Engineering topics

Dwek, E.

Publications and source records attributed to Dwek, E..

61 records · Page 4

NASA NTRS1981

Where is the ice in comets

JHKL photometry of comets P/Tuttle, Meier 1980q, P/Stephan-Oterma, and Bowell 1980b has yielded J-H and H-K colors, uncontaminated by thermal emission, which are nearly identical for all four comets. The colors are inconsistent with the reflection spectrum from a cloud of icy particles, if the particles are composed primarily of any of the ices (H2O, CO2, CH4, or NH3) commonly assumed to be present in the nuclei of comet. Circular variable filter spectra of comet Stephan-Oterma confirm the absence of any absorption features due to these ices. The reflection spectra of these comets appear most like those of Cand S-type asteroids and the ring of Jupiter. Mie calculations for core-mantle particles suggest that ices could be present as thin mantles on inherently reddish, refractory cores, such as magnetite.

Ahearn, M. F.↗

NASA NTRS1981

The infrared emission from supernova condensates

The possibility of detecting grains formed in supernovae by observations of their emission in the infrared is examined. The basic processes determining the temperature and infrared radiation of grains in supernova environments are analyzed, and the results are used to estimate the infrared emission from the highly metal enriched fast moving knots in Cas A. The predicted fluxes lie within the reach of current ground-based facilities at 10 microns, and their emission should be detectable throughout the infrared band with cryogenic space telescopes.

Dwek, E.↗

NASA NTRS1980

Excitation mechanisms for the unidentified infrared emission features

Infrared and radio observations of various objects are analyzed to put observational constraints on the mechanism which gives rise to the unidentified emission features at 3.3, 3.4, 6.2, 7.7, 8.6, and 11.3 microns. The results show that gas-grain collisions or fluorescence is not likely to be the excitation mechanism responsible for the observed features. Thermal emission by dust is reanalyzed and it is concluded that this mechanism can explain the emission features. A simple model in which the emission features arise in a population of small, hot, interstellar grains is constructed. These grains are very efficient radiators, and the emitting materials only need be a minor grain constituent to provide the power that is emitted in the features. The model offers, therefore, a simple explanation for the absence of these features in absorption.

Dwek, E.↗

NASA NTRS1978

Proton-associated alpha-irradiation in the early solar system - A possible K-41 anomaly

It is shown that alpha-particle fluences associated with proton fluences sufficiently high to produce the Al-correlated excess of Mg-26 found in the Allende meteorite produce equally significant amounts of Ca-41. The Ca-41 is produced by Ar-38(alpha,n)Ca-41 reactions occurring in the gas and condenses into the Ca-Al-rich grains forming at that time. After the decay of Ca-41, these grains will have a Ca-correlated excess of K-41. The question is considered whether the proposed scheme is capable of producing an observable excess of K-41. A calculation is conducted of the magnitude of the K-41 excess expected to be found in Ca-Al-rich grains for various ratios of Ca/K by developing an irradiation-condensation model. It is shown that on the basis of the currently available data it is not possible to come to firm conclusions regarding the validity of the irradiation model.

Dwek, E.↗

NASA NTRS1977

Isotopic anomalies and proton irradiation in the early solar system

Nuclear cross sections relevant to the various isotopic-abundance anomalies found in solar-system objects are evaluated in an attempt to set constraints on the hypothesized mechanism of irradiation of forming planetesimals by energetic protons from the young sun. A power-law proton spectrum is adopted, attention is restricted to proton energies less than about 20 MeV, and average cross sections are calculated for several reactions that might be expected to lead to the observed anomalies. The following specific anomalies are examined in detail: Al-26, Na-22, Xe-126, I-129, Kr-80, V-50, Nb-92, La-138, Ta-180, Hg-196, K-40, Ar-36, O-17, O-18, N-15, C-13, Li, Be, and B. It is suggested that the picture of presolar-grain carriers accounts for the facts more naturally than do irradiation models.

Clayton, D. D.↗

NASA NTRS1976

Gamma-ray emission and nucleosynthesis of lithium by young pulsars

It is proposed that Li-7 is produced in the Galaxy primarily by alpha-alpha collisions surrounding newly born pulsars. About 10 percent of the pulsar energy losses are converted to medium-energy alpha-particles which collide in a dominantly He nebula. The problem of the origin of lithium would be solved by the scenario, and clear-cut tests by nuclear gamma-ray astronomy are described.

Clayton, D. D.↗

NASA NTRS1975

Solar models of low neutrino-counting rate - The depleted Maxwellian tail

Evolutionary sequences for the sun are presented which confirm that the Cl-37 neutrino counting rate will be greatly reduced if the high-energy tail of the Maxwellian distribution of relative energies is progressively depleted. Thermonuclear reaction rates and pressure are reevaluated for a distribution function modified by the correction factor suggested by Clayton (1974), and the effect of the results on solar models calculated with a simple Henyey code is discussed. It is shown that if the depletion is characterized by a certain exponential dependence on the distribution function, the counting rate will fall below 1 SNU for a distribution function of not less than 0.01. Suggestions are made for measuring the distribution function in the sun by means of neutrino spectroscopy and photography.

Clayton, D. D.↗