Browsing by Author "Clayton, Donald W."
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Item Accuracy of the vector magnetometer as an attitude sensing device for auroral sounding rockets(1979) Robinson, Robert M.; Cloutier, Paul A.; Clayton, Donald W.; Reiff, Patricia H.A Terrier-Malemute sounding rocket was launched over a stable auroral arc from Poker Flat, Alaska on March 9, 1978 U. T. Among the instruments carried by the payload were a cesium vapor magnetometer and a solid-state star sensor. The star sensor, designed and built at INIK, Lulea, Sweden, measured the time and magnitude of individual star transits during the flight. These data were used to determine the attitude and rotational dynamics of the payload to very high accuracy. Similar information was not available for previous Rice University payloads so that attitude reconstruction for these flights was done using magnetometer data alone. However, the procedure for extracting spin and coning parameters from magnetometer data requires certain assumptions about the direction of the geomagnetic field. Since independent attitude information was available for the present flight, it was possible to test the accuracy of the magnetometer as an attitude sensing device. The following errors were discovered. First, the direction of the rocket coning center as measured by the magnetometer was in error by about 3 degrees of arc. Second, the payload spin frequency computed from the magnetometer data differed by as much as .1 degrees per second from that measured by the star sensor. With regard to the detection of ionospheric currents for this and the previous flights, these errors suggest that (1) unless the rocket coning center is in the plane of the trajectory the orientation of the inferred current system may be uncertain by as much as 15 degrees, and (2) the effects of large scale field-aligned currents may be misinterpreted as a gradual variation in the spin rate of the payload.Item Aggregation of grains in a turbulent presolar disk(1983) Wieneke, Bernhard; Clayton, Donald W.; Michel, F. Curtis; Reiff, Patricia H.The growth and evolution of grains in the protostellar nebula is investigated within the context of turbulent-low mass disk models developed by previous investigators. Because of grain collisions promoted by the turbulent velocities, particles aggregate to millimeter size in times of order 1^3 yrs. During the growth the particles acquire a large inward radial velocity due to gas drag (Weidenschilling, 1977) and spiral into the sun. The calculations indicate that the final size of the particles does not exceed a few centimeters. This result is not very sensitive to the specific nebula parameters. For all conditions investigated it seems impossible to grow meter- or kilometer-sized bodies which could decouple from the gas motion. An additional argument is given which shows that only particles smaller than centimeter size can survive drift into the growing sun by being transported radially outwards by turbulent mixing. This agrees well with the maximum size of inclusions and chondrules. Since sedimentation of grains and subsequent dust disk instability is effectively inhibited by turbulent stirring, the formation of planetesimals and planets can not be explained in the above scenario without further assumptions.Item Cosmic ray muon production of isotopic anomalies in the Earth's crust(1977) Morgan, John Adrian; Clayton, Donald W.The recent discovery of naturally occurring radioactive niobium isotopes, of atomic weight 92 and 94, in commercially prepared niobium admits an explanation in terms of production by the capture of cosmic ray muons by terrestrial molybdenum, and encourages the examination of (u-,xn) mechanisms for their possible role in the origin of other trace isotopic effects.Item Digital analysis of narrow band imagery of the Cygnus loop(1983) Hester, John Jeffrey; Dufour, Reginald J.; Clayton, Donald W.; Few, Arthur A.Digital analysis of narrow passband direct imagery of a field in the southeast part of the Cygnus Loop SNR is presented. Calibrated surface brightness and spectral line ratio maps involving emission lines from six different ionic species are shown. The detailed morphology of the remnant and correlations between various line ratios are discussed in the context of recent shock model calculations. The spatial structure is categorized on the basis of the presence or absence of features in images isolating different emission lines, and the correlation or lack thereof between these features and features in the maps of spectral line ratios such as [ III] 57/Ha. For a common type of structure, features present in line ratio maps are not associated with surface brightness features. These same structures also demonstrate spectral behavior consistent with that predicted by steadyflow shock calculations, indicating the presence of steady-flow shocks with velocities from <= 6 km/sec to ≈ 12 km/sec. Differences in the morphologies of the remnant as viewed in different emission lines and gradients in spectral line ratios are interpreted as a continuous transition with distance behind the blast wave from non-steady-flow shocks with velocities >= 13 km/sec to steady-flow shocks with velocities ≈ 6 km/sec .Item Instantaneous recycling approximation in chemical evolution of galaxies(1985) Pantelaki, Irini A.; Clayton, Donald W.; Wolf, Richard A.; Dufour, Reginald J.The purpose of this study is to determine the range of validity of the instantaneous recycling approximation (IRA) in the models for the chemical evolution of galaxies. To achieve this and in order to be able to follow the evolution of the gas mass and the gas metallicity with time without making the instantaneous recycling assumption, I constructed a simple numerical code. This same code was later converted to a form assuming the instantaneous recycling approximation and the two groups of solutions (with and without IRA) were compared. The results show that in the case of a star formation rate which at early times builds up to a maximum and later declines, a physically important departure from the instantaneous recycling approximation is observed in: a) The evolution of the gas mass at late times when the remaining gas mass density becomes a small fraction of the total density and b) the time evolution of the abundances in the interstellar gas of the elements contributed by the low mass stars. A less significant discrepancy between the two solutions (with and without IRA) appears at late times in the cases of the abundances of the elements whose origin are the intermediate and high mass stars.Item Patterns of soil maturation and mixing at the Apollo 16 landing site: surface soils(1979) Ray, James R.; Heymann, Dieter; Clayton, Donald W.; Freeman, John W.An analysis of the Apollo 16 surface soil samples is presented in an effort to distinguish the separate effects due to mixing of unlike fines from those attributable purely to regolithic maturation processes. In establishing a background needed to accomplish this task, aspects of large-scale stratigraphy and geologic sequence in the central highlands are reviewed. Likewise, a comprehensive discussion of the interrelated phenomena associated with a soil's residence in the upper millimeter of the regolith follows. Data for major element chemistry and trapped inert gases are assembled from the literature and used to infer the provenance of archetypical soils and kinship relationships of remaining surface samples. The hypothesis of maturation domination of inert gas variations is considered and its consequences are delineated. It is found that this assumption, while adequate in explaining the gas properties of most soils, fails to account for about one-fifth of the samples, particularly soil 61221. An alternative approach is then presented which views the distinctive soils as remnants of ancient solar wind irradiation. In this context, unambiguous maturation effects are discernible only for the evolution of absolute gas concentrations and the elemental ratio 2^Ne/36^Ar. The unique soil 61221 is found to have excess argon relative to its very low state of maturity by a factor of about ten after correction for outgassing. This anomaly is ascribed to an irradiation between 3.5 and 4. Gy ago by a solar wind with particle flux greater than present by the same factor. A similar gas-rich component occurs in less pure form in other surface samples.Item Some beta decay lifetimes of importance to the s-process(1973) Newman, Michael John; Clayton, Donald W.The standard V-A theory of terrestrial beta decay is reviewed. Modifications required for treatment of negatron emission, positron emission, and electron capture under stellar conditions are discussed. Ionization states, population of nuclear excited states, and lifetimes against the appropriate beta decay processes are calculated for 36 beta-active nuclides lying on or near the canonical s-process path, for a range of temperatures and densities characteristic of the shell helium burning phase of red giant stars. An example is given showing the applicability of these results to determinations of the time scale of the s-process.Item The effects of a depleted Maxwellian tail on thermonuclear reactions, pressure, and solar neutrinos(1975) Dwek, Eli; Clayton, Donald W.Nuclear reaction rates and pressure are calculated using a Maxwellian distribution function which is depleted at its high energy tail. The results are applied to nuclear reactions occuring within the solar interior, showing that the neutrino counting rate can be significantly reduced.Item The time scale of the s-process(1974) Ward, Richard Arland; Clayton, Donald W.The canonical s-process theory of heavy element formation is reformulated to allow for competition between beta decay and neutron capture at various nuclei along the path. Solutions to the resulting network equations at each branch are presented that do not require the assumption of steady flow of the neutron current. An approximate expression is also developed for the complete s-process that includes the effects of all possible branches. From the observed solar system abundances and measured capture cross sections several key branching ratios are inferred. Using recently calculated temperature-dependent beta decay rates, comparisons of adjacent branches then yield an estimate for the appropriate temperature and half-life against neutron capture at selected branch points. Possible extensions of the results to observations of freshly produced s-process material in the surfaces of individual stars are also discussed.