Probing conditions at ionized/molecular gas interfaces with high resolution near-infrared spectroscopy
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Regions 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.