Engineering topics

Mccray, Richard

Publications and source records attributed to Mccray, Richard.

At least 19 records

NASA NTRS1995

The He I emission lines of SN 1987A

We model the evolution of the He I lines, especially He I 1.083 micrometers and He I 2.058 micrometers, from the envelope of SN 1987A during the first 3 yr after the explosion. The He I metastable 2(sup 3)S and 2(sup 1)S states are populated by fast electrons resulting from energy deposition by gamma rays and depopulated by electron collisions and two-photon decay. The 2.058 micrometer line is excited mainly by nonthermal electrons at all times. The 1.083 micrometer line is excited mainly by thermal electron impact for t less than or approximately = 450 days and by recombination afterward. We can fit the observed evolution of He I 2.058 micrometers with a model in which approximately 3 solar mass of nearly pure helium and approximately 11 solar mass of hydrogen mixed with primordial helium occupy the volume emitting most of the light (v less than or approximately = 2500 km/s). For t less than or approximately = 450 days, trace carbon in the inner part of the helium suppresses the He I 2.058 micrometer line by photoabsorbing He I lambda 584 and depopulating the 2(sup 1) P(sup 0) state. The observed 1.08 micrometer line is a blend of He I 1.083 micrometers and (S I) 1.082 micrometers. We estimate that He I 1.083 micrometers contributes 40% of the observed flux for t less than or approximately = 550 days and approximately 60% thereafter.

Li, Hongwei↗

NASA NTRS1994

The impact of SN 1987A with its circumstellar ring

The envleope of SN 1987A will strike its circumstellar ring in 12 +/- 3 yr after explosion (A.D. 1999+/-3), the exact time depending weakly on the uncertain density of diffuse gas between the supernova and the ring. The impact will drive a radiative shock into the ring with velocity approximatley 200-400 km s(exp -1). The shocked ring will become a bright optical and ultraviolet emsiison-line source. A bright arc will suddenly appear at the near side of the ring and grow into an entire ring about 11 months later. The luminosities of the brightest lines, H-alpha H-alpha, N v lambda lambda 1238, 1242, and O VI lambda lambda 1032, 1038 will rise rapidly to approxminately 10(exp 36)-10(exp 37) ergs s(exp -1) and remain bright for several years after impact. The emission lines from the shocked ring will have FWHM approximatley 300-600 km s(exp -1) and complex profiles that will depend on position and will be sensitive to the details of the density distribution of gas in the ring. Strong EUV radiation from the shock will photoionize the unshocked ring, causing emission of narrow FWHM equivalent to 15 km s(exp -1) H-alpha, H-beta and (O III) lambda lambda 4959, 5007 lines with luminosities approximatley 10(exp 35) ergs s(exp -1).The EUV radiation will probably cause the nebulosity beyond the ring to become visable again. The EUV radiation may also illuminate the unshocked outer supernova envelope, causing visible emission of broad FWHM equivalent to 10(exp 4) km s(exp -1) H-alpha and H-beta lines.

Luo, Ding↗

NASA NTRS1993

On the origin of the soft X-ray background

The angular autocorrelation function and spectrum of the soft X-ray background is studied below a discrete source detection limit, using two deep images from the Rosat X-ray satellite. The average spectral shape of pointlike sources, which account for 40 to 60 percent of the background intensity, is determined by using the autocorrelation function. The background spectrum, in the 0.5-0.9 keV band (M band), is decomposed into a pointlike source component characterized by a power law and a diffuse component represented by a two-temperature plasma. These pointlike sources cannot contribute more than 60 percent of the X-ray background intensity in the M band without exceeding the total observed flux in the R7 band. Spectral analysis has shown that the local soft diffuse component, although dominating the background intensity at energies not greater than 0.3 keV, contributes only a small fraction of the M band background intensity. The diffuse component may represent an important constituent of the interstellar or intergalactic medium.

Wang, Q. D.↗

NASA NTRS1993

The Ca II emission lines of SN 1987A

We study the evolution of the forbidden Ca II 7300 A and Ca II 8600 A emission lines in the spectrum SN 1987A. We can explain the observations with a model in which (1) the lines come from about 1.7 x 10 exp -4 solar mass of calcium distributed with filling factor about 0.1 in a sphere expanding with velocity 2500 km/s; (2) the electron number density is given by 4 x 10 exp 9/cu cm/(t/100d)-cubed; (3) the gas temperature decreases from T roughly 6400 K at 200 days to T roughly 3900 K at 500 days; and (4) ultraviolet pumping in the Ca II 3934 A, 3969 A (K, H) lines is required to explain the line ratios for t greater than about 350 days. The mass of calcium in our model is consistent with the LMC abundance of calcium in about 5 solar mass of hydrogen in the inner envelope of SN 1987A. If more calcium is produced by supernova nucleosynthesis, it must be in dense clumps that are too cool to emit the forbidden Ca II 7300 A and Ca II 8600 A lines.

Li, Hongwei↗

NASA NTRS1993

Supernova 1987A revisited

Updates on research on SN 1987A are provided, with emphasis on the aftermath of the explosion: the interpretation of the light curve and spectrum, the physics of the envelope, and the interaction of the supernova with its circumstellar envelope. UVOIR light curves, gamma rays and hard X-rays, and dust formation are examined, and explosion dynamics is explored. Attention is given to optical and IR spectrum development, with emphasis on the photospheric spectrum, nebular spectrum, ionization, emission-line profiles, line strengths, and iron, cobalt, and nickel. The circumstellar environment is discussed, with special reference to progenitor mass loss, circumstellar ring and nebulosity, radio emission, and soft X-rays.

Mccray, Richard↗

NASA NTRS1992

The semicircular shell of CTB 109

The radio and X-ray images of the supernova remnant CTB 109 have the morphology of a semicircular shell. It is shown that such a structure is a natural result of a supernova explosion that occurred 3,000 yr ago near an interface between the diffuse interstellar medium and a dense molecular cloud. The calculated X-ray, infrared, and radio fluxes agree fairly well with the observed values.

Wang, Zhenru↗

NASA NTRS1992

Hydrogen recombination at high optical depth and the spectrum of SN 1987A

A general theory is presented for hydrogen recombination line formation in an expanding medium in which some of the lines are optically thick. This theory is used to calculate the time evolution of the hydrogen lines of SN 1987A at t equal to or greater than 150 days, assuming that the supernova envelope is a homologously expanding uniform sphere. The theoretical luminosities and ratios of the recombination lines agree remarkably well with the observations. For the first 2 yr, the supernova envelope is optically thick to Balmer continuum. For t equal to or less than 400 days, hydrogen is ionized primarily from the n = 2 level by Balmer continuum photons, which are provided partly by the two-photon decay of the 2s state and partly by emission lines of heavy elements.

Xu, Yueming↗

NASA NTRS1991

The circumstellar shell of SN 1987A

An hourglass-shaped circumstellar shell, with radius about 0.65 lt-yr at the equator and about 2.4 lt-yr at the poles, surrounds SN 1987A. The shell is a bubble blown by the wind of the blue giant progenitor of SN 1987A into an equatorially concentrated wind of a prior red giant stage. The bubble was ionized by the initial UV flash of the supernova, and the emission-line region is concentrated in a ring at the waist of the hourglass. In about 2002 AD the supernova envelope will strike the shell. The resulting young supernova remnant will become a luminous source of soft X-rays and broad optical and ultraviolet emission lines.

Luo, Ding↗

NASA NTRS1991

Energy degradation of fast electrons in hydrogen gas

An equation is derived for calculating the energy distribution of fast electrons in a partially ionized gas and a method is provided to solve for the electron degradation spectrum and the energy deposition in different forms (ionization, excitation, or heating). As an example, the energy degradation of fast electrons in a gas of pure hydrogen is calculated, considering excitations to the lowest 10 atomic levels. The Bethe approximation and the continuous slowing-down approximation are discussed and it is concluded that these approximations are accurate to the order of 20 percent for electrons with initial energy of greater than about keV. The method and results can be used to determine heating, excitations, and ionizations by high-energy photoelectrons or cosmic-ray particles in various astrophysical circumstances, such as the interstellar medium, supernova envelopes, and QSO emission-line clouds.

Xu, Yueming↗

NASA NTRS1991

Supernova remnant 1987 A

A dense circumstellar shell, with radius of about 0.5 lt-yr, surrounds SN 1987 A. In 16 yr or less after outburst, the expanding debris of SN 1987 A will strike this shell. When it does, the hot gas and relativistic electrons resulting from the forward and reverse shocks will radiate X-rays, infrared radiation, and nonthermal radio waves. The remnant of SN 1987 A will then become one of the brightest X-ray and radio sources in the LMC.

Luo, Ding↗

NASA NTRS1991

Comptonization of gamma rays by cold electrons

An analytic method is developed for calculating the emergent spectrum of gamma-rays and X-rays scattered in a homogeneous medium with low-temperature electrons. The Klein-Nishina corrections of the scattering cross section and absorption processes are taken in account. The wavelength relaxation and the spatial diffusion problems are solved separately, and the emergent spectrum is calculated by convolving the evolution function of the spectrum in an infinite medium with the photon luminosity resulting from the spatial diffusion in a finite sphere. The analytic results are compared with that of Monte Carlo calculations and it is concluded that the analytic result is quite accurate.

Xu, Yueming↗

NASA NTRS1991

Policy opportunities

Recommendations are given regarding National Science Foundation (NSF) astronomy programs and the NASA Space Astrophysics program. The role of ground based astronomy is reviewed. The role of National Optical Astronomy Observatories (NOAO) in ground-based night-time astronomical research is discussed. An enhanced Explored Program, costs and management of small and moderate space programs, the role of astrophysics within NASA's space exploration initiative, suborbital and airborne astronomical research, the problems of the Hubble Space Telescope, and astronomy education are discussed. Also covered are policy issues related to the role of science advisory committees, international cooperation and competition, archiving and distribution of astronomical data, and multi-wavelength observations of variable sources.

Mccray, Richard↗

NASA NTRS1990

X-rays from colliding stellar winds

A stellar wind from a massive OB or Wolf-Rayet star in a binary system will strike the surface or stellar wind of its companion, forming shocked gas that can radiate X-rays. The X-ray spectrum from the shocked winds will vary in a predictable way with orbital phase, owing to photoelectric absorption by the stellar winds. Detailed models are calculated for the hydrodynamics and X-ray emission from two such systems. In one of these systems (HD 165052), the winds are nearly identical in strength. In the other (V444 Cygni), the wind of the Wolf-Rayet star overwhelms and crushes that of its companion. The calculated X-ray luminosities agree fairly well with the observed values for HD 165052 and for V444 Cygni. These results can be scaled to other such systems.

Luo, Ding↗

NASA NTRS1989

Superbubble blowout dynamics

Multiple supernovae and stellar winds from OB associations carve large holes filled with hot gas in the galactic disk. These superbubbles sweep up H I into cold, thin, dense shells and eventually grow large enough to blow completely out of the galactic H I disk. When superbubbles blow out of the disk, they vent hot gas and supernova energy into the galactic corona. In this paper ZEUS, a two-dimensional hydrodynamics code, is used to model the blowout of a superbubble from exponential and Gaussian models for the vertical density stratification. The results are compared to those from the Kompaneets (thin-shell) approximation. It is found that this approximation works very well, and that most of the mass of the shell remains in the plane, with 5 percent of it accelerating upward. The venting of the hot gas and the stability of the shell depends strongly on the model of the density distribution. It is suggested that the low galactic halo actually consists of a froth of merged superbubbles.

Mac Low, Mordecai-Mark↗

NASA NTRS1988

Bow shocks and bubbles are seen around hot stars by IRAS

Examination of the IRAS all-sky imagery reveals extended, arcuate, and ringlike features associated with hot luminous stars. They fall into a number of classes: stellar wind bow shocks, stellar wind bubbles, dust shells, dust heated by isolated B stars, bright rims, and dust in H II regions. Here, some objects are discussed in which the star exercises structural control over the spatial distribution of dust: bow shocks, bubbles, and radiation pressure-driven shells. A list of the 15 most prominent objects is presented, a few prototypes are shown, and their characteristics are explained in terms of thermal emission processes and gasdynamics.

Van Buren, Dave↗

NASA NTRS1988

X-rays from supernova 1987A

Detailed calculations of the development of the X-ray spectrum of 1987A are presented using more realistic models for the supernova composition and density structure provided by Woosley. It is shown how the emergence of the X-ray spectrum depends on the parameters of the model and the nature of its central energy source. It is shown that the soft X-ray spectrum should be dominated by a 6.4 keV Fe K(alpha) emission line that could be observed by a sensitive X-ray telescope.

Xu, Yueming↗

NASA NTRS1988

Superbubbles in disk galaxies

Correlated supernovae from an OB association create a superbubble: a large, thin, shell of cold gas surrounding a hot pressurized interior. Because supernova blast waves usually become subsonic before reaching the walls of the shell or cooling radiatively, the energy input from supernovae may be reasonably approximated as a continuous luminosity. Using the Kompaneets (thin-shell) approximation, the growth of superbubbles in various stratified atmospheres is numerically modeled. A dimensionless quantity predicts whether a superbubble will blow out of the H I disk of a spiral galaxy (and begin to accelerate upward) or collapse. Superbubbles blow out of the H I layer when they have a radius in the plane between one and two scale heights. They blow out only one side of a disk galaxy if their centers are more than 50-60 p above the plane and the gas layer has density and scale height typical of the Milky Way. Fingers of warm interstellar gas intrude into the hot interior when the superbubble overtakes dense clouds.

Mac Low, Mordecai-Mark↗

NASA NTRS1988

Supernova 1987A

Supernova 1987A (February 23, 1987) in the Large Magellanic Cloud is the brightest supernova to be observed since SN 1604 AD (Kepler). Detection of a burst of neutrinos indicates that a neutron star was formed. Radioactive decay of about 0.07 solar mass of Co-56 is responsible for the observed optical light as well as hard X-rays and gamma-ray lines. Ultraviolet, optical, and infrared 'light echoes' and soft X-rays provide information on the distribution of circumstellar matter and the evolution of the progenitor star.

Mccray, Richard↗