Seeing Stars Like Never Before: A Long-term Interferometric Imaging Survey of Red Supergiants

Red supergiants (RSGs) are cool, luminous stars with radii that can exceed 1000 R⊙. Indeed, such is their size that nearly every advance in stellar imaging has used the closest RSG, Betelgeuse, as a test case! These objects represent a late stage in the evolution of some massive stars, and, via their mass-loss and eventual demise in supernovae, they play an important role in the chemical evolution of the Universe. Moreover, their high luminosities have made them an object of interest for astronomers studying nearby galaxies. As a result of their increasingly broad use in astronomy, a solid understanding of RSGs and in the limitations of models of these objects is important. One of the biggest challenges in modeling red supergiants is convection. In RSGs, granules and convection cells are quite large relative to the size of the star---with granules roughly 0.10-0.30 R*. and convection cells at least 0.50 R*. This results in large surface features that can be studied using optical interferometry, but which can also corrupt measurements of parallax and other stellar parameters. Increasingly, there exist models of RSGs which take into account this behavior, but it is important to constrain these models with actual observations. In this dissertation, we present a long-term study of surface features on RSGs using the Michigan InfraRed Combiner (MIRC/MIRC-X after 2016) at the Center for High Angular Resolution (CHARA) Astronomy Array on Mt. Wilson. Images resulting from these data are among the highest resolution obtained for any star (apart from the Sun). Fitting to model spectra, we derive Teff= 3989±117 K and log(g)=0.29±0.26 for the RSG AZ Cyg and Teff= 3650±50 K and log(g)=0.30±0.26 for the RSG SU Per. We also determine radii for 17 RSGs including AZ Cyg and SU Per. We reconstruct images of AZ Cyg from 2011, 2012, 2014, 2015 and 2016, and reconstruct images of SU Per from 2015 and two months in 2016. In both cases, we find evidence of long lived (\u3e1 year) features roughly 0.50 R* in size and short lived (\u3c1 year) features roughly 0.10 R* in size. We compare these observations to predictions from 2D and 3D models. We also discuss future directions for studying RSGs using optical interferometric imaging

A High Elevation Record of the Star-nosed Mole (Condylura cristata) in Northeastern Vermont

A single male Star-nosed Mole, Condylura cristata, was captured in a rock pile at the top of East Mountain (elevation 1042 m; 3240 ft) in northeastern Vermont. Although known from high elevations (up to 1676 m; 5500 ft) in southern U.S. states, this species was not known to occur above 573 m (1880 ft) in the northern part of its range. This record is also important in confirming that C. cristata has some climbing ability when travelling above ground

Detection of Lead in the Carbon-Rich, Very Metal-Poor Star LP625-44: A Strong Constraint on s-Process Nucleosynthesis at Low Metallicity

We report the detection of the Pb I 4057.8A line in the very metal-poor ([Fe/H]=-2.7), carbon-rich star, LP625-44. We determine the abundance of Pb ([Pb/Fe] = 2.65) and 15 other neutron-capture elements. The abundance pattern between Ba and Pb agrees well with a scaled solar system s-process component, while the lighter elements (Sr-Zr) are less abundant than Ba. The enhancement of s-process elements is interpreted as a result of mass transfer in a binary system from a previous AGB companion, an interpretation strongly supported by radial velocity variations of this system. The detection of Pb makes it possible, for the first time, to compare model predictions of s-process nucleosynthesis in AGB stars with observations of elements between Sr and Pb. The Pb abundance is significantly lower than the prediction of recent models (e.g., Gallino et al. 1998), which succeeded in explaining the metallicity dependence of the abundance ratios of light s-elements (Sr-Zr) to heavy ones (Ba-Dy) found in previously observed s-process-enhanced stars. This suggests that one should either (a) reconsider the underlying assumptions concerning the 13C-rich s-processing site (13C-pocket) in the present models, or (b) investigate alternative sites of s-process nucleosynthesis in very metal-poor AGB stars.Comment: 10 pages, 3 figures, Astrophysical Journal Letters, in pres

Phylogenetic Relationships and Divergence Times in Rodents Based on Both Genes and Fossils

Molecular and paleontological approaches have produced extremely different estimates for divergence times among orders of placental mammals and within rodents with molecular studies suggesting a much older date than fossils. We evaluated the conflict between the fossil record and molecular data and find a significant correlation between dates estimated by fossils and relative branch lengths, suggesting that molecular data agree with the fossil record regarding divergence times in rodents. Our approach includes a correction for tree hierarchy involving simulating the random appearance of fossils. We also present a ghost lineage approach that attempts to incorporate the potential for the discovery of older fossils into a Bayesian analysis of divergence dates. Applying this approach to a set of Eocene rodent fossils, we estimated the earliest divergence in rodents appears to have occurred at approximately the K/T boundary, but interordinal splits were estimated to have taken place late in the Cretaceous. We propose that some molecular clock studies may overestimate divergence times due to periods of accelerated molecular evolution across multiple lineages or due to saturation of data that is not adequately corrected by the evolutionary model. We have sequenced the complete mitochondrial genomes of three rodent species, Anomalurus beecrofti, Castor canadensis, and Dipodomys ordii, and attempt to resolve phylogenetic relationships within rodents using the mitochondrial genome, a nuclear dataset of comparable size, and a combined analysis containing 26 kbp of sequence data. The combined analysis recovered a Sciuromorpha – Hystricomorpha clade with strong support. Our data suggest that increased character sampling improves resolution at these early nodes while better taxon sampling of mitochondrial genomes has led to better supported clades that converge on conclusions obtained from nuclear datasets. Several molecular studies have concluded that the zokors, genus Myospalax, evolved from within the rodent subfamily Cricetinae. We tested this conclusion using mitochondrial data and determined that Myospalax is sister to a clade containing the subfamilies Spalacinae and Rhizomyinae, and all three of these lineages appear to be basal to the superfamily Muroidea. Based on the position of these three lineages, we suggested that they be placed in a distinct family, the Spalacidae. The murine genera Mus and Rattus are thought to have diverged about 12 million years ago (Ma) based on a series of fossils from the Siwaliks of Pakistan, but assumptions of murid relationships that led to this conclusion have been shown to be false by molecular data. Equally parsimonious hypotheses can be proposed which place the 12 million year old Progonomys fossil at the base of the family Muridae, basal to the subfamily Murinae, or at the Mus - Rattus divergence. We here test the dates of evolutionary divergences in murids. Our results indicate that the family Muridae probably diverged earlier than the Siwalik fossils, but Mus and Rattus diverged at the same time or prior to the 12 Ma fossil date. We also cannot reject the hypothesis that the 12 Ma date represents the oldest split in the Murinae instead of the more derived Mus – Rattus date. We also recovered phylogenetic results suggesting that Taterillus is related to the tribe Gerbillini and not to other genera that are treated as Taterillini and that Gerbillurus evolved from within Gerbilliscus

Lithium abundances of halo dwarfs based on excitation temperature. I. LTE

The discovery of the Spite plateau in the abundances of 7Li for metal-poor stars led to the determination of an observationally deduced primordial lithium abundance. However, after the success of the Wilkinson Microwave Anisotropy Probe (WMAP) in determining the baryon density, OmegaBh^2, there was a discrepancy between observationally determined and theoretically determined abundances in the case of 7Li. One of the most important uncertain factors in the calculation of the stellar 7Li abundance is the effective temperature, Teff. We use sixteen metal-poor halo dwarfs to calculate new Teff values using the excitation energy method. With this temperature scale we then calculate new Li abundances for this group of stars in an attempt to resolve the 7Li discrepancy. Using high signal-to-noise (S/N ~ 100) spectra of 16 metal-poor halo dwarfs, obtained with the UCLES spectrograph on the AAT, measurements of equivalent widths from a set of unblended FeI lines are made. These equivalent widths are then used to calculate new Teff values with the use of the single line radiative transfer program WIDTH6, where we have constrained the gravity using either theoretical isochrones or the Hipparcos parallax, rather than the ionization balance. The lithium abundances of the stars are calculated with these temperatures. The physical parameters are derived for the 16 programme stars, and two standards. These include Teff, log g, [Fe/H], microturbulence and 7Li abundances. A comparison between the temperature scale of this work and those adopted by others has been undertaken. We find good consistency with the temperatures derived from the Halpha line by Asplund et al. (2006), but not with the hotter scale of Melendez & Ramirez (2004).Comment: 13 pages, 9 figure

Lithium abundances in CEMP stars

Carbon-enhanced metal-poor (CEMP) stars are believed to show the chemical imprints of more massive stars (M > 0.8 Msun) that are now extinct. In particular, it is expected that the observed abundance of Li should deviate in these stars from the standard Spite lithium plateau. We study here a sample of 11 metal-poor stars and a double-lined spectroscopic binary with -1.8 <[Fe/H]< -3.3 observed with VLT/UVES spectrograph. Among these 12 metal-poor stars, there are 8 CEMP stars for which we measure or constrain the Li abundance. In contrast to previous arguments, we demonstrate that an appropriate regime of dilution permits the existence of "Li-Spite plateau and C-rich" stars, whereas some of the "Li-depleted and C-rich" stars call for an unidentified additional depletion mechanism that cannot be explained by dilution alone. We find evidence that rotation is related to the Li depletion in some CEMP stars. Additionally, we report on a newly recognized double-lined spectroscopic binary star in our sample. For this star, we develop a new technique from which estimates of stellar parameters and luminosity ratios can be derived based on a high-resolution spectrum alone, without the need for input from evolutionary models.Comment: 62 pages, 16 figures, accepted for publication in Ap

Two-Dimensional Phononic Crystal Simulation and Analysis

This thesis presents the design of a two-dimensional phononic band gap crystal simulator, and phononic crystal analysis. Phononic crystals and their application to microwave acoustic filters are studied. Wave mechanics is introduced. A two-dimensional phononic crystal simulator is developed. Simulator operation is validated through comparison with published data. Design parameters for phononic crystal band gap engineering are outlined. Digital signal processing and wave mechanics are utilized to analyze fractal and circular inclusion based phononic crystals. Topics for further study are given. Phononic crystal band structure is found to be sensitive to inclusion boundary geometry. Fractal inclusion based crystals provide multiple pass band characteristics. The evolution of a fractal inclusion in a phononic crystal may cause band gap widening and the formation of new band gaps. Circular inclusion based phononic crystals have piecewise-linear phase characteristics and quality factors up to 600

Making Molecules for Light-Driven Water Splitting

To meet growing global energy demand, methods to transform solar energy into chemical fuels are necessary. Photosynthesis provides a blueprint: using electrons from the photo-driven oxidation of water to generate reductive equivalents, which ultimately convert CO2 to carbohydrates. Key to any artificial photosynthetic device that attempts to mimic this process is the light-driven water oxidation reaction, 2H2O + 4hv → O2 + 4e-. Incorporation of a chromophoric material for absorption of light and a catalyst for water oxidation are therefore required. Several monomeric, single-site Ru polypryidyl catalysts have been investigated for this transformation. Electrochemical or Ce?IV? oxidation results in homogeneous catalytic water oxidation. Catalytic rates and onset potentials were found to be highly tunable based on the ligand environment. Water oxidation catalysts built with first-row transition metals were also investigated. Electrochemical studies of these complexes indicated complexes of Fe and Cu were catalysts for water oxidation at elevated pH. In addition to water oxidation catalysts, chromophore/redox mediator (CRM) complexes are necessary for light-driven water oxidation. A series of nine Ru polypyridyl complexes as well as four organic derivatives of perylene diimide (PDI) were synthesized, and their electrochemical and photophysical properties were investigated for possible use in an artificial photosynthetic device. Control of conjugation and heteroatoms in the polypyridyl ligands led to highly tunable redox potentials, UV/vis absorptions, and emission energies for the Ru complexes, while the PDI derivatives were found to undergo rapid charge separation upon excitation. For artificial photosynthetic devices to operate in aqueous media, it is necessary to form stable anchors to metal oxide surfaces. New synthetic procedures for making phosphonic acid-derivatized bipyridine ligands and their Ru polypyridyl complexes were devised, and these routes allow easy access to important ligands and to complexes that can bind to metal oxide surfaces. Finally, CRM complexes and catalysts were combined in a molecular Ru-choromphore-Ru-catalyst assembly. Solution studies led to the discovery of a redox mediator effect in water oxidation driven by CeIV. Detailed electrochemical and photophysical studies of the assembly anchored to metal oxide surfaces were performed, and the assembly was shown to be the first molecular chromophore-catalyst assembly capable of light-driven water oxidation.Doctor of Philosoph