Browsing by Subject "Interstellar medium"
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Item Probing conditions at ionized/molecular gas interfaces with high resolution near-infrared spectroscopy(2017-08) Kaplan, Kyle Franklin; Dinerstein, Harriet L. (Harriet Lenore); Jaffe, D. T.; Evans, Neal J; Ferland, Gary J; Lacy, John HRegions of star formation and star death in our Galaxy trace the cycle of gas and dust in the interstellar medium (ISM). Gas in dense molecular clouds collapses to form stars, and stars at the end of their lives return the gas that made up their outer layers back out into the Galaxy. Hot stars generate copious amounts of ultraviolet photons which interact with the surrounding medium and dominate the energetics, ionization state, and chemistry of the gas. The interface where molecular gas is being dissociated into neutral atomic gas by far-UV photons from a nearby hot source is called a photodissociation or photon-dominated region (PDR). PDRs are found primarily in star forming regions where O and B stars serve as the source of UV photons, and in planetary nebulae where the hot core of the dying star acts as the UV source. The main target of this dissertation is molecular hydrogen (H₂), the most abundant molecule in the Universe, made from hydrogen formed during the Big Bang. H₂ makes up the overwhelming majority of molecules found in the ISM and in PDRs. Far-UV radiation absorbed by H₂ will excite an electron in the molecule. The molecule then either dissociates (~10% of the time; Field et al. 1966) or decays into excited ro- tational and vibrational (“rovibrational”) levels of the electronic ground state. These excited rovibrational levels then decay via a radiative cascade to the ground rovibrational state (v = 0, J = 0), giving rise to a large number of transitions observable in emission from the mid-IR to the optical (Black & van Dishoeck, 1987). These transitions provide an excellent probe of the excitation and conditions within the gas. These transitions are also observed in warm H₂, such as in shocks, where collisions excite H₂ to higher rovibrational levels. High resolution near-infrared spectroscopy, with its ability to see through dust, and avoid telluric absorption and emission, serves as an effective tool to detect emission from ions, atoms, and molecules within PDRs. The Immersion Grating INfrared Spectrometer (IGRINS), with a high spectral resolution of ~45,000 and simultaneous wavelength coverage of the near infrared H and K bands (1.45–2.45 μm) has proven to be an excellent instrument for such studies. Over 200 H₂ rovibrational transitions are observable within the wavelength coverage of IGRINS. In this dissertation, we use IGRINS on the 2.7m telescope at McDonald Observatory, to observe a variety of PDRs in the ISM and use the rovibrationally excited H₂ to probe the physical conditions within them. We fit our data with grids of Cloudy models (Ferland et al., 2013), which reproduce the observed H₂ rovibrational level populations, to determine the physical parameters in the gas such as temperature, density, and UV field intensity. This dissertation is split into five chapters. In the first chapter, we introduce our science questions and explain our observations, data processing, and how to analyze H₂ emission. In the second chapter, we present a deep near-infrared spectrum of the Orion Bar PDR. In the third chapter, we analyze several other PDRs in star forming regions in a similar fashion to the Orion Bar, finding significant differences in their H₂ excitation and conditions. In the fourth chapter, we use the high spectral resolution of IGRINS to reveal kinematically and energetically distinct components of H₂ emission in three planetary nebulae (M 1-11, Vy 2-2, and Hen 2-459) consisting of UV-excited (PDR) H₂ and red- and blue-shifted thermal H₂ “bullets” that likely represent shocked molecular gas that is distinct from the UV-excited PDR components. In the fifth chapter, we summarize this dissertation, discuss the broader implications of this work, and suggest future directions for near-IR ISM research.Item Study of galactic clumps with millimeter / submillimeter continuum and molecular emission : early stages of massive star formation(2014-08) Merello Ferrada, Manuel Antonio; Evans, Neal J.Massive stars play a key role in the evolution of the Galaxy; hence they are important objects of study in astrophysics. Although they are rare compared to low mass stars, they are the principal source of heavy elements and UV radiation, affecting the process of formation of stars and planets, and the physical, chemical, and morphological structure of galaxies. Star clusters form in dense "clumps" (~few parsecs in size) within giant molecular clouds, while individual stars form in cores (subparsec scale). An important step in the observational study of massive star formation is the identification and characterization of clumps. More detailed studies can then show how these clumps fragment into cores. Studies of clumps in our Galaxy will provide fundamental guidelines for the analysis of other galaxies, where individual clumps and cores cannot be resolved, and provide a catalog of interesting sources for observations of the Milky Way with a new generation of instruments, such as the Atacama Large Millimeter/Submillimeter Array. Large-scale blind surveys of the Galactic plane at millimeter and submillimeter wavelengths have recently been completed, allowing us to identify star forming clumps and improve our understanding of the early stages of massive stars. One of these studies, the Bolocam Galactic Plane Survey (BGPS), mapped the continuum emission at 1.1 mm over a large region of the northern Galactic plane at a resolution of 33'', identifying 8559 compact sources throughout the Galaxy. In this dissertation, I present observations of a sample of sources from the BGPS catalog, obtained with the Submillimeter High Angular Resolution Camera II (SHARC-II). I present in this work 107 continuum emission maps at 350 microns at high angular resolution (8.5'') toward clump-like sources and construct a catalog of BGPS substructures. I estimate clump properties such as temperatures and multiplicity of substructures, and compare my results with 350 microns continuum maps from the Hi-GAL survey. I also present a detailed analysis, using molecular line and dust continuum observations, of the region G331.5-0.1, one of the most luminous regions of massive star formation in the Milky Way, located at the tangent region of the Norma spiral arm. Molecular line and millimeter continuum emission maps reveal the presence of six compact and luminous molecular clumps, with physical properties consistent with values found toward other massive star forming sources. This work includes the discovery of one of the most energetic and luminous molecular outflows known in the Galaxy, G331.512-0.103. For this high-speed outflow, I present ALMA observations that reveal a very compact, extremely young bipolar outflow and a more symmetric outflowing shocked shell surrounding a very small region of ionized gas. The source is one of the youngest examples of massive molecular outflows associated with the formation of a high-mass star.Item Studying star formation at low and high redshift with integral field spectroscopy(2011-05) Blanc, Guillermo; Gebhardt, Karl; Evans, Neal J.; Hill, Gary J.; Bromm, Volker; Gawiser, EricIn this thesis I focus mainly in studying the process of star formation in both high redshift, and local star forming galaxies, by using an observational technique called integral field spectroscopy (IFS). Although these investigations are aimed at studying the star formation properties of these objects, throughout this work I will also discuss the geometric, kinematic, and chemical structures in the inter-stellar medium of these galaxies, which are intimately connected with the process of star formation itself. The studies presented here were conducted under the umbrella of two different projects. First, the HETDEX Pilot Survey for Emission Line Galaxies, where I have studied the properties of Ly-alpha emitting galaxies across the 2