spacecraft are studied as well. It is suggested that this class of pulsations is most likely to be detected post local noon and when the solar wind speed is low. A correlation between the frequency of dayside pulsations and the solar wind speed is found.

Mchenry, Mark A.↗
NASA NTRS1990

Short-term FUV flux variability and phase-dependent winds in Be stars

FUV (Far Ultraviolet) light curves and the behavior of the C IV wind line versus phase are discussed for four Be stars. The IUE (International Ultraviolet Explorer) data acquired during three multiwavelength campaigns aimed at determining the cause for short term optical light variability in Be stars and whether it is linked with the mass loss mechanism is presented. The observations confirm that it is a modulation in the star's photospheric temperature that causes the flux variability. A phase dependent variation is seen in the equivalent width and profile in the C IV wind line. The wind absorption tends to be strongest at maximum light and in a Be shell star the absolute flux of its C IV emission component appears to be anticorrelated with the absorption strength and the level of the FUV flux. The observations suggest that nonradial pulsations are responsible for the photometric variability and the generic model of a star pulsating in a sectorial mode.

Peters, G. J.↗