Engineering topics

Raymond, John C.

Publications and source records attributed to Raymond, John C..

48 records · Page 3

NASA NTRS1992

Microflare heating of the galactic halo

Recent estimates of the strength of the random component of the galactic magnetic field suggest that magnetic reconnection is probably important in heating the galactic halo. A microflare model of impulsive energy release has been proposed for the solar corona by Sturrock and collaborators. When the solar microflare model is scaled to a heating rate and pressure appropriate to the galactic halo, it predicts soft X-ray background emission, column densities of highly ionized species, and diffuse UV line emission in reasonable agreement with observed values. Column densities and surface brightnesses of low-ionization species are more ambiguously predicted by the model, but magnetic heating could contribute to them as well.

Raymond, John C.↗

NASA NTRS1991

Evidence for a thermally unstable shock wave in the Vela supernova remnant

The emission and absorption line signatures of supernova remnant shock waves provide complementary diagnostic capabilities. This paper presents IUE spectra of the nebulosity and new spectra of HD 72088. Models of the emission and absorption lines from shocked gas are used to derive a shock velocity and elemental depletions. There is evidence from the absorption-line strengths and widths for thermally unstable cooling behind a 150 km/s shock. The shock velocity and swept-up column density estimates of Wallerstein and Balick (1990) are confirmed, and evidence is found for a nonthermal contribution to the pressure.

Raymond, John C.↗

NASA NTRS1990

Predicted extreme-ultraviolet and X-ray spectrum of a microflare-heated corona

Sturrock et al. (1990) have proposed that a distribution of microflares produces the solar coronal and transition-region emission. This paper presents computed EUV and X-ray emission spectra from that distribution, including the effects of time-dependent ionization. The overall agreement of the predicted spectrum with observations is fairly good, although the predicted emission from the upper transition region is high by a factor of 2. Time-dependent ionization greatly enhances the He II 304-A line, improving the agreement with observation compared with ionization equilibrium models. The predicted effects on temperature-sensitive line ratios of other ions cannot yet be tested against observations due to uncertainties in instrumental calibration and atomic rates.

Raymond, John C.↗

NASA NTRS1990

UV observations of x ray binaries

IUE (International Ultraviolet Explorer) has observed both high and low mass x ray binaries throughout its life. The UV spectra of high mass systems reveal the nature of the massive companion star and the effects of the x ray illumination of the stellar wind. In loss mass systems, the x ray illuminated disk or companion star dominates the UV light. System parameters and the characteristics of the accretion disk can be inferred.

Raymond, John C.↗

NASA NTRS1989

Ultraviolet spectrophotometry and optical and infrared photometry of the old nova GK Persei

IUE observations in the 1150-3250-A region were obtained of GK Per during the rise, at the maximum, and during the decline of the 2.5-mag optical outburst in 1981. The results support previous predictions for the interaction of an accretion disk with the magnetic field of an accretion star. The luminosity at minimum is found to be 2.3 solar luminosities, with 1/3 of this being attributed to the cool secondary star.

Wu, Chi-Chao↗

NASA NTRS1989

The ultraviolet spectrum of an oxygen-rich supernova remnant in the Small Magellanic Cloud

Ultraviolet and optical spectra data are presented for the oxygen-rich supernova remnant 1E 0102-7219 in the SMC. These UV data are the first to seriously constrain the UV emission from an oxygen-rich remnant. Emission lines of O I, forbidden O II, semiforbidden O III, semiforbidden O IV, C IV, forbidden Ne IV, and Mg II have been unambiguously detected in the UV. Shock models for material with abundances appropriate to massive stellar ejecta are calculated, and a number of differences between these models and the observations are found. The UV lines are observed to be weaker relative to optical lines than expected from models. Models of photoionization by the remnant's X-ray emission are calculated, and these provide a better qualitative match to the observed spectra. Approximate abundances in the ejecta are derived and the implications these have for the precursor star are discussed.

Blair, William P.↗

NASA NTRS1989

Wind-disk shocks around T Tauri stars

It is suggested that the winds of T Tauri stars are heated at distances of 50-100 AU by oblique shocks with circumstellar disks, producing the observed high-velocity forbidden-line emission. The low shock velocities characteristic of this model provide low-excitation emission and line profiles in reasonable agreement with observations. The blueshifted character of the observed emission is explained by a combination of disk occultation of the receding flow, plus a modest collimation of the observable wind by the disk envelope. Instabilities at the wind-disk interface may be important in producing the observed emission.

Hartmann, Lee↗

NASA NTRS1988

Interstellar absorption lines in high-resolution IUE spectra of cataclysmic variables

High-resolution ultraviolet spectra of five cataclysmic variables obtained with the IUE are used to investigate the character of the interstellar medium in the vicinity of the sun. These spectra reveal narrow absorption features of neutral and singly ionized interstellar species, and, in SS Cyg, narrow velocity-shifted absorption features of C IV, Si IV, and Si III. Using the column densities implied by the absorption features of the neutral and singly ionized species, values for the depletion of Si, Mg, Mn, and Fe from the gas phase of the interstellar medium in the vicinity of the sun are obtained. These hydrogen column densities are of particular importance in constraining the soft X-ray luminosity of cataclysmic variables because of the severe attenuation of the soft X-ray flux of cataclysmic variables by photoelectric absorption in the interstellar medium. In addition, using the column densities implied by the absorption features of C IV, Si IV, and Si III in the spectrum of SS Cyg, the existence of an expanding H II region of interstellar gas photoionized by he EUV and soft X-ray flux of this cataclysmic variable are inferred.

Mauche, Christopher W.↗

NASA NTRS1988

Measuring H0 and q0 with X-ray lines from galaxy clusters

A new method of measuring angular diameter distances, and hence H0 and q0, by means of X-ray observations of galaxy clusters is proposed. It resembles the method which combines maps of X-ray continuum emission with maps of the Suniaev-Zel'dovich effect, but substitutes measurements of X-ray absorption lines in the spectra of background quasars for the Suniaev-Zel'dovich effect. When the high resolution permitted by the AXAF microcalorimeter becomes available, this method should yield results with significantly smaller uncertainty.

Krolik, Julian H.↗

NASA NTRS1988

Improved bow shock models for Herbig-Haro objects - Application to HH 2A-prime

An improved version of the bow shock theory previously applied to Herbig-Haro objects is presented. The modifications provide a more accurate calculation of the ionization state of material entering the bow shock. The revised preionization does not drastically affect the emission-line predictions for a 200 km/s bow shock model, though the effects will be more severe for slower shock velocities. The line profiles of the new models resemble the observed profiles somewhat more closely, and the relative emission-line intensities typically differ by 30 percent from those predicted by the older models. The models agree well with new IUE spectra and existing optical data for HH 2A-prime.

Raymond, John C.↗

NASA NTRS1988

The Spartan-281 Far Ultraviolet Imaging Spectrograph

The U.S. Naval Research Laboratory's Far Ultraviolet Imaging Spectrograph (FUVIS), currently under development for flight as a Spartan shuttle payload, is designed to perform spectroscopy of diffuse sources in the FUV with very high sensitivity and moderate spatial and spectral resolution. Diffuse nebulae, the general galactic background radiation, and artificially induced radiation associated with the Space Shuttle vehicle are sources of particular interest. The FUVIS instrument will cover the wavelength range of 970-2000 A with selectable resolutions of 5 and 30 A. It is a slit imaging spectrograph having 3 arcmin spatial resolution along its 2.7 deg long slit.

Carruthers, George R.↗

NASA NTRS1987

IUE observations of the dwarf nova HL Canis Majoris and the winds of cataclysmic variables

An observational and theoretical study is conducted of the P Cygni profiles of cataclysmic variables, with attention to the profiles of the dwarf nova HL CMa, in light of the ionization structure of the wind for a given geometry. It is found that a spherically symmetric wind is capable of generating the observed profile shapes when the accretion disk is limb-darkened, provided that the acceleration of the wind is very low and that the wind represents a mass-loss rate of about 10 to the -11th/C IV ionization fraction of the solar mass per year; a wind of this magnitude, however, cannot be driven by radiation pressure.

Mauche, Christopher W.↗