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At least 271 records · Page 15
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NASA NTRS1991

An infrared analysis of Puppis A

A quantitative analysis of the infrared emission of the supernova remnant (SNR) Puppis A as seen by the Infrared Astronomical Satellite is presented. Physical parameters of the remnant (and the surrounding interstellar medium), including density, mass, temperature, and age are estimated. The infrared spectrum of Puppis A is fit by models of thermal emission from interstellar dust grains which have been swept up into the SNR where they are collisionally heated by the shocked gas. Evidence of an enhanced abundance of very small grains in the interstellar medium near Puppis A, and evidence of their destruction by the advancing blast wave, are found.

Arendt, Richard G.↗
NASA NTRS1991

Laser induced spark ignition of methane-oxygen mixtures

The use of laser-induced spark ignition in liquid-propellant rocket engines is investigated by studying the ignition of mixtures of methane and oxygen. Specific attention is given to minimum ignition energy, spark efficiency, the effect of ignition energy on flame kernels, the effect of laser wavelength, and comparisons with electrode-spark ignition. A pulsed frequency-doubled Nd:YAG laser is tested at atmospheric pressure and 296 K under both laminar-premixed and turbulent-incompletely-mixed conditions. Laser sparks of 10 and 40 mJ and an electrode spark of 6 mJ are measured for flame-kernel radius as a function of time with pulsed laser shadowgraphy. Initially, the flame-kernel size is similar to those predicted by the Taylor spherical-blast-wave model, and subsequent growth is characterized by rapid acceleration. The growth rate is significantly affected by the effect of incomplete fuel-oxidizer mixing.

Lee, T. W.↗
NASA NTRS1992

Pressure vessel burst test program - Progress paper No. 3

An updated progress report is provided on a program developed to study through test and analysis, the characteristics of blast waves and fragmentation generated by ruptured gas filled pressure vessels. Prior papers on this USAF/NASA/General Physics program were presented to the AIAA in July 1990 and June 1991. Ten pressure vessels have been burst using pneumatic pressure. Tests were designed to explore burst characteristics and used an instrumented arena. Data trends for current experiments are presented. This paper is the third progress report on the program and addresses: (1) a brief review of current methods for assessing vessel safety and burst parameters, (2) a review of pneumatic burst testing operations and testing results, including a comparison to current methods for burst assessment, and (3) a review of the basis for the current test program including planned testing.

Cain, Maurice R.↗
NASA NTRS1992

Global O VI line emission from the Cygnus Loop supernova remnant and direct kinematic measurement of the associated shock

A far-ultraviolet spectrophotometric emission-line mapping of the Cygnus Loop supernova remnant is presented. These are results from the first flight of the rocket-borne, High Resolution Emission Line Spectrometer. The spatial distribution of the emission is that of a limb-brightened shell, and similar to soft X-ray maps. The emission-line profiles, which are broader than the instrument resolution, were consistent with uniformly expanding shell models. Best-fit values give a radial expansion velocity to the emissive region of 185(+/-19) km/s and a reddening-corrected average surface brightness of 8.8(+/-3.6) x 10 exp -6 ergs/sq cm s sr in the doublet. Comparison of the observed brightness with predictions of both radiative and nonradiative shock models provides constraints for the global blast wave ram pressure as well as a "covering factor" of the intermediate velocity shock.

Rasmussen, Andrew↗
NASA NTRS1992

Ultraviolet Imaging Telescope observations of the Cygnus Loop

Attention is given to UV images of a 40-arcmin field in the northeast part of the Cygnus Loop made by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 mission in December 1990. The longest FUV (B5) exposure is compared with ground-based and X-ray observations and with models. It is inferred from SNR shock models that C IV 1550 A dominates for more typical conditions in SNRs, and IUE spectra exhibit a strong correlation between C IV 1550-A flux and UIT B5 flux. The B5 image resembles images in forbidden O III 5007 A, but to the east of the edge of the forbidden O III filaments there is a faint C IV emission, consistent with nonradiative shocked material behind the SNR blast wave. Conditions in shocked gases that lead to C IV emission are similar to those that produce forbidden O III. Hence the ratio of the B5 and forbidden O III intensities can be used to map the optical depths of the sheetlike regions that emit the C IV resonance line.

Cornett, Robert H.↗
NASA NTRS1992

Dynamics of Kepler's supernova remnant

Observations of Kepler's SNR have revealed a strong interaction with the ambient medium, far in excess of that expected at a distance of about 600 pc away from the Galactic plane where Kepler's SNR is located. This has been interpreted as a result of the interaction of supernova ejecta with the dense circumstellar medium (CSM). Based on the bow-shock model of Bandiera (1985), we study the dynamics of this interaction. The CSM distribution consists of an undisturbed stellar wind of a moving supernova progenitor and a dense shell formed in its interaction with a tenuous interstellar medium. Supernova ejecta drive a blast wave through the stellar wind which splits into the transmitted and reflected shocks upon hitting this bow-shock shell. We identify the transmitted shock with the nonradiative, Balmer-dominated shocks found recently in Kepler's SNR. The transmitted shock most probably penetrated the shell in the vicinity of the stagnation point.

Borkowski, Kazimierz J.↗
NASA NTRS1992

Far-ultraviolet observations of the supernova remnant N49 using the Hopkins Ultraviolet Telescope

The Hopkins Ultraviolet Telescope has been used to obtain the first sub-Lyman-alpha spectrum of an extragalactic SNR, N49, in the LMC. Emission from O VI 1032, 1038 has been detected in this spectral region. The measured fluxes of C IV, semiforbidden O IV, and O VI and the upper limit on N V provide stringent limits on the shocks responsible for the bulk of the O VI production. O VI cannot originate in the shocks with velocities of 140 km/s or less because the postshock temperature is not high enough. The nonradiative main blast wave cannot account for the brightness of the observed O VI emission. The majority of the O VI must originate in an optically faint system of shocks with velocities of 190-270 km/s and preshock densities of 20-40 cu cm. The emission from such high-velocity shocks is dominated by O VI.

Vancura, Olaf↗
NASA NTRS1993

Gasdynamics of relativistically expanding gamma-ray burst sources - Kinematics, energetics, magnetic fields, and efficiency

We calculate both analytically and numerically the evolution of highly relativistic fireballs through the stages of free expansion and coasting, and determine the dependence of the thermodynamic and radiation variables in the comoving and laboratory flames. The dynamics and the comoving geometry change at the (lab) expansion factors r/r(0) greater than eta and r/r(0) greater than eta-squared, respectively, where eta = E(0)/M(0)c-squared is the initial Lorentz factor. In the lab, the gas appears concentrated in a thin shell of width r(0) until r/r(0) of less than about eta-squared, and increases linearly after that. Magnetic fields may have been important in the original impulsive event. We discuss their effect on the fireball dynamics and also consider their effects on the radiation emitted when the fireball runs into an external medium and is decelerated. The inverse synchro-Compton mechanism can then yield high radiative efficiency in the reverse shock (and through turbulent instabilities and mixing also in the forward blast wave), producing a burst of nonthermal radiation mainly in the MeV to GeV range. The energy and duration depend on eta, the magnetic field strength, and the external density, and can match the range of properties observed in cosmic gamma-ray bursts.

Meszaros, P.↗
NASA NTRS1994

Radio and optical emission: Spectral shapes and breaks in GRB

Relativistic blast wave models of Gamma Ray Bursts (GRB) predict the spectrum of the emitted synchrotron radiation. The electrons in the shocked region are heated to a Wien distribution whose 'temperature' is 1/3 of the mean electron energy. This energy determines a characteristic (break) frequency of synchrotron radiation. At much lower frequencies, a spectrum F(sub nu) varies as nu(sup 1/3) is predicted independently of the details of the emitting region. This is consistent with the observed soft x ray emission of GRB. It implies low visible and radio intensities, unless there are collective emission processes.

Katz, J. I.↗
NASA NTRS1993

Relativistically expanding pair plasmas as bursting sources of cosmic gamma rays

High temperature shock heating of relativistically expanding plasmas produced in neutron star binary mergers provides a model for cosmic gamma ray burst sources. This requires the fireball resulting from the merger to have a very high entropy per baryon, mechanisms for which are discussed. The energy, temporal structure and spectrum produced by the blast wave and reverse shock as the fireball is decelerated in an external medium are comparable to those observed, as is the frequency of occurrence and the characteristics of the spatial distribution of the events. Difficulties common to all cosmological gamma ray burst scenarios concern the total amount of energy ultimately appearing at gamma ray energies, the time scales, the spectrum, and the great variety of time profiles. A very general mechanism which overcomes these problems is presented. Situations where the pair plasma is created in regions which are relatively free of baryons are discussed. The effect of the interaction of the expanding fireball with the external medium is considered.

Meszaros, Peter↗
NASA NTRS1993

Two populations and models of gamma ray bursts

Gamma-ray burst statistics are best explained by a source population at cosmological distances, while spectroscopy and intensity histories of some individual bursts imply an origin on Galactic neutron stars. To resolve this inconsistency I suggest the presence of two populations, one at cosmological distances and the other Galactic. I build on ideas of Shemi and Piran (1990) and of Rees and Mesozaros (1992) involving the interaction of fireball debris with surrounding clouds to explain the observed intensity histories in bursts at cosmological distances. The distances to the Galactic population are undetermined because they are too few to affect the statistics of intensity and direction; I explain them as resulting from magnetic reconnection in neutron star magnetospheres. An appendix describes the late evolution of the debris as a relativistic blast wave.

Katz, J. I.↗
NASA NTRS1994

A more direct measure of supernova rates in starburst galaxies

We determine ages for young supernova remnants in the starburst galaxies M82 and NGC 253 by applying Chevalier's model for radio emission from supernova blast waves expanding into the ejecta of their precursor stars. Absolute ages are determined by calibrating the model with radio observations of Cas A. We derive supernova rates of 0.10 and 0.08/yr for M82 and NGC 253, respectively. Assuming L (sub FIR) to be proportional to the supernova rate, we find r(sub SN) approximately equal 2 x 10(exp -12) x L(sub FIR), solar yr(exp -1) for these archetypal starburst galaxies. This approach is unique in that the supernova rate is derived from direct observation of supernova remnants rather than from star formation rates and an assumed initial mass function (IMF). We suggest that the approach presented here can be used to derive star-formation rates that are more directly related to observable quantities than those derived by other methods. We find that the supernova rate, far infrared (FIR) luminosity, and dynamical mass of the M82 starburst place few constraints on the initial mass function (IMF) slope and mass limits.

Van Buren, Dave↗
NASA NTRS1994

Test problems for radiation and radiation-hydrodynamics codes

A number of test problems for radiation and radiation-hydrodynamics computer codes are described. These include evolution to radiative equilibrium, cooling from radiative equilibrium, subcritical and supercritical radiating shocks, and a radiating blast wave in a power-law density distribution. For each test problem, example input parameters and plots of the results are presented. Some test problems for pure hydrodynamics are also suggested. The radiation-hydrodynamics code used to perform the example test problems and the equations it solves are described in some detail.

Ensman, Lisa↗
NASA NTRS1994

The outer shell of SNR 0540-69.3

The supernova remnant 0540-69.3 in the Large Magellanic Cloud has been observed with the Roentgen Satellite (ROSAT) HRI detector. The outer shell was resolved from the bright central region and its X-ray morphology and luminosity determined. One-quarter of the soft X-rays originate in this shell. This result and a simple blast-wave model imply that the energy release in the supernova was about 2 x 10(exp 51) ergs.

Seward, F. D.↗
NASA NTRS1994

Two populations and models of gamma-ray bursts

Gamma-ray burst statistics are best explained by a source population at cosmological distances, while spectroscopy and intensity histories of some individual bursts imply an origin on Galactic neutron stars. To resolve this inconsistency I suggest the presence of two populations, one at cosmological distances and the other Galactic. I build on ideas of Shemi & Piran (1990) and of Rees & Meszaros (1992) involving the interaction of fireball debris with surrounding clouds to explain the observed intensity histories in burst at cosmological distances. The distances to the Galactic population are undetermined because they are two few to affect the statistics of intensity and direction; I explain them as resulting from magnetic reconnection in neutron star magnetospheres. An appendix describes the late evolution of the debris as a relativistic blast wave.

Katz, J. I.↗
NASA NTRS1995

A ROSAT observation of the supernova remnant Kes 79

Kes 79 was observed with the ROSAT Position Sensitive Proportional Counter (PSPC) and High Resolution Imager (HRI). Some X-ray emission comes from a faint outer region, well correlated with the outer shell. Most emission is from a bright diffuse inner region where there are also bright radio filaments. The X-ray spectrum is fitted with a thermal model with temperature 1.3 keV. Variations in spectra from different regions arew small. There is no X-ray evidence for an internal neutron star. Remnant characteristics are derived using a simple blast-wave model. The observed X-ray absorption is less than expected. It is possible that the remnant is closer than the published distance of 10 kpc.

Seward, F. D.↗
NASA NTRS1995

The distance to the heliospheric VLF emission region

Two major episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager spacecraft in 1983/84 and 1992/3. This higher-frequency component is apparently triggered by solar wind transients with sufficiently large spatial extents and energies to continue to propagate as shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather, it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution the shock moves as time to the 2/3 power and in the other as time to the 4/5 power. Using these models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the 1992 emission is consistent with an emission region distance of approximately 130 AU (assuming no additional slowing of the shock in the heliosheath). If the termination shock was at approximately 70 AU when the transient shock collided with it, the true distance to the source region was probably closer to approximately 115 AU.

Mcnutt, R. L., Jr.↗
NASA NTRS1995

Origin of coronal mass ejection and magnetic cloud: Thermal or magnetic driven?

A fundamental problem in Solar-Terrestrial Physics is the origin of the solar transient plasma output, which includes the coronal mass ejection and its interplanetary manifestation, e.g. the magnetic cloud. The traditional blast wave model resulted from solar thermal pressure impulse has faced with challenge during recent years. In the MHD numerical simulation study of CME, the authors find that the basic feature of the asymmetrical event on 18 August 1980 can be reproduced neither by a thermal pressure nor by a speed increment. Also, the thermal pressure model fails in simulating the interplanetary structure with low thermal pressure and strong magnetic field strength, representative of a typical magnetic cloud. Instead, the numerical simulation results are in favor of the magnetic field expansion as the likely mechanism for both the asymmetrical CME event and magnetic cloud.

Zhang, Gong-Liang↗