Thesis (Ph. D.)--Northwestern University, 2007. / Includes vita. Includes references.

Thesis (Ph. D.)--Northwestern University, 2007. / Includes vita. Includes references.

The presence of turbulence in astrophysical magnetic fields can have a significant effect on the diffusion of particles and, therefore, should be taken into account when performing simulations involving particle propagation. After reviewing the constructionof the turbulent magnetic field component, we incorporate this feature in two separate projects. In the first, we consider the possible source(s) of hadronic cosmic rays thought to be responsible for the diffuse TeV gamma-ray emission in the vicinity ofthe Galactic center. Assuming a completely turbulent magnetic field with an average strength of 10-100microG, we find that relativistic protons do not travel far enough to produce gamma-rays spatially correlated with the giant molecular clouds, as seen by HESS,when injected into the interstellar medium by a single point source, such as the supermassive black hole Sagittarius A*. Increasing the number of point sources to five does improve the longitudinal extent of the emission but either shows only weak correlation with the molecular gas or highlights the source positions - both pictures areinconsistent with HESS observations. We conclude that protons must be accelerated throughout the Galactic center region via e.g. a second-order Fermi process in order to reproduce the HESS gamma-ray map if the magnetic field there is completely turbulent. Secondly, we examine the possible link between the asymmetric 511keV electron-positron annihilation emission from the inner Galactic disk and hard low mass X-ray binaries (LMXBs). Three different magnetic field configurations were considered: a completely turbulent field, a field in which the turbulent component has equal energy density as the mean component, and a strongly ordered field with little turbulence. Assuming the environment around each LMXB system is the same, we find that the LMXBs alone cannot account for all the positrons necessary to sufficiently fill the region regardless of the particular magnetic field structure chosen. Another transport mechanism (e.g. a galactic wind) in addition to the diffusive motion caused by the magnetic field fluctuations and/or allowing the LMXBs to be embedded in different phases of the interstellar medium is needed for the LMXB picture to remain a viable possibility.

The mysterious radio source N3 appears to be located within the vicinity of the Radio Arc region of the Galactic Center. To investigate the nature of this source, we have conducted radio observations with the VLA and the VLBA. Continuum observations between 2 and 50 GHz reveal that N3 is an extremely compact and bright source with a non-thermal spectrum. Molecular line observations with the VLA reveal a compact molecular cloud adjacent to N3 in projection. The properties of this cloud are consistent with other galactic center clouds. We are able to rule out several hypotheses for the nature of N3, though a micro-blazar origin cannot be ruled out. Robotic Telescope systems are now seeing widespread deployment as both teaching and research instruments. While these systems have traditionally been able to produce high quality images, these systems have lacked the capability to conduct spectroscopic observations. To enable spectroscopic observations on the Iowa Robotic Observatory, we have developed a low cost (~ $500), low resolution (R ~ 300) spectrometer which mounts inside a modified filter wheel and a moderate cost (~ $5000), medium resolution (R ~ 8000) fiber-fed spectrometer. Software has been developed to operate both instruments robotically and calibration pipelines are being developed to automate calibration of the data. The University of Iowa offers several introductory astronomy laboratory courses taken by many hundreds of students each semester. To improve student learning in these laboratory courses, we have worked to integrate active learning into laboratory activities. We present the pedagogical approaches used to develop and update the laboratory activities and present an inventory of the current laboratory exercises. Using the inventory, we make observations of the strengths and weaknesses of the current exercises and provide suggestions for future refinement of the astronomy laboratory curriculum.

Thesis (Ph. D.)--Massachusets Institute of Technology (MIT), 2006. / PDF copy of thesis. Includes bibliograpy.

published_or_final_version / abstract / Physics / Master / Master of Philosophy

The environmental conditions in the center of the Milky Way galaxy (Central Molecular Zone; CMZ) are much more extreme than in the disk of the galaxy with molecular gas properties similar to those seen in galaxies at z~2. While the CMZ hosts several massive star clusters, the formation of stars from these dense molecular clouds and the feedback of these massive stars on the ISM is not well understood. I present in my thesis a case study of a few regions in the CMZ using VLA continuum and spectral line observations. I explore feedback eects of recent star formation (a young massive stellar cluster) on the surrounding ISM (molecular and ionized gas). Using the kinematic information of molecular and ionized gas, obtained dierent species of spectral lines, I will determine the line of sight geometry of the dierent gas components. I will then establish the relationship of these components to the larger structure of molecular gas in the CMZ.

Thesis (M. Phil.)--University of Hong Kong, 2008. / Also available in print.

The purpose of this dissertation is to locate evolved massive stars within the central 50 pc of the Galactic Center. These stars are considered to be the descendants of O stars and should be less than 10 Myr old. They trace young star clusters within the Galactic Center. Through these stars and young star clusters, we hope to understand the star formation mode and history within the Galactic Center, as well as the properties of evolved massive stars in the high metallicity environment. We first study the Chandra X-ray deep survey of the Arches and Quintuplet clusters, two of the three young massive star clusters within the Galactic Center. The diffuse X-ray emission is used to constrain their initial mass function and we find a deficiency of low-mass stars, which could be explained by an ongoing collision between the clusters and the adjacent molecular clouds. We then perform a systematic search of young massive stars on a large scale within the Galactic Center through our new HST/NICMOS Paschen-alpha survey. We produce mosaic maps of the Paschen-alpha line and continuum emission, giving an unprecedentedly high resolution and high sensitivity panoramic view of stars and photo-ionized gas in the nuclear environment of the Galaxy. Many new HII regions and extended emission regions have been found. Combined with the archived HST snapshot observations and spectroscopic observations, we construct a sample of 180 potentially evolved massive stars. A multi-wavelength study of these stars is conducted. We find that young massive stars have continued to form within the Galactic Center during the last 10 Myr and some of the evolved massive stars may represent star formation in small groups or even in isolation, compared to the three massive star clusters within the Galactic Center

The interstellar medium (ISM) of our Galaxy, and of others, is pervaded by ultra low-density gas and dust, as well as magnetic fields. Embedded magnetic fields have been known to play an important role in the structure and dynamics of the ISM. However, the ability to accurately quantify these fields has plagued astronomers for many decades. Unfortunately, the experimental techniques for measuring the strength and direction of magnetic fields are few, and they are observationally challenging. The only direct method of measuring the magnetic field is through the Zeeman effect. The goal of this dissertation is to expand upon the current observational studies and understanding of the effects of interstellar magnetic fields across various regions of the Galaxy. Zeeman effect observations of magnetic fields in two dynamically diverse environments in the Milky Way are presented: (1) An OH and HI absorption line study of envelopes of molecular clouds distributed throughout the Galaxy, and (2) A study of OH absorption lines toward the Galactic center region in the vicinity of the supermassive black hole Sgr A*. We have executed the first systematic observational survey designed to determine the role of magnetic fields in the inter-core regions of molecular clouds. Observations of extragalactic continuum sources that lie along the line-of-sight passing through Galactic molecular clouds were studied using the Arecibo telescope. OH Zeeman effect observations were combined with estimates of column density to allow for computation of the mass-to-flux ratio, a measurement of the gravitational to magnetic energies within a cloud. We find that molecular clouds are slightly subcritical overall. However, individual measurements yield the first evidence for magnetically subcritical molecular gas. Jansky VLA observations of 18 cm OH absorption lines were used to determine the strength of the line-of-sight magnetic field in the Galactic center region. This study yields no clear detections of the magnetic field and results that differ from a similar study by Killeen, Lo, & Crutcher (1992). Our results suggest magnetic fields no more than a few microgauss in strength.