title: Ice Mapping In The JWST Era
Creators: name: Smith, Zak
Publisher: name: The Open University
Year: 2024

description: The focus of this thesis is the study of interstellar ices. In it, I aim to address two fundamental questions on the nature of these ices:<br>
<i>How are the major interstellar ices distributed on a local scale across a single star forming region? How do the interstellar ice abundances change with the local physical conditions?</i><br>
I answer these questions through the analysis of infrared and sub-mm observations.
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The primary set of infrared observations in this thesis were taken with the NIRCam instrument on board the James Webb Space Telescope (JWST), and used to directly study the distribution of water (H<sub>2</sub>O), carbon dioxide (CO<sub>2</sub>) and carbon monoxide (CO) in their solid state across the Chameleon I molecular cloud.
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The primary set of sub-mm observations in this thesis were taken with the Institut de Radioastronomie Millimétrique (IRAM) 30m and the Atacama Pathfinder EXperiment (APEX) 12m single dish telescopes, and used to study gas-phase materials, namely Diazenylium (N<sub>2</sub>H<sup>+</sup>), allowing physical constraints to be set on the onset conditions of solid state CO in the B35A star-forming region. Archival data from the AKARI infrared space telescope and the James Maxwell Clerk Telescope (JCMT) sub-mm single dish telescope are also included to complement my own observations.
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This thesis gives a brief background on the interstellar medium and the environments found within it. In addition, it presents a brief history of interstellar ice observations, the chemical processes associated with their formation and a technique called “ice mapping” which can be used to study the spatial distribution of interstellar ices.
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An overview of the JWST and its instruments is presented, along with a complete description of the NIRCam Wide-Field Slitless Spectroscopy (WFSS) observing mode. The challenges associated with ice mapping using this observing mode are outlined. A brief overview of the JWST programme from which my data are taken from is presented, along with a description of the adjustments I made to the NIRCam WFSS observing strategy.
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A novel data reduction technique was required to successfully extract hundreds of spectra from the NIRCam WFSS data. This thesis presents an in-depth description of such a technique that I developed. The spectra I extract using my method form the the dataset from which my ice maps are built. The analysis techniques that are subsequently applied in a uniform way to these hundreds of spectra are presented. In particular, I describe novel statistically robust methods that I have applied to improve on historical approaches. The primary goal of these analyses was to extract ice column densities from my extracted spectra.
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The resultant ice column densities from the analysis are then used to build H<sub>2</sub>O, CO<sub>2</sub> and CO ice maps of the Chameleon I molecular cloud. These ice column densities show that my observations are significantly more sensitive to CO<sub>2</sub> ice than to CO or H<sub>2</sub>O ice. I found that CO<sub>2</sub> ice forms very effectively in cloud regions where there is no detectable CO freeze-out. In addition, multiple distributions of ice column density are seen for each molecular species, suggesting different formation or destruction routes may be present across different regions of the Chameleon I molecular cloud.
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Additionally, gas-phase maps of N<sub>2</sub>H<sup>+</sup> emission within the B35A star-forming region are presented. The presence of this molecule is used as a tracer of gas-phase CO depletion into the solid-state. I present detections of N<sub>2</sub>H<sup>+</sup> emission towards three regions within B35A, two of which are novel detections. These observations motivate the need for follow-up solid state CO ice observations with JWST to provide the first set of direct and indirect detections of CO freeze-out within a molecular cloud.
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The observing, reduction and analysis methods presented in this thesis form the basis for future observations of interstellar ices with the JWST NIRCam WFSS observing mode. These will be applied again to my JWST Cycle 2 observations as part of the CHEERIO programme (PID: 4358).