Browsing by Author "Clayton, Donald D."
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Item A calculation of the mean age of interstellar dust particles(1989) Scowen, Paul Andrew; Clayton, Donald D.This thesis aims to compute the mean age of interstellar dust particles using a sputtering and recycling model originally formulated by Liffman and Clayton. The ages of the particles are evaluated after a period of 6 $\times$ 10$\sp9$ years, deemed here to be the age of our Galaxy, when the Solar System formed. I find a correlation between resulting particle sizes and their mean age. The consequence of this correlation is that if interstellar particles can be sorted dynamically by size, then the conglomerations of these size populations as dust grains will be composed of matter that is of differing ages. This age variation in grains of differing sizes will produce isotopic variations, or anomalies, due to the time dependent nature of secondary versus primary nucleosynthesis. The most important example of such an observed anomaly is the 5% enrichment of $\sp{16}$O in inclusions of Al$\sb2$O$\sb3$ in meteorite structures relative to the abundance of $\sp{16}$O in the solar gas. (Abstract shortened with permission of author.)Item Chemical evolution of dwarf irregular galaxies(1989) Pantelaki, Irini Andreas; Clayton, Donald D.I propose here an interesting historical scenario for galaxy formation that may find application to the problems of dwarf irregular galaxies in general and of I Zw18 in particular. My purpose is to present calculations of the C:N:O ratios within this simple model in an attempt to explain the apparent paradox, that characterizes I Zw18, of having very low concentrations of C, N and O in HII regions of a current starburst (some forty times smaller than solar) and yet having nearly solar ratios for C/O and N/O. I first calculate concentrations in a hot ambient medium that suffers a "Hubble-like flow" wind in a galaxy that has experienced several bursts of star formation. This hot matrix contains HI clouds whose collisions initiate the starbursts. The ambient gas which is found to have large and variable X$\sb{\rm O}$, X$\sb{\rm C}$ and X$\sb{\rm N}$ concentrations, is mixed very slowly into the clouds, so that today a few percent of the cloud mass has been gathered from the hot medium by admixing, resulting in cloud concentrations comparable to those found in I Zw18. Different assumptions were tried for the details of the galactic history. The resulting ratios of the abundances in the clouds were found to be insensitive to these details.Item Excited-state populations of singly ionized sulphur in gaseous nebulae(1968) Weisheit, Jon Carleton; Clayton, Donald D.A detailed study is made of the populations of singly ionized sulphur (SII) metastable states. These four states, 2D3/2, 2D5/2, 2P1/2, 2P3/2, and the ground state, 4S3/2, arise from a 3p3 electron configuration. Rates of populating the five states by collisional and radiative processes prominent in gaseous nebulae are examined. Inelastic electron-SII collision cross sections are estimated using a scaling technique suggested by Osterbrock (1965). The quantum defect method is used to calculate the photoionization cross sections of neutral and singly ionized sulphur. With these and related data, population rate equations for all SII (3p3) states are solved for numerous combinations of electron number densities and temperatures in the ranges 500<=Ne (cm-3)<10^9; and 5,000Item Nuclear statistical equilibrium and the iron peak(1972) Hainebach, Kem Lawrence; Clayton, Donald D.The equations governing the abundance distribution of the nuclear species (atomic nuclei) under conditions of statistical equilibrium of the strong nuclear interaction are rederived and discussed. An algorithm for their solution on a computer is presented, giving abundance distributions as a function of temperature, density and overall neutron-proton ratio. A computer search of nuclear species abundance distributions and mixtures of two distributions is carried out in an attempt to match equilibrium distributions, after weak interaction decay, of certain "iron peak" species to their observed solar system distributions. A mixture is found which reproduces the solar system abundance distribution of Fe 54,56,57,58. Ninety-nine percent of the Fe56 is produced in nuclear statistical equilibrium as Ni56, lending hope to the proposed observation by gamma ray astronomy of the decay of Ni56 to Fe56 following a supernova event.Item Nucleosyn by rapid neutron capture during explosive carbon burning(1970) Howard, William Michael; Clayton, Donald D.The purpose of this calculation is to attempt to account for the solar system abundances of certain neutron-rich nuclei by rapid neutron capture upon various seed nuclei (Fe56, Ni58, Cr Ca Ca40 and A36) in the iron peak region during explosive carbon burning. By assuming a Population I abundance of iron peak seed exposed to the temperatures and neutron densities of explosive carbon burning, we may account for the observed solar system abundances of certain neutron-rich nuclei (C137, A40, Ca46, Ca48, Ti50, Ni64, Zn66, Zn70 and Ge72). In addition, we will discuss the results of exposing a small abundance of iron peak nuclei to the conditions of explosive oxygen burning.Item Positron survival in type II supernovae(1990) Sturner, Steven J.; Clayton, Donald D.In this work I investigate the possibility of Type II supernovae being the origin for positrons producing observed annihilation radiation observed toward the Galactic center. It was my contention that the decay of $\sp{56}$Co coupled with falling densities would allow for the production and extended existence of positrons in the supernova outflow. Supernova 1987A has prompted many people to construct models of supernova outflow. I use the results of two existing models as the initial conditions in my models. I have created both an analytic and a computer model for the survival of positrons. These models show that while Type II supernovae fall short of the needed production of surviving positrons, the lower densities existing in Type I supernovae may be a more promising source.Item Stochastic histories of dust grains in the interstellar medium(1988) Liffman, Kurt; Clayton, Donald D.The purpose of this thesis is to study an evolving system of SUperNOva CONdensateS (SUNOCONS) within the Interstellar Medium (ISM). This is done via a Monte Carlo process where refractory dust grains formed within supernova remnants are subjected to the processes of sputtering and collisional fragmentation in the diffuse phase and accretion within the cold molecular cloud phase. In order to record chemical detail, we take each new particle to consist of a superrefractory core plus a more massive refractory mantle. The particles are allowed to transfer to and fro between the different phases of the ISM until either the particles are destroyed or the program finishes. The resulting chemical and size spectrum(s) are then applied to various astrophysical problems with the following results: (1) after six thousand million years roughly 10 to 20% by mass of the most refractory material (Al$\sb2$O$\sb3$) survives the rigors of the ISM intact, which leaves open the possibility that 'fossilized' isotopically anomalous material may have been present within the primordial solar nebula. (2) structured or layered refractory dust grains within our model cannot explain the observed interstellar depletions of refractory material. (3) fragmentation due to grain-grain collisions in the diffuse phase plus the accretion of material in the molecular cloud phase can under certain circumstances cause a bimodal distribution in grain size.Item Supernovae light curves(1972) Falk, Sydney Westervelt; Clayton, Donald D.The characteristics of the light curves of supernovae, (luminosities as functions of time) are an old problem in Astrophysics. Characteristic half-widths (in the visual spectrum) are of the order of 20 - 50 days, with peak (integrated) luminous energies at least of the order of 1050 ergs. In addition, spectroscopy reveals Doppler shifts suggesting associated velocities of expansion of the order of 103 - 104 km/sec in the outer regions of the exploding star. The problem of constructing a hydrodynamic model of such an object, which duplicates the features of the light curve, especially the energetics and the temporal characteristics, is further complicated by the presence of a variety of distinct types of supernovae, with widely varying light curves. An attempt is made to explore the hydrodynamic structure required to produce such characteristic luminosities, with emphasis upon the case of radiative diffusion as the dominant mode of energy transport. The models investigated will be of concentric spherical mass shells expanding radially at velocities of order 103 - 104 km/sec. Effects of the outer zones becoming optically thin will be examined, as well as effects of varying masses, compositions, ionization, shock wave heating, and radioactive decay of the (possible) nuclear products of the "explosion". It is hoped that the successful specification of such parameters as mass, composition, spatial size, as well as the mechanism responsible for the supernova outburst, will add to our understanding of the processes of stellar evolution, nucleosynthesis, and galactic evolution.Item Temperature enhanced alpha-decay and the S process(1970) Perrone, Frank Anthony; Clayton, Donald D.In this paper temperature enhanced alpha-decay at high stellar temperatures is investigated in conjunction with s-process nucleosynthesis. Temperature dependent alpha-decay is assumed to be the result of more energetic, hence faster, decay from thermally excited nuclear states. An analytical formula for the temperature dependent half-life is derived as a function of the atomic number and weight, temperature, and the alpha-decay Q-value of the isotope. The effect of such alpha-decay upon s-process abundances is discussed and information about s-process neutron flux strengths is deduced based on the calculated alpha-decay half-lives of the isotopes Nd 144 and Sm 150. The neutron flux-temperature relationships are compared to specific constant temperature carbon burning models. The carbon burning phases of massive stars are considered to be possible sources of solar system s-process abundances. Finally, information regarding the time between s-process termination and final interstellar injection of s-process products is derived using the decay calculations.