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At least 595 records · Page 33

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NASA NTRS1991

A search for gamma-ray line and low energy continuum emission from Sco X - 1

Results are presented from the analysis of the first high-resolution gamma-ray observations of Sco X - 1 by the HEAO 3 experiment. Flux upper limits are presented for line emission at 511 keV (positron annihilation), 0.847 (Fe-56), 1.23 (Fe-56), 2.2 (deuterium formation), 4.4 (C-12) and 6.2 (O-16) MeV. The 2sigma upper flux limits on continuum emission are also given. The relevance of the results within the context of the low-mass X-ray binary model of Sco X - 1 is discussed. The possibility of detecting gamma-rays from this source using the instruments on board the Gamma-Ray Observatory is discussed.

Bhattacharya, Dipen↗

NASA NTRS1991

Gamma-ray line and anomalous continuum emission from the Crab

The observations of transient gamma-ray line and sporadic continuum emissions from the Crab over the last two decades are reviewed, and these measurements are used to explore the emission mechanisms and regions. When examined in detail, the measurements require an emission region(s) considerably more complex than predicted by simple synchrotron models, suggesting behavior reminiscent of gamma-ray bursts.

Owens, Alan↗

NASA NTRS1992

A comparison of modeled and observed intensity profiles for C2, C3, CN, and the continuum for P/Halley

Model intensity profiles have been compared with those observed for the C2, C3, CN, and continuum emissions of Comet Halley, in order to constrain the dust and gas parameters. While most of the parameters were consistent with expected values, the lifetimes of C3 and its parent were unexpectedly small. The day:night production rate ratio, which was about 1:1 for the gas, covered the 4:3-1:0 range in the case of the dust.

Ellis, Tracy A.↗

NASA NTRS1992

Monte Carlo simulation of a near-continuum shock-shock interaction problem

A complex shock interaction is calculated with direct simulation Monte Carlo (DSMC). The calculation is performed for the near-continuum flow produced when an incident shock impinges on the bow shock of a 0.1 in. radius cowl lip for freestream conditions of approximately Mach 15 and 35 km altitude. Solutions are presented both for a full finite-rate chemistry calculation and for a case with chemical reactions suppressed. In each case, both the undisturbed flow about the cowl lip and the full shock interaction flowfields are calculated. Good agreement has been obtained between the no-chemistry simulation of the undisturbed flow and a perfect gas solution obtained with the viscous shock-layer method. Large differences in calculated surface properties when different chemical models are used demonstrate the necessity of adequately representing the chemistry when making surface property predictions. Preliminary grid refinement studies make it possible to estimate the accuracy of the solutions.

Carlson, Ann B.↗

NASA NTRS1992

Solution of the Burnett equations for hypersonic flows near the continuum limit

The INCA code, a three-dimensional Navier-Stokes code for analysis of hypersonic flowfields, was modified to analyze the lower reaches of the continuum transition regime, where the Navier-Stokes equations become inaccurate and Monte Carlo methods become too computationally expensive. The two-dimensional Burnett equations and the three-dimensional rotational energy transport equation were added to the code and one- and two-dimensional calculations were performed. For the structure of normal shock waves, the Burnett equations give consistently better results than Navier-Stokes equations and compare reasonably well with Monte Carlo methods. For two-dimensional flow of Nitrogen past a circular cylinder the Burnett equations predict the total drag reasonably well. Care must be taken, however, not to exceed the range of validity of the Burnett equations.

Imlay, Scott T.↗

NASA NTRS1992

A far wing line shape theory and its application to the foreign-broadened water continuum absorption. III

The far wing line shape theory developed previously and applied to the calculation of the continuum absorption of pure water vapor is extended to foreign-broadened continua. Explicit results are presented for H2O-N2 and H2O-CO2 in the frequency range from 0 to 10,000/cm. For H2O-N2 the positive and negative resonant frequency average line shape functions and absorption coefficients are computed for a number of temperatures between 296 and 430 K for comparison with available laboratory data. In general the agreement is very good.

Ma, Q.↗

NASA NTRS1992

Radio continuum and X-ray properties of the coronae of RS Canum Venaticorum and related active binary systems

Data from circular polarization measurements are reported which yield radio-continuum and X-ray characterizations of the coronae of active binaries including RS CVn. The circular polarization measurements are taken at 6 cm for 28 binary systems detected at flux densities of more than 0.7 mJy, and the data are examined in conjunction with those by Drake et al. (1989). Optical positions are derived for 8 RS CVn objects, three previous detections are confirmed, and seven sources are shown to have circular polarization at levels of 2-13 percent. A total of 49 X-ray sources are confirmed in a total of 190 positions, and the correlation between X-ray and radio emissions is analyzed. No observational evidence exists to support the notion that radio and X-ray emissions are due to nonthermal electrons, and the data provide evidence for and against thermal gyrosynchrotron radio emission.

Drake, Stephen A.↗

NASA NTRS1991

X-ray and radio continuum correlation in late-type galaxies

X-ray observations give us a direct view of galaxian components that would otherwise remain unknown. These components include the end products of stellar evolution, the hot phase of the interstellar medium, and phenomena related to nuclear activity. Here, comparisons of X-ray and radio continuum data are presented, which suggest the possibility of a link between the emission in these two wavebands.

Fabbiano, G.↗

NASA NTRS1992

The evolution of the quasar continuum

We now have in hand a large data base of Roentgen Satellite (ROSAT), optical, and IR complementary data. We are in the process of obtaining a large amount of the International Ultraviolet Explorer (IUE) data for the same quasar sample. For our complementary sample at high redshifts, where the UV was redshifted into the optical, we have just had approved large amounts of observing time to cover the quasar continuum in the near-IR using the new Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) array spectrographs. Ten micron, optical, and VLA radio, data also have approved time. An ISO US key program was approved to extend this work into the far-IR, and the launch of ASTRO-D (early in 1993) promises to extend it to higher energy X-rays.

Elvis, M.↗

NASA NTRS1992

High spatial resolution continuum and H-alpha imaging of the high-redshift IRAS source FSC 10214+4724

New observations are reported which support the view that the high-redshift IRAS source FSC 10214+4724 is a rare extreme example of the class of ultraluminous infrared galaxies observed in the local universe. The presence of two continuum components and a compact H-alpha source suggests that a merger between two or more gas-rich systems has triggered the production of a powerful dust-enshrouded quasar which is responsible for the bulk of the luminosity.

Soifer, B. T.↗

NASA NTRS1992

A surface brightness correlation between carbon monoxide and nonthermal radio continuum emission in the galaxy

The relation between the projected face-on velocity-integrated CO (1-0) brightness ICO and the 20 cm nonthermal radio continuum brightness T20 is examined as a function of radius in the Galactic disk. Averaged in 1 kpc annuli, the ratio ICO/T20 is nearly constant with a mean value of 1.51 +/- 0.34 km/s from 2 to 10 kpc. The manner in which ICO and T20 are derived for the Galaxy is different in several significant respects from the more direct observational determinations possible in nearby galaxies. The fact that the Galaxy also follows this correlation further strengthens the generality of the result.

Allen, R. J.↗

NASA NTRS1992

Multilevel optimization using a continuum model for structures

The new concept of using a continuum model in the multilevel decomposition of structural optimization problems is presented. The practicality of this concept is demonstrated by application in a scheme for the optimization of beam-like space structures. This scheme is tested against traditional optimization procedures for savings in computational cost. Results from both optimization methods are presented for comparison.

Yates, K.↗

NASA NTRS1992

Software issues in three dimensional continuum shape optimization employing boundary formulations

The paper addresses the issue of how individual computational techniques for the accurate and economical calculation of information required for large-scale 3D continuum structural shape optimization via boundary element analysis (BEA) formulations can be prudently selected and incorporated in large-scale BEA programs, employing either direct or iterative equation solvers. A series of techniques that allows for the incorporation of economical shape design sensitivity analysis capability with a minimal amount of software development is described. The use of reanalysis is shown to be the key ingredient associated with each of these techniques. It is concluded that, from a software engineering perspective, capabilities that facilitate 3D shape optimization can be implemented in BEA programs with only modest investments in program development.

Kane, James H.↗

NASA NTRS1993

Detection of radio continuum emission from Procyon

We have detected the F5 IV-V star Procyon as a weak and variable 3.6 cm radio continuum source using the VLA. The inferred radio luminosity is similar to, though some-what higher than, the X-band luminosity of the active and flaring sun. The 33 micro-Jy flux density level at which we detected Procyon on four of five occasions is close to the 36 micro-Jy radio flux density expected from a model in which the radio emission consists of two components: optically thick 'stellar disk' emission with a 3.6 cm brightness temperature of 20,000 K that is 50 percent larger than the solar value, and optically thin coronal emission with an emission measure the same as that indicated by Einstein and EXOSAT X-ray flux measurements in 1981 and 1983. The maximum mass-loss rate of a warm stellar wind is less than 2 x 10 exp -11 solar mass/yr. An elevated flux density of 115 micro-Jy observed on a single occasion provides circumstantial evidence for the existence of highly localized magnetic fields on the surface of Procyon.

Drake, Stephen A.↗

NASA NTRS1993

Electron excitation of the H2(a 3Sigma(g)(+) - b 3Sigma(u)(+)) continuum in the vacuum ultraviolet

The electron excitation function for the H2(a 3Sigma(g)(+) - b 3Sigma(u)(+)) continuum emission has been measured from 11 to 30 eV at 195.0 nm. The emission cross, section data have been combined with electron energy loss cross section measurements to extend the experimental data to 60 eV. The excitation cross section has been estimated by normalizing the data to the electron energy loss cross section at 20 eV and using a model to account for emission from the entire band system. The spin forbidden excitation of the a 3Sigma(g)(+) state is a major dissociative channel of H2 at low energy with a peak cross section of 1.7 +/- 0.85 x 10 exp -17 sq cm at 15.5 eV. The FWHM of the cross section is 7 eV. The high energy dependence of the cross section has a rapid 1/E-cubed fall off with electron energy, E, above 50 eV. A modified Born approximation analysis was applied to the data to provide an analytic model.

Ajello, J. M.↗

NASA NTRS1993

Ultraviolet continuum variability and visual flickering in the peculiar object MWC 560

High-speed U-band photometry of the peculiar emission object MWC 560 obtained with the ground-based instrumentation, and V-band photometry obtained with the International Ultraviolet Explorer-Fine Error Sensor indicates irregular brightness variations are quasi-periodic. Multiple peaks of relative brightness power indicate statistically significant quasi periods existing in a range of 3-35 minutes, that are superposed on slower hourly varying components. We present a preliminary model that explains the minute and hourly time-scale variations in MWC 560 in terms of a velocity-shear instability that arises because a white dwarf magnetosphere impinges on an accretion disk. We also find evidence for Fe II multiplet pseudocontinuum absorption opacity in far-UV spectra of CH Cygni which is also present in MWC 560. Both CH Cyg and MWC 560 may be in an evolutionary stage that is characterized by strong UV continuum opacity which changes significantly during outburst, occurring before they permanently enter the symbiotic nebular emission phase.

Michalitsianos, A. G.↗

NASA NTRS1993

A high-sensitivity survey of radio continuum emission from Herbig Ae/Be stars

Results of a high-sensitivity VLA/Australia Telescope survey of radio continuum emission from the 57 Herbig Ae/Be stars and candidates in the 1984 catalog of Finkenzeller and Mundt are presented. Twelve stars were detected at the primary observing wavelength of 3.6 cm, on the basis that not less than 4 sigma radio sources lie within 1 arcsec of the optical positions. It is suggested that the radio emission is predominantly thermal and in many cases wind-related. The unusual eclipsing binary TY CrA is an exception and is classified as a nonthermal radio source on the basis of its decidedly negative spectral index (alpha = -1.2). A simple spherically symmetric free-fall accretion model is used to show that the predicted radio fluxes due to accretion at rates, estimated in the literature, of about 10 exp -6 to 10 exp -5 solar mass/yr are one to four orders of magnitude larger than observed.

Skinner, Stephen L.↗

NASA NTRS1993

The contribution of disks and envelopes to the millimeter continuum emission from very young low-mass stars

We investigate the question of disk formation during the protostar phase. We model the dust continuum emission from the dense cloud core using the cloud-collapse models of Terebey et al. (1984) and show that dust emission from the dense core is important when measured with large single-dish telescopes at 1.3 mm, but nearly negligible with interferometers at 2.7 mm. From published and new data, we conclude that massive disks are also seen toward a number of other sources including L1448 IRS 3, whose disk mass is estimated to be 0.5 solar mass. However, 1.3 mm data show that massive disks are relatively rare, occurring around perhaps 5 percent of young embedded stars. This implies that either massive disks occur briefly during the embedded phase or that relatively few young stars form massive disks. The median 1.3 mm flux density of IRAS-Dense cores in our sample is nearly the same as T Tauri stars in the sample of Beckwith et al. (1990). We conclude that the typical disk mass is not significantly higher during the embedded phase than during the later T Tauri phase.

Terebey, S.↗