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The Orion Trapezium is one of the youngest and closest star-forming regions within our Galaxy. With a dynamic age of ~3 × 10[superscript 5] yr, it harbors a number of very young hot stars, which likely are on the zero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectra in the wavelength range of 1.5-25 Å of three of its X-ray-brightest members (Θ1 Ori A, C, and E) obtained with the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra X-Ray Observatory. We measured X-ray emission lines, calculated differential emission measure distributions (DEMs), and fitted broadband models to the spectra. The spectra from all three stars are very rich in emission lines, specifically from highly ionized Fe, which includes emission from Fe XVII to Fe XXV ions. A complete line list is included. This is a mere effect of high temperatures rather than an overabundance of Fe, which in fact turns out to be underabundant in all three Trapezium members. Similarly there is a significant underabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appear close to solar. The DEM derived from over 80 emission lines in the spectrum of Θ[superscript 1] Ori C indicates three peaks located at 7.9, 25, and 66 MK. The emission measure varies over the 15.4 day wind period of the star. For the two phases observed, the low-temperature emission remains stable, while the high-temperature emission shows significant differences. The line widths seem to show a similar bifurcation, where we resolve some of the soft X-ray lines with velocities up to 850 km [superscript s-1] (all widths are stated as half-width at half-maximum), whereas the bulk of the lines remain unresolved with a confidence limit of 110 km s[superscript -1]. The broadband spectra of the other two stars can be fitted with several collisionally ionized plasma model components within a temperature range of 4.3-46.8 MK for Θ[superscript 1] Ori E and 4.8-42.7 MK for Θ[superscript 1] Ori A. The high-tempe, Smithsonian Astrophysical Observatory (Contract SV-61010), Smithsonian Astrophysical Observatory (Contract NAS 8-39073)

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009., Cataloged from PDF version of thesis., Includes bibliographical references (p. 115-126)., The exact structure of layered inorganic nanotubes is difficult to determine, but this information is vital to using atomistic calculations to predict nanotube properties. A multi-walled nanotube with a circular cross section will have either a mostly incoherent interface or a large amount of tensile strain to accommodate a coherent interface, but a polygonal cross section could result in a coherent interface with considerably less strain. An energy component model is parameterized with atomistic calculations to compare nanotubes with a circular and polygonal cross section. The model shows that for TiS2 nanotubes with some chiralities the radius at which a polygonal shape becomes energetically favorable is approximately 15 A. Due to the higher strain energy and lower interfacial energy the critical radius for polygonal formation of MoS2 nanotubes is 36 A. Both of these values are below the typical radius of TiS2 and MoS 2 nanotubes seen experimentally, indicating that for certain chiralities polygonal nanotubes should form. We also investigate the potential of inorganic nanotubes as energy storage materials. First principles calculations on curved surfaces and distorted slabs are used to analyze the effect of curvature and stacking on voltage and diffusion properties. The effect is qualitatively and quantitatively dependent on the material and structure. The Li voltage on the surface of TiS2 nanotubes decreases with a decreasing radius whether lithium is inside or outside of the nanotube. On the surface of MoS2, the voltage decreases with decreasing radius when Li is inside the tube, but increases with decreasing radius when Li is outside the tube., (cont.) The activation barrier for lithium diffusion increases with decreasing radius whether Li is outside or inside the nanotube while the barrier decreases in either case for MoS 2. When the stacking is disordered the lithium voltage and activation barrier between TiS2 layers decreases, although the decrease in voltage is not as large as the decrease in activation barrier because the stable lithium site changes from the octahedral site to the tetrahedral site at some stacking arrangements., by Kevin Tibbetts., Ph.D.

Multiwalled inorganic nanotubes with circular cross sections must have either an incoherent interface or a large amount of strain. However, nanotubes with a polygonal cross section can have a coherent interface with considerably less strain. We present a model for polygonal nanotubes with no defects where the chirality of the nanotube determines the shape of the cross section. Circular and polygonal nanotubes are compared based on their strain energy and interfacial energy. We have used first-principles calcuations to parameterize strain and interfacial energy for TiS2 nanotubes. These calculations show that the polygonal model is energetically favorable to the circular model when the inner radius is above a critical radius, 6.2 Å for a TiS2 nanotube with ten layers. These results should provide insight into further investigations of nanotube structure and allow computational studies to more accurately predict nanotube properties., Vehicle Technologies of the U.S. Department of Energy, National Science Foundation MRSEC Program

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004., Includes bibliographical references (p. 41-42)., Determining the stable structure types of an alloy is critical to determining many properties of that material. This can be done through experiment or computation. Both methods can be expensive and time consuming. Computational methods require energy calculations of hundreds of structure types. Computation time would be greatly improved if this large number of possible structure types was reduced. A method is discussed here to predict the stable structure types for an alloy based on compiled data. This would include experimentally observed stable structure types and calculated energies of structure types. In this paper I will describe the state of this technology. This will include an overview of past and current work. Curtarolo et al. showed a factor of three improvement in the number of calculations required to determine a given percentage of the ground state structure types for an alloy system by using correlations among a database of over 6000 calculated energies.I will show correlations among experimentally determined stable structure types appearing in the same alloy system through statistics computed from the Pauling File Inorganic Materials Database Binaries edition. I will compare a method to predict stable structure types based on correlations among pairs of structure types that appear in the same alloy system with a method based simply on the frequency of occurrence of each structure type. I will show a factor of two improvement in the number of calculations required to determine the ground state structure types between these two methods. This paper will examine the potential market value for a software tool used to predict likely stable structure types. A timeline for introduction of this product and an analysis of the market for such a tool will be included. There is no established market for structure type prediction software, but the market will be similar to that of materials database software and energy calculation software.The potential market is s, by Kevin Tibbetts., M.Eng.