Engineering topics
Publications and source records attributed to Donn, B..
Amorphous magnesium silicate smoke particles were condensed from hydrogen and argon atmospheres containing Mg and Si0. A wide range of initial compositions were observed but all particles could be recrystallized into forsterite (Mg2Si04), by heating to 1000 C in vacuum. The amount of smoke formed decreased rapidly with temperatures between 300 and 800 K at reactant partial pressures of about 1 torr.
The case of condensation from a gas in the absence of any preexisting grains or nuclei upon which condensation can occur is considered. Some of the analysis is expected to apply to condensation on grains already present. An attempt is made to examine the basic assumptions of the various procedures used to describe condensation and to examine their applicability to astronomical systems. Attention is given to thermodynamic equilibration calculations, nucleation theory, kinetic mechanisms, the theoretical mechanisms, and condensation experiments.
A Fischer Tropsch type mechanism has been proposed for the synthesis of organic compounds in the primitive solar system. The evidence for hydrocarbon formation, especially those of higher molecular weight, under simulated conditions of the early solar nebula has been presented (Studier et al., 1968, 1972; Gelpi et al., 1970). In this paper, we report studies on the formation of specifically the lower molecular weight hydrocarbons. By using an open flow reaction system and two closed systems, several factors which may affect the production of these compounds have been examined.
Laboratory investigations of CN radical formation by photodissociation of parent molecules have suggested the possibility of observing emission lines in cometary spectra from newly formed CN radicals. These laboratory studies have shown that high initial internal excitation of CN is the rule with excitation of rotational levels N up to 70. In the collisionless environment of the cometary atmosphere this initial excitation would yield a corresponding distribution for the lowest vibrational level of the ground X(2) Sigma (+) state. Our calculations show that it is feasible with present observational techniques to detect photochemically excited lines with N approx. equal to 30 in the 0-0 band of the violet system.
The continuum to molecular emission intensity ratio in cometary spectra is examined as an indicator of initial composition of the comets. This ratio presumably measures the dust to gas ratio in comets, or the component of gas yielding visible emissions. The spectral relationships and compositions of new comets and more evolved ones are compared. No significant difference in the continuum to emission ratio was noted between new and more evolved comets, which is in contradiction to Oort's conclusion. This suggests that no systematic change occurs in the continuum to emission ratio as comets evolve from new to periodic. Perihelion passage does not seem to produce a general weakening of the continuum, although it occurs in individual cases.
The intensity ratio of the continuum to the molecular emissions was estimated in the spectra of eight-five comets. Four conclusions are drawn: (1) There is no readily apparent difference in continuum to emission intensity ratio between new and more evolved comets; (2) an intrinsic distribution of this characteristic does occur; (3) periodic comets with weak continua are derived from new comets with the same property; and (4) no weakening of the continuum in general occurs following perihelion passage. The infrared evidence for comet Encke suggests that the faintness of its continuum may be caused by a size distribution containing only particles larger than about 10 um.
Papers are presented dealing with observations of comets. Topic discussed include: photometry, polarimetry, and astrometry of comets; detection of water and molecular transitions in comets; ion motions in comet tails; determination of comet brightness and luminosity; and evolution of cometary orbits. Emphasis is placed on analysis of observations of comet Kohoutek.
Results of observations of comet Bradfield using the Haystack telescope are summarized with emphasis on the detection of the 1.35 cm emission line of water in the comet. The excitation of water and methyl cyanide in comets is briefly considered.
Flyby missions and systematic observations of comets are projected for studying comet nuclei and cometary dust tail structures.
The association of comets with star formation in clusters is elaborated. This hypothesis is also used to explain origin and evaluation of the Oort cloud, the composition of comets, and relationships between cometary and interstellar molecules.
On January 5, 1974, the ultraviolet emissions from Comet Kohoutek (1973f) in the spectral range from 1200 to 3200 A were studied with the aid of a rocket carrying two spectrophotometers. In addition to the Lyman alpha line of atomic hydrogen at 1216 A and the (0,0) and (1,1) bands of OH at 3090 and 3142 A, both atomic oxygen (1304 A) and atomic carbon (1657 and 1561 A) were detected.
Examination of the relations between the solar-wind and Comet Bennett during the period from Mar. 23 to Apr. 5, 1970. A large kink was observed in the ion tail of the comet on April 4, but no solar-wind stream was observed in the ecliptic plane which could have caused the kink. Thus, either there was no correlation between the solar wind at the earth and that at Comet Bennett (which was 40 deg above the ecliptic) or the kink was caused by something other than a high-speed stream. The fine structure visible in photographs of the kink favors the second of these alternatives. It is shown that a shock probably passed through Comet Bennett on March 31, but no effect was seen in photographs of the comet. A stream preceded by another shock and a large abrupt change in momentum flux might have intercepted the comet between March 24 and March 28, but again no effect was seen in photographs of the Comet.
The particular characteristics of chemistry in interstellar space are determined by the unique environmental conditions involved. Interstellar matter is present at extremely low densities. Large deviations from thermodynamic equilibrium are, therefore, to be expected. A relatively intense ultraviolet radiation is present in many regions. The temperatures are in the range from 5 to 200 K. Data concerning the inhibiting effect of small activation energies in interstellar clouds are presented in a table. A summary of measured activation energies or barrier heights for exothermic exchange reactions is also provided. Problems of molecule formation are discussed, taking into account gas phase reactions and surface catalyzed processes.
Gas phase processes involving interstellar molecules are considered. The primary process is photodissociation in the ambient radiation field. In obscured regions, a combination of Johnson's (1965) and Stecher's (1969) Perseus extinction curves was used to obtain the radiation field as a function of the extinction at 5,500 A. On the basis of laboratory measurements, it is concluded that the molecules of water, ammonia, methane, and formaldehyde have lifetimes less than one hundred years.
Quantitative discussion of the photochemistry and lifetime against photodecomposition of five interstellar molecules: H2CO, NH3, H2O, CH4, and CO. For the first four molecules, primary photochemical decomposition processes yielding atomic and molecular hydrogen are considered in addition to photoionization. The quantum yield for decomposition of these molecules is unity. For CO, only decomposition to ground-state carbon and oxygen atoms occur, but the quantum yield is not known and may be considerably less than unity. The radiation field in obscuring clouds is estimated, using an interstellar extinction curve for the Perseus region.
Examination of the question of chemical evolution. Observational evidence for the occurrence of some organic compounds in space is presented, and the likely existence of more complex molecules which are as yet undetected or unidentified is pointed out. This implies that massive solid objects, i.e., planets and asteroids, were accumulated from material which already contained a variety of organic compounds. The objects or regions of the galaxy studied are comets, interstellar space, prestellar nebulae, and cool stellar atmospheres.
The formation of OH by inverse predissociation from continuum levels to the v = 1, k = 1 vibrational level of a repulsive molecular state originating from an asymptotic atomic ground state is discussed. A rate constant of (1 to 3) x 10 to the minus twentieth power cu cm/sec is calculated for temperatures greater than 20 K. Predicted OH densities are found to be consistent with observations in dense heavily obscured clouds, but appear to be somewhat low for H I clouds.
Microwave water emission in Comet Bennett, deriving column density for various temperatures of cometary gas