Engineering topics

Donn, B.

Publications and source records attributed to Donn, B..

At least 55 records · Page 3

NASA NTRS1981

A systematic program of cometary spectroscopy

Some early results of a systematic program of observing the spectroscopic behavior of comets as a function of heliocentric distance are presented. An ultraviolet sensitive microchannel plate intensifier spectrograph was used to record the 3000-5000A spectrum of comets brighter than magnitude 17 with a spectral resolution of 8 or 16A, followed by direct image for better interpretation of the spatial distribution of spectral features. Although the goals of the program require much more time and data, some interesting results from Comets Schwassmann-Wachmann, 1, Bradfield and Bowell were obtained.

Larson, S. M.↗

NASA NTRS1981

Laboratory Research

To properly interpret the rapidly growing body of data from comet observations, many types of laboratory measurements are needed. These include: (1) molecular spectroscopy in the visible, ultraviolet, infrared and microwave region of the spectra; (2) laser fluorescent spectroscopy of photofragments; (3) laboratory cross-section or reaction rate measurements using flow tube techniques, fluorescent spectroscopy detection for neutrals and ion-molecule reaction techniques; (4) experiments to simulate solar-wind interactions with comets; (5) studies of the properties and behavior of ice mixtures; (6) experiments on the sublimation rate of ice, and the phase transition from amorphous to crystalline ice; (7) investigations of the irradiation of ice; and (8) the electron impact dissociation and excitation of molecules of cometary interest. A nearly completed experiment on the proton irradiation of ice is described.

Donn, B.↗

NASA NTRS1981

Structure and origin of cometary nuclei

There is strong evidence that a comet nucleus consists of a single object whose basic structure is Whipple's icy conglomerate. A number of cometary phenomena indicate that the nucleus is a low density, fragile object with a large degree of radial uniformity in structure and composition. Details of the ice-dust pattern are more uncertain. A working model is proposed which is based on theories of accumulation of larger objects from grains. This nucleus is a distorted spherical aggregate of a hierarchy of ice-dust cometesimals. These cometesimals retain some separate identity which lead to comet fragmentation when larger components break off. The outer layers of new comets were modified by cosmic ray irradiation in the Oort Cloud. The evidence for meteorite-comet association is steill controversial. Current dynamical studies do not seem to require a cometary source of meteorites.

Donn, B.↗

NASA NTRS1981

Vibrational disequilibrium in low pressure clouds

Steady state vibrational populations of SiO and CO in several dilute blackbody radiation fields have been calculated as a function of total pressure, kinetic temperature, and chemical composition of the gas. An approximate calculation for polyatomic molecules has also been carried out. Vibrational disequilibrium becomes increasingly significant as the total pressure and radiation density decrease. Many regions of postulated grain formation are found to be far out of thermal equilibrium before the onset of condensation. The use of classical nucleation theory and equilibrium thermodynamics to predict the onset of condensation and the composition of the solids or the molecular composition of the gas phase in such regions therefore is very questionable and needs to be examined for each case.

Nuth, J. A.↗

NASA NTRS1981

A spectroscopic study of intermediates in the condensation of refractory smokes - Matrix isolation experiments of SiO

The infrared and Raman spectra of N2 matrix-isolated silicon oxides were investigated. The vibrational frequencies of SiO, Si2O2, and Si3O3 were identified and assigned on the basis of normal coordinate analyses. Heating the solid to approximately 50 K (evaporating the matrix) leaves a residue whose infrared spectrum is identical to that of a smoke condensed at ambient temperatures. Further heating of the sample to approximately 500 K leads to significant changes in the band shapes. Investigations of the infrared spectra at several stages of the diffusion process enable a mechanism to be proposed for the transition from molecular properties to those of the residue (bulk) material, which has been characterized as Si2O3.

Khanna, R. K.↗

NASA NTRS1981

Fluorescence equilibrium in the ultraviolet spectra of Comets Seargent (1978 m) and Bradfield (1979 l)

Detailed fluorescence calculations for OH including the Swings effect were performed. These calculations were used to reproduce the high resolution spectra of Comets Seargent and Bradfield taken with IUE. The calculations also provide the OH fluorescence efficiencies (g factors), as a function of heliocentric radial velocity, which are needed to derive OH abundances from measured fluxes. A close examination of the spectra shows no sign of the corresponding emission bands of OD allowing upper limits to be placed on the ratio N(OD)/N(OH). Preliminary attempts to reproduce the CO+ band structure by fluorescence are discussed.

Donn, B.↗

NASA NTRS1981

A search for interstellar molecules in the spectra of highly reddened stars

A total of ten stars were observed with cameras of the International Ultraviolet Explorer (IUE) in both high and low dispersion. One star, X Persei (HD 24534, 6.0 BE), was analyzed in detail. Ultraviolet observations of the column densities of CO match those derived from the radio to within a factor of 4, with the difference probably due to the larger beam size of the radio measurement and the assumption of a thermal population in the rotational levels of CO. Upper limits are given to the log column densities for OH, HCl, and CH2 of 14.0, 12.3 and 12.8. The carbon abundance was found to be about solar with a possible depletion of about a factor of 2. With precautions concerning both noise and correct background, the IUE can be used for studies of interstellar molecules.

Lien, D.↗

NASA NTRS1981

Comet nucleus - Some characteristics and a hypothesis on origin and structure

The spectroscopic properties of comets are discussed, and consideration is given to the problems of the cosmic ray irradiation and the origin and structure of cometary nuclei. The classification of cometary spectra as continuum dominant, molecular emissions dominant and CO(+) dominant is examined, and changes in spectral characteristics with heliocentric distance although not with cometary age are pointed out. Spectral evidence for a variety of organic compounds in the cometary nucleus is noted, and a scenario is presented whereby these complex molecules may form as a result of cosmic ray irradiation of cometary ices in the Oort cloud. Finally, a model is proposed for the accumulation of small icy grains to form a cometary nucleus composed of cometesimals of size distribution following a -5th power dependence on mass.

Donn, B.↗

NASA NTRS1980

IUE observations of the UV spectrum of Comet Bradfield

Comet Bradfield (1979), discovered shortly after perihelion on Christmas day 1979, is characterized by a retrograde orbit of period approximately equal to 250 yr. This orbit was very favorable for observation by the IUE satellite, and the first observations, the preliminary results of which are reported here, were made on 10 and 11 January 1980. Previously, comprehensive vacuum UV cometary spectra were available only for Comet West (1976VI) from rocket observations and Comet Seargent (1978m) with IUE, hence three comets have now been observed in the wavelength region where most of the major constituents are detectable spectroscopically. The UV spectra of these comets are remarkably similar, despite large differences between them in the gas-to-dust ratio and the different heliocentric distances at which the observations were made, and although the statistical basis is still very small, the implication for cosmogony is that nearly all comets have the same primary composition and origin.

Feldman, P. D.↗

NASA NTRS1980

A laboratory study of magnesium-tetrabenz-porphyrin - Lack of agreement with diffuse interstellar bands

The visible and infrared spectra and thermal behavior of the bis-pyridal-magnesium-tetrabenz-porphyrin molecule proposed as the carrier of the diffuse interstellar bands were measured. Of the six band coincidences reported by Johnson (1977), only one, 4430 A, occurs in these experiments. This coincidence requires a special environment, not likely to occur in interstellar space but the infrared spectrum does not support Johnson's vibrational scheme. These spectroscopic and thermal measurements contradict the hypothesis that this molecule causes the diffuse bands.

Donn, B.↗

NASA NTRS1980

Laser spectroscopy of the CN radical and astrophysical applications

The formation of CN radicals by flash photolysis of a number of parent molecules was monitored by laser induced fluorescence in the B2Sigma(plus)-X2Sigma(plus) transition of CN. The rotational and vibrational energy of the newly formed radicals in the X state was measured directly. Formation of CN(A2Pi) in the lower vibrational levels was determined by an indirect method based on resonant energy transfer to higher vibrational levels of the ground state. These laboratory studies have shown that high initial internal excitation of CN with rotational levels of maximum N equals 50-70 is the rule. In general, the formation of simple molecules in excited states is commonly the case. There appear to be astrophysical systems where radiation from these excited levels may be detectable. Such observations would serve as a probe of molecular formation.

Cody, R. J.↗

NASA NTRS1979

Some aspects of condensation in clouds

Several general features of nucleation characteristics of low-density cosmic clouds are discussed. These are: (1) tendency for metastable condensates to form; (2) nonoccurrence of nominal refractory molecules in the gas; (3) a strong temperature dependence of condensation at relatively low temperatures; and (4) significant vibrational disequilibrium in cosmic clouds. These support previous analyses which indicate that equilibrium calculations have restricted applicability. A kinetic treatment of condensation is required for cosmic grains, and the possibility of formulating such an analysis is pointed out.

Donn, B.↗

NASA NTRS1979

An analysis of the infrared continuum of comets. II - Comet Kohoutek

An analysis of the infrared observations of comet Kohoutek has been carried out on the basis of silicate grain models. The 'observed' grain temperature has been obtained as a function of the heliocentric distance, and it is compared with the expected distribution. In the visible region, one requires for the refractive index, (m = n-ik), n of about 1.6 and k less than 0.05. The total number of grains varies essentially as the inverse square of radial distance; the size of the particles in the antitail of the comet Kohoutek is greater than 5 microns. The ratio of infrared flux from grains to the incident solar flux at wavelengths of a few microns is calculated.

Swamy, K. S. K.↗

NASA NTRS1979

The ultraviolet spectrum of Comet Seargent 1978m

UV spectrograms of Comet Seargent (1978m) have been obtained with the IUE satellite in the low- (1.8-nm) and high- (0.1-nm) dispersion modes. In the long-wavelength region (189-340 nm) emission bands of OH, CS, CO2(+), NH, and CO(+) can be identified, and the rotational structure of OH could clearly be resolved. In the short-wavelength region (119-192 nm) emission lines of H(Lyman-alpha), O I, C I, and S I appear. These observations demonstrate that the IUE observatory can effectively be used to obtain high-quality spectra of comets as faint as ninth magnitude.

Jackson, M. W.↗

NASA NTRS1978

Condensation of nonequilibrium phases of refractory silicates from the vapor

Silicon monoxide solid was evaporated in a bell jar containing reducing, neutral, or oxidizing atmospheres at pressures of a few torr. The vapor invariably condensed as smoke-sized particles of silicon sesquioxide, Si2O3. The condensation of a solid whose composition differs from that of the parent gas and is apparently the least stable of the three solid species illustrates the importance of specific nucleation effects in the condensation process. This result has significant implications for theories of formation of grains in space.

Day, K. L.↗

NASA NTRS1978

An experimental investigation of the condensation of silicate grains

Results are presented for a series of laboratory experiments designed to investigate the nucleation of small silicate grains from a vapor of astrophysically significant elements and compounds. In the experiments, magnesium silicate grains were condensed by simultaneously evaporating Mg and SiO solids into an atmosphere of argon or hydrogen at a pressure of a few torr. The results show that at low temperatures (up to a few hundred degrees C) the condensates are amorphous grains and have widely varying stoichiometries. The thermodynamically most stable compounds (Mg2SiO4, MgSiO3, SiO2) do not form readily, but all initial condensates can be converted to crystalline forsterite (Mg2SiO4) by heating to 1000 C in vacuum. At higher temperatures (above 700 K) it becomes more difficult to nucleate any silicates, and those that do form are amorphous, indicating that surface energies and kinetic effects are very important in determining under what conditions condensation will occur. The IR spectra of the experimentally produced magnesium silicates are found to have a strong resemblance to those observed in many astronomical clouds.

Day, K. L.↗

NASA NTRS1978

On the detection of newly created CN radicals in comets

A description is presented of laboratory measurements concerning the amount and the distribution of energy released into newly formed radicals by the photodissociation process. The experimental method used involved flash photolysis of the parent compound followed by laser induced fluorescence detection of the CN fragment. This method allowed the detection of individual rotational lines and, thereby, a monitoring of the relative populations of the vibrational-rotational levels of the CN ground state. The results obtained in the investigation suggest the possibility of observing emission lines in cometary spectra from newly formed CN radicals. Calculations show that it is feasible with present observational techniques to detect photochemically excited lines in the 0-0 band of the violet system.

Donn, B.↗

NASA NTRS1978

Grain formation in astronomical systems: A critical review of condensation processes

An analysis is presented of the assumption and the applicability of the three theoretical methods for calculating condensations in cosmic clouds where no pre-existing nuclei exist. The three procedures are: thermodynamic equilibrium calculations, nucleation theory, and a kinetic treatment which would take into account the characteristics of each individual collision. Thermodynamics provide detailed results on the composition temperature and composition of the condensate provided the system attains equilibrium. Because of the cosmic abundance mixture of elements, large supersaturations in some cases and low pressures, equilibrium is not expected in astronomical clouds. Nucleation theory, a combination of thermodynamics and kinetics, has the limitations of each scheme. Kinetics, not requiring equilibrium, avoids nearly all the thermodynamics difficulties but requires detailed knowledge of many reactions which thermodynamics avoids. It appears to be the only valid way to treat grain formation in space. A review of experimental studies is given.

Donn, B.↗