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At least 217 records · Page 12
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NASA NTRS1982

Einstein Observatory solid state spectrometer observations of M87 and the Virgo cluster

X-ray observations of the galaxy M87 and of a region in the Virgo cluster displaced 7 arcmin from the center of M87 are reported. X-ray spectra were obtained at these two locations with the Solid State Spectrometer onboard the Einstein Observatory. Emission lines were observed in both locations, indicating the presence of heavy elements at abundances near solar. There is no strong abundance gradient, within the errors. A temperature gradient has been detected: T increases from about 1.4 keV at the position of M87 to about 3.35 keV 7 arcmin away. There is lower temperature thermal emission at the center of M87 with T about 0.6 keV, consistent with models for cooling flows in this cluster. A mass infall rate of nearly 20 solar masses/yr has been obtained. In addition to the thermal emission, a power-law component has been detected in the spectrum of M87, consistent with that observed by HEAO 1, indicating that this component probably originates in the galaxy itself. Comparison of the results with those of other observers and with theoretical models for this source indicates the presence of intracluster gas having density near 1/1000 per cu cm and temperature of about 30,000,000 K.

Lea, S. M.↗
NASA NTRS1983

Mass loss, levitation, accretion, and the sharp-lined features in hot white dwarfs

A study has been conducted of eight white dwarfs, including seven DA and one He-rich types. The study is based on high-resolution observations conducted with the aid of the International Ultraviolet Explorer. Four of the dwarfs show features related to heavy elements which are not interstellar in origin. It is tentatively suggested that, at least in the hottest low-gravity DA white dwarfs, the observed narrow-lined features are formed in expanding halos or winds associated with the white dwarfs. Theoretically, stable white dwarf halos should actually be coronae with temperatures in excess of 1,000,000 K. However, the observed narrow-lined features do not suggest such high temperatures. The observed radial velocities suggest weak stellar winds in two hot white dwarfs, namely, G191-B2B and 2111+49. It is tentatively proposed that radiative levitation can explain the appearance of the observed metallic lines in the hot DA white dwarfs.

Bruhweiler, F. C.↗
NASA NTRS1982

Chemical evolution of OB associations

It is determined that the existence of Al-26 and Pd-107 in meteorites in the early solar system indicates that our solar system probably formed inside an OB association that had been contaminated by the debris of at least one supernova. In addition to these radioactive tracers, the contamination of the material out of which the solar system formed would have significantly enriched the heavy element composition of the solar system relative to that of the average interstellar medium. It is found that the solar system would be enriched in those isotopes which are produced by the more massive stars, such as O-16, C-12, Ne-20, and some other r-process material. It is proposed that specific isotopic ratios and elemental ratios reflecting these differences would include the Ne-20/Ne-22 ratio, which would be higher in the solar system than in the interstellar medium and the cosmic rays, the C-12/C-13 ratio which would be higher in the solar system than in the interstellar medium, and the oxygen-to-carbon ratio, which would also be higher in the solar system than in the typical interstellar medium.

Schramm, D. N.↗
NASA NTRS1983

Turbulent heating in solar cosmic ray theory

The heating of minor ions in solar flares by wave-wave-particle interaction with Langmuir waves, or ion acoustic waves, can be described by a diffusion equation in velocity-space for the particle distribution function. The dependence of the heating on the ion charge and mass, and on the composition of the plasma, is examined in detail. It is found that the heating mechanisms proposed by Ibragimov and Kocharov cannot account for the enhanced abundances of heavy elements in the solar cosmic rays.

Weatherall, J.↗
NASA NTRS1983

Shock processing of interstellar grains

Theoretical and observational evidence is presented that shock processing of interstellar dust grains by supernova blast waves affects both heavy element depletions and ultraviolet extinction curves. By coupling a realistic model of grain sizes and populations with a radiative shock code, significant grain destruction at velocities as low as 40 km/s is demonstrated. Nonthermal sputtering and grain-grain collisions destroy relatively more large grains than small, and more silicates than graphite. Consequently, both the 2175 A extinction 'bump' and the far-ultraviolet normalized extinction are increased in strength. Ultraviolet extinction studies with the International Ultraviolet Explorer of nine stars near three supernova remnants (the Monoceros Loop, Shajn 147, and Vela) exhibit strong 2175 A bumps and normal or high far-ultraviolet extinction. Diffuse bands, if they are created by small grains, should show little correlation with such activity.

Seab, C. G.↗
NASA NTRS1984

Turbulent heating in solar cosmic-ray theory

The heating of minor ions in solar flares by wave-wave-particle interaction with Langmuir waves, or ion acoustic waves, can be described by a diffusion equation in velocity-space for the particle distribution function. The dependence of the heating on the ion charge and mass, and on the composition of the plasma, is examined in detail. It is found that the heat mechanisms proposed by Ibragimov and Kocharov cannot account for the enhanced abundances of heavy elements in the solar cosmic rays.

Weatherall, J.↗
NASA NTRS1985

Multiple Scattering Effects in de/dx-E Instruments for Isotopic Composition Studies

The development of cosmic ray telescopes to separate individual isotopes of heavy elements using the dE/dx-vs.-E technique depends on the incorporation of precise trajectory sensing elements into these systems. In typical implementations the particle trajectory is derived from a set of position measurements made prior to a particle's entering the first energy loss detector. The use of the trajectory obtained in this way to correct energy loss signals for the actual pathlengths through the detectors depends on the assumption that the particle trajectory is a straight line. In order to resolve iron isotopes the angles of incidence, theta the particle track must be known rather accurately.

Wiedenbeck, M. E.↗
NASA NTRS1984

X-ray emission from supernova remnants MSH 14 - 63 and RCW 103

Electron-ion collisions, heavy element enrichment of emissions from reverse-shocked ejecta, abundances in the interstellar medium (ISM), the correspondence between the projected and measured age, ISM particle density, and distance, and classification factors for Type I or II supernovae are examined using a nonequilibrium ionization (NEI) model. The NEI model accounts for the absence of collisional ionization equilibrium and possibly thermal equilibrium between electrons and ions behind the shock fronts, and describes the emission spectra from ejecta and the shocked ISM. When applied to data from the SNR MSH 14 - 63 a non-Coulomb, ion-electron interaction is obtained that is consistent with a shocked ISM. For the SNR RCW 103, however, the processes behind the shock front are not discernable, although the elemental abundances will have solar values if the interactions are non-Coulomb. The age of RCW 103 must be approximated from physical data, as Chinese astronomers would not have been able to see the event from their position on earth two millenia ago.

Nugent, J. J.↗
NASA NTRS1984

Ultraviolet analysis of the peculiar supergiant HD 112374 = HR 4912

The ultraviolet energy distribution of the metal-poor supergiant HD 112374 is analyzed based on observations from the International Ultraviolet Explorer (IUE) satellite for the region between 1200 and 2000 A. A discontinuity was found in the UV spectra at 2600 A which confirmed the low-abundance of heavy elements found by Luck et al. (1983). Values for effective temperature and log g in HD112374 were consistent with the star being a very luminous Population II semi-regular variable. The full observational results are presented in a table.

Bohm-Vitense, E.↗
NASA NTRS1985

CNO abundances resulting from diffusion in accreting nova progenitors

The configurations of potential nova progenitors at the onset of a thermonuclear runaway (TNR) are obtained by evolving a series of white dwarf (WD) models through the quasi-static accretion phase preceding the TNR. Special attention is paid to the effect of diffusion on composition profiles. The initial parameters of the accretion phase are the WD mass, the WD luminosity, and the accretion rate. They lead to a wide range of combinations of accreted mass and heavy element content of the envelope Z(env). It is found that the WD mass is the most important parameter in determining the accreted mass. A lower limit exists for the mass of a WD capable of producing a nova-like outburst which decreases with decreasing WD mass. There is a strong correlation between the accretion rate and Z(env), higher Z(env) values resulting from lower accretion rates. Changes in WD luminosity have no dramatic effect on the outcome of the accretion phase.

Kovetz, A.↗
NASA NTRS1985

What have we learned from modeling giant planet interiors?

Models of the giant planets are reviewed. The theoretical techniques used in computing the models are described, and the observational and experimental inputs are summarized. Special emphasis is placed on uncertainties in these input data. The models are then examined and the results of various authors presented. It is demonstrated that all the planets have heavy-element enhancements of between 10 and 40 earth masses, with a large fraction of this material residing in the core. It is also shown that the ratio of ice to rock in Uranus and Neptune is on the order of three. The implications of these results for theories of the origin of the solar system are discussed.

Podolak, M.↗
NASA NTRS1985

The infrared echo of Type II supernovae with circumstellar dust shells. II - A probe into the presupernova evolution of the progenitor star

This paper studies the spectral appearance and evolution of the infrared light curve, also referred to as the infrared echo, of Type II supernovae embedded in carbon- or oxygen-rich circumstellar dust shells. The distinct spectral signature of the echo and its temporal evolution can be used to estimate the mass of the shell and identify the composition of the dust. Since the shell mass and dust composition are determined by the combined effect of stellar mass loss and the dredging of newly synthesized heavy elements to the stellar surface, observations of the infrared echo may provide useful clues to the presupernova evolution of the progenitor star.

Dwek, E.↗
NASA NTRS1985

The atmospheres of icy bodies

Ices are considered as the major volatile-carrying solid in the outer solar system. As such, they play a major role in forming atmospheres of icy satellites and the comas of comets and they have probably contributed to the enrichment of heavy elements in outer planet atmospheres. A possible role for clathrate hydrates in causing the activity of distant comets and sub-surface processes on icy satellites seems worthy of further study.

Owen, T.↗
NASA NTRS1986

The X-ray spectrum of SN 1006

The supernova (SN) of A.D. 1006 represents the brightest in recorded history. The X-ray observations of its remnant appear to be full of puzzles. The present paper has the objective to reevaluate published X-ray data on SN 1006, using detailed nonequilibrium spectral computations and a self-consistent hydrodynamic SNR model. In a summary of the main conclusions, it is pointed out that an observed power-law continuum can be understood as mainly free-free emission from a reverse shock into SN ejecta with an initially uniform density profile. The spectral index is correctly predicted by such a model. Most of the continuum is produced by fully ionized gas in the dense outer layers of ejecta. The X-ray spectrum is not consistent with a solar composition or a uniform composition but is consistent with a layered composition of ejecta containing carbon on the outside, and processed heavy elements on the inside. There is no difficulty in including a large mass of iron.

Hamilton, A. J. S.↗
NASA NTRS1986

The X-ray spectrum of Tycho

Baade (1945) has identified Tycho's supernova (SN) of A.D. 1572 as Type I, taking into account an analysis of the light curve recorded by Tycho. However, in spite of the obvious interest of Tycho's spectrum, to date the spectrum remains poorly understood. The present study is concerned with a reevaluation of the X-ray spectrum of Tycho. Attention is given to detailed nonequilibrium calculations and a self-consistent hydrodynamic model for ejecta consisting of pure heavy elements moving into a uniform external medium. The model provides a satisfactory fit to the spectrum, and avoids any requirement for a large mass of material in Tycho. It is demonstrated that the observed iron line emission is compatible with the presence of perhaps 0.7 solar mass of iron, of which 0.3 solar mass is shocked.

Hamilton, A. J. S.↗
NASA NTRS1986

The chemical composition of the cool white dwarf LP 701-29

New observations and a new synthetic spectrum of the cool white dwarf LP 701-29 are presented. The analysis suggests that the atmosphere of this star is helium dominated, with enhanced metal abundances compared to the usual abundances of white dwarfs. The best fit is achieved with an effective temperature of 4500 K and an abundance of heavy elements (especially Ca) of the order 0.01 of the solar ratio of metals to helium.

Kapranidis, S.↗
NASA NTRS1985

CNO abundances in novae ejecta

Isotope production, particularly CNO, which are enhanced in novae events and the interstellar medium (ISM) by novae are discussed in terms of emission-line data taken a few weeks after novae events. The emission-line abundances emerge only after the fading of absorption spectrum, and are also detectable in thin shells which sometimes, though rarely, form after novae. CNO abundances in these shells are equivalent to those observed in fading novae, with CNO/H ratios being 100 times solar values. Possible heavy element production pathways in the events and their shrinking are discussed, along with observation techniques for detecting CNO isotopes through absorption data on atomic transitions in novae ejecta and the ISM.

Williams, R. E.↗
NASA NTRS1986

The (32)S/(33)S abundance as a function of galactocentric radius in the Milky Way

Astration of heavy elements by the stars of the Milky Way forms a fossil record which may preserve spacial distribution of the mass function for the stars in the galaxy. Sulfur is among the last common element for which the relative abundance of its various isotopes have yet to be completely measured within our galaxy. Explosive oxygen burning in massive stars is thought to be the process which dominates sulfur production within stars. There models predict that the various isotopes (S-32, S-33, S-34) are formed in relative abundance which depend strongly upon the mass of the parent star. This relative abundance is thought to be unaffected by subsequent stellar procesing since all important sinks of sulfur destroy it without regard for isotopic form. Hence the spacial variation of the mass function (MF) can be studied by measuring the abundance variation of sulfur isotopes in the galaxy provided that the product yields for these isotopes are known accurately as a function of stellar mass.

Greenhouse, M. A.↗