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At least 721 records · Page 40
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NASA NTRS2016

Rapidly Rotating, X-Ray Bright Stars in the Kepler Field

We present Kepler light curves and optical spectroscopy of twenty X-ray bright stars located in the Kepler field of view. The stars, spectral type F-K, show evidence for rapid rotation including chromospheric activity 100 times or more above the Sun at maximum and flaring behavior in their light curves. Eighteen of our objects appear to be (sub)giants and may belong to the class of FK Com variables, which are evolved rapidly spinning single stars with no excretion disk and high levels of chromospheric activity. Such stars are rare and are likely the result of W UMa binary mergers, a process believed to produce the FK Com class of variable and their descendants. The FK Com stage, including the presence of an excretion disk, is short lived but leads to longer-lived stages consisting of single, rapidly rotating evolved (sub)giants with high levels of stellar activity.

stars: chromospheres – stars: evolution – star↗
NASA NTRS1970

Investigation of an Optimum Detection Scheme for a Star-Field Mapping System

An investigation was made to determine the optimum detection scheme for a star-field mapping system that uses coded detection resulting from starlight shining through specially arranged multiple slits of a reticle. The computer solution of equations derived from a theoretical model showed that the greatest probability of detection for a given star and background intensity occurred with the use of a single transparent slit. However, use of multiple slits improved the system's ability to reject the detection of undesirable lower intensity stars, but only by decreasing the probability of detection for lower intensity stars to be mapped. Also, it was found that the coding arrangement affected the root-mean-square star-position error and that detection is possible with error in the system's detected spin rate, though at a reduced probability.

Aldridge, M. D.↗
NASA NTRS1972

Energy Distributions and spectra of Orion B stars

New MK spectral types and energy distributions are presented for B stars in Orion for which far ultraviolet flux excesses have recently been discovered. Significant differences between HD spectral energy distributions show the Orion late B stars to have smaller Balmer discontinuities than do field stars of the same spectral types. For the late B stars, these effects cause the 1500 A fluxes to be under-estimated by approximately 0.5 mag. No comparable systematic effects were found for the early B stars.

Schild, R. E.↗
NASA NTRS1972

Balmer continuous emission and polarization in Be stars

Observations by OAO-2 of two Be stars, gamma Cassiopeiae and 60 Cygni, are analyzed to show that unpolarized radiation produced by Balmer continuous emission in the extended atmospheres of Be stars is probably a significant factor in explaining the polarization observed shortwards of the Balmer discontinuity. The ratio of the energy flux of Be stars to normal B stars with the same amount of reddening varies from about 1.4 at 3600 A to 1.05 at 2500 A. This is interpreted as due to Balmer continuous emission in the Be stars. For gamma Cas the polarizations at 3300 and 3600 A are respectively 0.6 and 0.7 of the values one expects from interstellar polarization.

Coyne, G. V.↗
NASA NTRS1972

The nature of the Herbig Ae- and Be-type stars associated with nebulosity.

A quantitative spectrophotometric study of the Herbig Ae and Be stars associated with nebulosity demonstrates that the majority have surface gravities appropriate to pre-main-sequence stars while the remainder have surface gravities characteristic of the zero-age main seee. This evidence, along with their gross spectroscopic and pc similarity to T Tauri-like stars and the fact that all these stars appear to be surrounded by circumstellar dust, places them among the youngest known stellar objects. The geometry of the circumstellar shells is discussed and the evidence seems to favor a nonspherical, possibly disklike shape. Finally, photographs taken with a 1-micron image-tube system reveal a large number of red stellar objects surrounding many of these stars. This suggests that the Herbig Ae and Be objects may be the brightest representatives of very recently formed stellar groups.-

Strom, S. E.↗
NASA NTRS1972

The association of X-ray sources with bright stars.

Comparison of selected Uhuru X-ray sources with the SAO star catalog, revealing ten stars within the positional error of the X-ray sources where only about two are expected on the basis of chance. Three of these are already under study as optical candidates for X-ray sources. The new results lend confidence to these candidates as being correct and yield candidate stars for seven other X-ray sources. The association of bright stars with X-ray sources is expected on the basis of the model which invokes mass transfer from a massive star to a compact companion.

Gursky, H.↗
NASA NTRS1972

Slowly rotating relativistic stars. VI - Stability of the quasi-radial modes.

Dynamical analysis of the quasi-radial modes of slowly and rigidly rotating, relativistic stars. The quasi-radial modes are those modes which would be radial if the star were not rotating. They are the crucial modes for determining the stability of the rotating star and also the modes which store energy for the longest time against dissipation by gravitational radiation. An equation is derived that makes it possible to calculate the effects of the slow and rigid star rotation on the frequency of the quasi-radial modes of oscillation of relativistic stars. The rotation is treated to second order in the angular velocity, but no other approximations are made.

Hartle, J. B.↗
NASA NTRS1972

Corrections to star catalogues from satellite observations.

Theoretical investigation of the possibility of obtaining systematic corrections to star catalogs from observations of artificial satellites. A model is established to represent the system of equations formed when such corrections are determined simultaneously with corrections to the geocentric position of the observer. An analytical expression for the covariance matrix is set up for the two-dimensional (planar) case. The resulting correlations and weights are discussed. Since this method presents an independent way of obtaining the systematic errors in star catalogs, apart from the orientation to the equatorial system, it is related to the measurement of all quantities which depend on star positions. This institutes a completely new principle of obtaining systematic corrections to star catalogs. It is based not on rotation, but on comparing the angle subtended at the satellite by two stars to the angle subtended at the satellite by two points on the surface of the earth.

Williams, C. A.↗
NASA NTRS1973

Luminosity functions for K giant stars derived from the two-micron sky survey.

Description of a method for determining either the space density or the luminosity function from star counts covering large areas of sky. Space density is assumed to vary only in the direction perpendicular to the galactic plane. The method extends that derived for use with the star counts in the Selected Areas by allowing for an integration over a wide and continuous range of galactic latitudes, and is therefore applicable to surveys where the number of stars per square degree is small but the area surveyed is an appreciable fraction of the sky. The catalog (IRC) produced from the 2-micron sky survey at Caltech is such a survey. Application of the method to a selection of IRC stars dominated by K giants shows that if these stars obey Oort's determination of their normalized space density perpendicular to the galactic plane than the dispersion of their 2.2-micron luminosity distribution must be large - i.e., on the order of plus or minus 1.0 min within a single spectral subtype. This result is in accord with conclusions recently set forth by Jung (1970).

Hughes, E. E., Jr.↗
NASA NTRS1973

A study of B 6 stars.

Because detailed information about the spectra of normal late B type stars is deficient, a study of the spectrum of zeta-Draconis, beta-Sextantis, and alpha-Leonis was undertaken to fill this lack and to provide data for comparison with observations of Bp stars, many of which are of late B spectral type. The study was made from high dispersion spectrograms which cover the spectral region 3100 to 6700 A, and from OAO-II spectral scans covering the spectral region 1100 to 3600 A. Profiles, equivalent widths, and central intensities of many lines in the spectrum of zeta-Draconis and of the major lines of beta-Sextantis are presented. The star alpha-Leonis rotates too rapidly for these measurements to be made from high resolution spectrograms. The star beta-Sextantis is the most massive and hottest of the three stars studied.

Underhill, A. B.↗
NASA NTRS1973

UV and radiofrequency observations of Wolf-Rayet stars.

Available spectrometric and photometric observations of Wolf-Rayet stars by the OAO 2 spacecraft in the UV range are discussed along with radio astronomical observations of W stars with symmetrical nebulae around them. The scanned spectrum of the WN5 star HD 50896 between 1200 and 1900 A is illustrated together with the photometered spectrum of the WN6 star HD 192163 from 1330 to 3320 A. RF observations of NGC 6888 around HD 192163 are examined relative to interpretation of the properties of a WN6 star ejecting mass into a nebular shell.

Johnson, H. M.↗
NASA NTRS1975

Time variations of UV emission features of Be stars

The UV spectra of three Be stars (gamma Cas, sigma Tau, eta Cen) were studied. Of the six Be stars observed in the first four lines of the Balmer series, three stars showed at least one of the Balmer lines to be variable in the equivalent width amounting to a few percent with time scales of 3 to 30 minutes. Photoelectric spectrum scans of five southern Wolf-Rayet stars showed night-to-night variations. A simple model is proposed to account for the behavior of these emission lines. Scans of gamma square Vel showed rapid variations of emission strengths of He II 4686 and C III - IV 4650. These variations have time scales of 1 minute and longer. Night-to-night variations were also found. Scans of four Be stars in H alpha showed a definite variation of 3 to 4 percent, with time scales of 1 minute and longer in sigma Tau. In 48 Per and kappa Dra the variations are not as well established. No variation of any significance was found for nu Gem.

Bahng, J. D. R.↗
NASA NTRS1975

Slowly braked, rotating neutron stars

A slowly braked, rotating neutron star is believed to be a star which rapidly rotates, has no nebula, is nonpulsing, and has a long initial braking time of ten thousand to a million years because of a low magnetic field. Such an object might be observable as an extended weak source of infrared or radio wave radiation due to the scattering of low-frequency strong-wave photons by accelerated electrons. If these objects exist abundantly in the Galaxy, they would act as sources of relatively low-energy cosmic rays. Pulsars (rapidly braked neutron stars) are shown to have difficulties in providing an adequate amount of cosmic-ray matter, making these new sources seem necessary. The possibility that the acceleration mechanism around a slowly braked star may be not a direct acceleration by the strong wave but an acceleration due to plasma turbulence excited by the strong wave is briefly explored. It is shown that white dwarfs may also be slowly braked stars with braking times longer than 3.15 million years.

Sato, H.↗
NASA NTRS1975

Luminous stars in galactic supernova remnants

It is shown that luminous stars should be expected in the vicinity of supernova remnants. Luminous stars within the maximum optical areas of supernova remnants are tabulated from catalogs, and an excitation parameter is correlated with the spectral classification of such stars when their UV flux excites H II regions to produce nebular thermal bremsstrahlung. The correlations indicate that the presence of a ZAMS star of type 08 to 05 is necessary to produce thermal bremsstrahlung equal to the observed flux density at 1 GHz. Optical components of seven other supernova remnants are discussed, and it is concluded that gross uncertainties in the distance moduli of both stars and supernova remnants prevent any associations from being made.

Johnson, H. M.↗
NASA NTRS1974

Neutron stars - General review

A review is presented of those properties of neutron stars upon which there is general agreement and of those areas which currently remain in doubt. Developments in theoretical physics of neutron star interiors are summarized with particular attention devoted to hyperon interactions and the structure of interior layers. Determination of energy states and the composition of matter is described for successive layers, beginning with the surface and proceeding through the central region into the core. Problems encountered in determining the behavior of matter in the ultra-high density regime are discussed, and the effects of the magnetic field of a neutron star are evaluated along with the behavior of atomic structures in the field. The evolution of a neutron star is outlined with discussion centering on carbon detonation, cooling, vibrational damping, rotation, and pulsar glitches. The role of neutron stars in cosmic-ray propagation is considered.

Cameron, A. G. W.↗
NASA NTRS1975

Observations and theory of Mg II lines in early type stars. II - Theory and predicted profiles. III - The observations and a comparison with the predictions

Profiles of the UV Mg II lines in the spectra of early type stars are computed in a grid of model atmospheres with effective temperatures between 8000 and 35,000 K and log g values of 2.5 to 4.0 using the LTE and non-LTE theories of line formation. The theoretical results indicate that the line cores are strengthened by non-LTE effects over the entire temperature range, resonance-line wings are weaker in the cooler models than in the LTE case, and very large deviations from LTE occur in the hot low-gravity models. These predictions are compared with the equivalent widths of the UV Mg II lines in 106 stars and of the visual lines in 48 stars (spectral types O4 to A3). The observed equivalent widths of normal stars in luminosity classes II through V are found to agree with the predictions over the entire range from O8 to A2 if a certain Mg/H abundance is adopted. The line intensities observed in supergiants, Be, Bp, and Ap stars are discussed.

Snijders, M. A. J.↗
NASA NTRS1974

Precision Attitude Determination System (PADS) system design and analysis: Single-axis gimbal star tracker

The feasibility is evaluated of an evolutionary development for use of a single-axis gimbal star tracker from prior two-axis gimbal star tracker based system applications. Detailed evaluation of the star tracker gimbal encoder is considered. A brief system description is given including the aspects of tracker evolution and encoder evaluation. System analysis includes evaluation of star availability and mounting constraints for the geosynchronous orbit application, and a covariance simulation analysis to evaluate performance potential. Star availability and covariance analysis digital computer programs are included.

Source record↗
NASA NTRS1976

Solution of a large hydrodynamic problem using the STAR-100 computer

A representative hydrodynamics problem, the shock initiated flow over a flat plate, was used for exploring data organizations and program structures needed to exploit the STAR-100 vector processing computer. A brief description of the problem is followed by a discussion of how each portion of the computational process was vectorized. Finally, timings of different portions of the program are compared with equivalent operations on serial machines. The speed up of the STAR-100 over the CDC 6600 program is shown to increase as the problem size increases. All computations were carried out on a CDC 6600 and a CDC STAR 100, with code written in FORTRAN for the 6600 and in STAR FORTRAN for the STAR 100.

Weilmuenster, K. J.↗