Astrochemistry in Star-forming Regions: LaboratoryMillimeter-Submillimeter Spectroscopy and BroadbandAstronomical Line Surveys Restricted; Files Only

Zou, Luyao (Fall 2017)

Permanent URL: https://etd.library.emory.edu/concern/etds/1544bq04h?locale=en
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Abstract

The interstellar medium in star-forming regions is highly molecular, evidenced by the detection of nearly 200 molecules via radio astronomy. Despite the confirmed chemical complexity in star-forming regions, mechanisms that drive the formation of this chemical complexity are far from well understood. The widely accepted overall picture is that ion-molecular reactions dominate the gas phase, while gas-grain chemistry dominate in the icy mantle on the dust surface. Still, when narrating reaction pathways leading to specific interstellar molecules, astronomical observations, laboratory experiments, and modeling often draw convoluted, even contradictory results and interpretations. Models suggest that ions and radicals, the unstable molecules under terrestrial conditions, play key roles in the gas-phase chemistry and ice chemistry, respectively. In order to elucidate these astrochemistry processes, it is insightful to combine perspectives from laboratory rotational spectroscopy and broadband astronomical line surveys. In this dissertation, I first describe a millimeter-submillimeter spectrometer and the development of a fast-sweep technique that facilitates the study of unstable molecules, and then demonstrate the application of these techniques to rotational spectroscopy and astronomy. The performance of the fast-sweep technique was fully evaluated, and its application was demonstrated by the spectral acquisition and analysis of trans-HO3 radical and Ar-H2O dimer. Meanwhile, quantitative abundance and temperature of interstellar molecules were determined from a global analysis of broadband astronomical line surveys of a large sample of star-forming regions. This analysis constructs the correlations between molecules in a variety of interstellar environment and evolutionary stage. It is also a first step to search for new interstellar molecules, preferably the reaction intermediates whose spectra can be measured using the laboratory millimeter-submillimeter spectroscopy described in this dissertation. The preliminary results of global analysis, as well as the search for trans-HO3 , were presented.

Table of Contents

1 Introduction ................................ 1

1.1 Rotational spectroscopy.......................... 1

1.2 The environment and chemistry in star-forming regions . . . . . . . . 3

1.3 Millimeter–submillimeter spectroscopy and its application to astrochem-istryinstar-formingregions........................ 5

1.4 Organization of this dissertation..................... 6

2 Experimental Design.......................... 8

2.1 Thejet-cooled molecular source ..................... 8

2.1.1 Supersonic expansion ....................... 8

2.1.2 High voltage pulsed discharge source. . . . . . . . . . . . . . . 10

2.2 The millimeter–submillimeter spectrometer . . . . . . . . . . . . . . . 12

2.2.1 Themultipass spectrometer.................... 12

2.2.2 Detection of molecula rsignal................... 15

2.3 Fast-sweep data acquisition technique. . . . . . . . . . . . . . . . . . 16

2.3.1 Concept............................... 16

2.3.2 Instrument settings and practical consideration. . . . . . . . . 20

2.3.3 Benchmark with known methanol (CH3OH) transitions . . . . 22

2.3.4 Concerns regarding radio frequency interference (RFI). . . . . 37

2.4 Summary.................................. 38

3 The Weakly Bound Radical: HO3 ................ 39

3.1 Introduction ................................ 39

3.2 Experimental details............................ 43

3.3 Results................................... 45

3.3.1 HO3andDO3detection...................... 45

3.3.2 New spectral features ....................... 48

3.4 Discussion.................................. 51

3.5 Summary.................................. 53

4 The Vibration-Rotation-Tunneling Spectrum of Ar–H2O.................................... 54

4.1 Introduction ................................ 54

4.2 Experimental details............................ 57

4.3 Model.................................... 58

4.4 Result and discussion ........................... 60

4.4.1 Observedbands........................... 60

4.4.2 Coriolis interaction......................... 66

4.5 Summary.................................. 72

5 Global Optimization Broadband Analysis Software for Interstellar Chemistry (GOBASIC) . . . . . . . . . . . . . . . 73

5.1 An outline of GO BASIC.......................... 73

6 Broadband Line Surveys of Star-forming Regions Using the Caltech Submillimeter Observatory (CSO) . . . . . . 75

6.1 Introduction ................................ 75

6.2 Observations andd at a reduction..................... 77

6.3 Line identification andanalysis...................... 80

6.4 Results................................... 87

6.5 Discussion.................................. 95

6.6 Search for HO3 instar-forming regions ................. 101

6.7 Summary.................................. 103

7 Conclusion and Future Outlook.................. 105

A Python Script for the Fast-sweep Technique . . . . . . . . 107

B SPFIT Assignment and Output File for HO3 and DO3 121

B.1 SPFITassignmentandoutputfileforHO3 . . . . . . . . . . . . . . . 121

B.2 SPFITassignmentandoutputfileforDO3 . . . . . . . . . . . . . . . 124

C Full Analysis Result for the CSO Surveys in a Machine-readableFormat ............................. 130

Bibliography.......... 141

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