Since the first successful measurements of stellar trigonometric parallax in the 1830s, the study of nearby stars has focused on the highest proper motion stars (mu > 0.18"/yr). Those high proper motion stars have formed the backbone of the last 150 years of study of the Solar Neighborhood and the composition of the Galaxy. Statistically speaking, though, there is a population of stars that will have low proper motions when their space motions have been projected onto the sky. At the same time, over the last twenty years, populations of relatively young stars (less than ~100 Myr), most of them with low proper motions, have been revealed near (<100 >pc) the Sun. This dissertation is the result of two related projects: A photometric search for nearby (<25 >pc) southern-hemisphere M dwarf stars with low proper motions (mu < 0.18"/yr), and a search for nearby (

Massive stars profoundly influence the evolution of the Universe, from Galactic dynamics and structure to star formation. They are often found with bound companions. However, our knowledge of O-type multiple systems with periods in the range from years to thousands of years is incomplete due their great distances. We present results from a high angular resolution survey to find angularly resolved companions using the Fine Guidance Sensor (FGS) on the Hubble Space Telescope and using ground-based adaptive optics at Gemini North. We observed 75 O- and early B-type stars in Cyg OB2 and determined that 42% of the sample have at least one companion that meets a statistical criterion for gravitationally bound status. As a case study, we present an examination of high resolution, ultraviolet spectroscopy from Hubble Space Telescope of the photospheric spectrum of the O-supergiant in the massive X-ray binary HDE 226868 = Cyg X-1. We analyzed the ultraviolet and ground-based optical spectra to determine the effective temperature and gravity of the O9.7 Iab supergiant. Using non-LTE, line blanketed, plane parallel models from the TLUSTY grid, we obtain Teff = 28.0 +/- 2.5 kK and log g > 3.00 +/- 0.25, both lower than found in previous studies. The optical spectrum is best fit with models that have enriched He and N abundances. We fit the model spectral energy distribution for this temperature and gravity to the UV, optical, and IR fluxes to determine the angular size of and extinction towards the binary. By assuming that the supergiant rotates synchronously with the orbit, we can use the radius - distance relation to find mass estimates for both components as a function of the distance and the ratio of stellar to Roche radius. Our results indicate masses of 23+8-6 solarmasses for the supergiant and 11+5-3 solarmasses for the black hole. These results agree with subsequent mass estimates Orosz et al. (2011) based on the trigonometric parallax distance measurements of Reid et al. (2011). The results of this survey provide fundamental information on the impact of environment on massive binaries and also the role multiplicity has on massive star formation and evolution.

Transient absorption spectroscopy was used to study light induced electron transfer processes in Type 1 photosynthetic reaction centers. Flash induced absorption changes were probed at 800, 703 and 487 nm, and on multiple timescales from nanoseconds to tens of milliseconds. Both wild type and menB mutant photosystem I reaction centers from the cyanobacterium Synechocystis sp. PCC 6803 were studied. Photosystem I reaction centers from the green algae Chlamydomonas reinhardtii, and the newly discovered chlorophyll-d containing organism Acaryochloris marina, were also studied. The flash induced absorption changes obtained for menB mutant photosystem I reaction centers are distinguishable from wild type at 800 nm. MenB mutant photosystem I reaction centers displays a large amplitude decay phase with lifetime of ~50 ns which is absent in wild type photosystem I reaction centers. It is hypothesized that this ~50 ns phase is due to the formation of the triplet state of primary electron donor.

Be stars are rapidly rotating B-type stars that eject large amounts of material into a circumstellar disk. Evidence of the presence of a disk is found through hydrogen emission lines in their spectra, IR excess flux, and linear intrinsic polarization. In this dissertation, we report the first simultaneous interferometric and spectroscopic observations of circumstellar disks around 24 bright Be stars made using the techniques of long baseline interferometry and moderate resolution spectroscopy in the near infrared. The goal of the project is to characterize the fundamental geometrical and physical properties of the emitting regions that are responsible for the IR flux excesses detected in the K-band in our sample stars. This observational work has been conducted with both the Center for High Angular Resolution Astronomy (CHARA) Array at Mount Wilson Observatory, and the Mimir spectrograph at Lowell Observatory. The visibility measurements were interpreted with different geometrical and physical disk models in order to determine the spatial extension of the disk, the inclination angle, the position angle, and the density profile of the disk. We find that the spatial extension of the circumstellar disk in the K-band is only about a few stellar radii, and that the density structure of the disk is consistent with a radially decreasing function with a density exponent that ranges between 2.5 and 3.5. The resulting disk densities are in a good agreement with those derived from the Infrared Astronomical Satellite (IRAS) measurements, and the resulting disk geometries are consistent with previous polarimetric measurements. We find that the K-band sizes of the emitting regions in the disk are smaller by a factor of two than the Hα sizes, and we show that this is due to the lower opacity of the continuum in the disk. By combining recent measurements of the projected rotational velocities with the disk inclination angles derived from interferometry, we were able to estimate the actual equatorial linear rotational velocities of the Be stars in our sample. The obtained linear rotational velocities indicate that Be stars are rapid rotators with an equatorial velocity that is about 0.7 - 0.9 of their critical velocities.

J/psi production is considered a very important probes for studying the properties of quark-gluon plasma (QGP). At the PHENIX experiment at Brookhaven National Laboratory, a large suppression of J/psi production in Au+Au collisions at 200 GeV center of mass energy as compared to the binary collision scaled p+p collisions was observed. The level of suppression is similar to that observed at other energies at CERN's SPS and LHC experiments. This work addresses the PHENIX J/psi measurements at sqrt{s_{NN}}= 39 and 62 GeV Au+Au collisions. These allow for the energy dependent J/psi suppression measurements in order to disentangle the important contributing factors of J/psi production. J/psi results over a wide range of center of mass energies (39-200 GeV) from PHENIX are discussed, in addition to a comprehensive comparison with other experiments.

This document presents a study of astronomical observing conditions of Hard Labor Creek Observatory. Analysis of factors such as sky brightness, astonomical seeing, and patterns in the level of cloudiness at the site are presented. Characteristics of the observatory's Apogee Alta U230 camera are also measured and calculated. These characteristics include loss of linearity in the CCD's response to light, read noise, gain, dark current, and stability in the camera's bias levels. The camera is also used in conjunction with the 20-inch RC Optics telescope to determine the system's pixel scale and a set of limiting magnitudes for the Johnson-Cousins photometric filters that are used with the camera. Observations of a transit of known transiting exoplanet Qatar-2 b as well as observations of the open cluster Messier 29 are also performed to demonstrate the ability of the equipment to perform precise photometric observations.

In this research the growth of InN epilayers by high-pressure chemical vapor deposition (HPCVD) and structural, optical properties of HPCVD grown InN layers has been studied. We demonstrated that the HPCVD approach suppresses the thermal decomposition of InN, and therefore extends the processing parameters towards the higher growth temperatures (up to 1100K for reactor pressures of 15 bar, molar ammonia and TMI ratios around 800, and a carrier gas flow of 12 slm). Structural and surface morphology studies of InN thin layers have been performed by X-ray diffraction, low energy electron diffraction (LEED), auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS) and atomic force microscopy (AFM). Raman spectroscopy, infrared reflection, transmission, photoluminescence spectroscopy studies have been carried out to investigate the structural and optical properties of InN films grown on sapphire and GaN/sapphire templates. InN layers grown on a GaN (0002) epilayer exhibit single-phase InN (0002) X-ray diffraction peaks with a full width at half maximum (FWHM) around 200 arcsec. Auger electron spectroscopy confirmed the cleanliness of the surface, and low energy electron diffraction yielded a 1×1 hexagonal pattern indicating a well-ordered surface. The plasmon excitations are shifted to lower energies in HREEL spectra due to the higher carrier concentration at the surface than in the bulk, suggesting a surface electron accumulation. The surface roughness of samples grown on GaN templates is found to be smoother (roughness of 9 nm) compared to the samples grown on sapphire. We found that the deposition sometimes led to the growth of 3 dimensional hexagonal InN pyramids. Results obtained from Raman and IR reflectance measurements are used to estimate the free carrier concentrations, which were found in the range from mid 10^18 cm-3 to low 10^20 cm-3. The optical absorption edge energy calculated from the transmission spectra is 1.2 eV for samples of lower electron concentration. The Raman analysis revealed a high-quality crystalline layer with a FWHM for the E2(high) peak around 6.9 cm^-1. The results presented in our study suggest that the optimum molar ratio might be below 800, which is due to the efficient cracking of the ammonia precursor at the high reactor pressure and high growth temperature.

A two-scintillator paddle muon telescope with variable angular acceptance at the earth's surface was used to study correlations between flux distribution and barometric pressure. The detector was placed in 2 different locations around Georgia State University with varying paddle separations of 0, 7, and 14 inches. Correlation and anti-correlation analyses were conducted by using the muon count from the detector along with the barometric pressure, surface temperature, stratospheric temperature and solar activity. It was observed that there was a short and long-term variation relationship between cosmic ray counts and barometric pressure and also cosmic ray counts and temperature. No significant relationship was found between cosmic ray flux and solar activity. A new two-scintillator paddle telescope with larger detecting area was constructed in order to observe a stronger correlation between cosmic ray flux and pressure.

Synchronized oscillations of ensembles of neurons in the brain underlie human cognition and behaviors. Neuronal network oscillations can be described by the physics of coupled dynamical systems. This dissertation examines the dynamic network activities in two distinct neurocognitive networks, the salience network (SN) and the ventral temporal cortex-dorsolateral prefrontal cortex (VTC-DLPFC) network, during perceptual decision-making (PDM). The key nodes of the SN include the right anterior insula (rAI), left anterior insula (lAI), and dorsal anterior cingulate cortex (dACC) in the brain. When and how a sensory signal enters and organizes within the SN before reaching the central executive network including the prefrontal cortex has been a mystery. Second, prior studies also report that perception of visual objects (face and house) involves a network of the VTC—the fusiform face area (FFA) and para-hippocampal place area (PPA)—and the DLPFC. How sensory information enters and organizes within the VTC-DLPFC network is not well understood, in milliseconds time-scale of human’s perception and decision-making. We used clear and noisy face/house image categorization tasks and scalp electroencephalography (EEG) recordings to study the dynamics of these networks. We demonstrated that beta (13–30 Hz) oscillation bound the SN, became most active around 100 ms after the stimulus onset, the rAI acted as a main outflow hub within the SN, and the SN activities were negatively correlated with the difficult tasks. We also uncovered that the VTC-DLPFC network activities were mediated by beta (13-30 Hz) and gamma (30-100 Hz) oscillations. Beta activities were enhanced in the time frame 125-250 ms after stimulus onset, the VTC acted as main outflow hub, and network activities were negatively correlated with the difficult tasks. In contrast, gamma activities were elevated in the time frame 0-125 ms, the DLPFC acted as a main outflow hub, and network activities—specifically the FFA-PPA pair—were positively correlated with the difficult tasks. These findings significantly enhance our understanding of how sensory information enters and organizes within the SN and the VTC-DLPFC network, respectively in PDM.

This dissertation presents the results of a study to (1) determine the census of the nearby southern M dwarf stellar population via three types of distances and (2) determine the multiplicity rate of nearby M dwarfs using two different search methodologies. The first part of this work reports three types of distance calculations (photographic, photometric, and trigonometric) for 1748 southern M dwarfs. Distances were estimated for 500 red dwarfs using photographic plate BRI magnitudes from SuperCOSMOS, while estimates were made for 667 stars using CCD VRI magnitudes. Both BRI and VRI were combined with 2MASS infrared JHK magnitudes. Distances for an additional 581 red dwarfs were derived from trigonometric parallaxes, 124 of which were measured for the first time during this work. For the second portion of this thesis, an all-sky sample of 1122 M dwarfs, known via trigonometric parallaxes to lie within 25 pc of the Sun, was surveyed for stellar companions at separations of 2" to 600". I-band images using primarily the CTIO/SMARTS 0.9m and the Lowell 42in telescopes were obtained in order to search these systems for companions at separations of 2" to 180". A complementary reconnaissance of wider companions to 600" was also done via blinking SuperCOSMOS BRI images. We find a stellar multiplicity fraction of 27.4 $\pm$ 1.3% for M dwarfs. Using this new gauge of M dwarf multiplicity near the end of the stellar main sequence, we calculate a multiplicity fraction of 30.1% for stellar systems of all types, implying that most systems are single. We find a peak in the separation distribution of the companions at 26 AU, i.e., distances on the scale of our Solar System, with a weak trend of smaller projected separations for lower mass primaries. A hint that M dwarf multiplicity may be a function of age/composition was revealed, with faster moving (and generally older) systems being multiple slightly less often. We calculate that at least 16% of M dwarf mass is made up of the stellar companions of multiple systems. Finally, we show that the mass function for M dwarfs increases to the end of the main sequence.